Alkanes are
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s that consist only of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms and are bonded exclusively by single bonds (i.e., they are
saturated compoundsIn chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
) without any cycles (or loops; i.e.,
cyclic structureIn chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...
). Alkanes belong to a
homologous seriesIn chemistry, a homologous series is a series of compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and showing a gradation in physical properties as a result of increase in molecular size and mass...
of organic compounds in which the members differ by a constant relative
molecular massThe molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
of 14.
Each carbon atom must have 4 bonds (either C-H or C-C bonds), and each hydrogen atom must be joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the
carbon skeletonThe skeletal formula of an organic compound is a shorthand representation of its molecular structure, developed by the organic chemist, Friedrich August Kekulé von Stradonitz. Skeletal formulae are ubiquitous in organic chemistry, because they are relatively quick and simple to draw. Carbon and...
or carbon backbone. In general, the number of carbon atoms is often used to define the size of the alkane (e.g., C
2-alkane).
An alkyl group, generally abbreviated with the symbol R, is a
functional groupIn organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms, for example a methyl or
ethyl groupIn chemistry, an ethyl group is an alkyl substituent derived from ethane . It has the formula -C2H5 and is very often abbreviated -Et.Ethylation is the formation of a compound by introduction of the ethyl functional group, C2H5....
.
The simplest possible alkane (the parent molecule) is
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, CH
4. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is
saturatedIn chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
, and is a
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
. Saturated oils and waxes are examples of larger alkanes where the number of carbons in the carbon backbone tends to be greater than 10.
Alkanes are not very reactive and have little
biological activityIn pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or pharmacophore but can be modified by the other...
. Alkanes can be viewed as a molecular tree upon which can be hung the more biologically active/reactive portions (functional groups) of the molecule.
Structure classification
Saturated hydrocarbons can be:
- linear (general formula ) wherein the carbon atoms are joined in a snake-like structure
- branched (general formula , n > 3) wherein the carbon backbone splits off in one or more directions
- cyclic
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...
(general formula , n > 2) wherein the carbon backbone is linked so as to form a loop.
According to the definition by
IUPACThe International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
, the former two are alkanes, whereas the third group is called
cycloalkaneCycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...
s. Saturated hydrocarbons can also combine any of the linear, cyclic (e.g., polycyclic) and branching structures, and they are still alkanes (no general formula) as long as they are acyclic (i.e., having no loops).They also have single covalent bonds between their carbons
Isomerism
Alkanes with more than three carbon atoms can be arranged in numerous ways, forming different structural isomers. An isomer, in part, similar to a chemical
anagramAn anagram is a type of word play, the result of rearranging the letters of a word or phrase to produce a new word or phrase, using all the original letters exactly once; e.g., orchestra = carthorse, A decimal point = I'm a dot in place, Tom Marvolo Riddle = I am Lord Voldemort. Someone who...
but unlike an anagram, may contain varying number of atoms and components, for which in a
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
can be structurally arranged in a multitude of different combinations and permutations. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the
n-isomer (
n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms . For example:
- C1: no isomers: methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
- C2: no isomers: ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
- C3: no isomers: propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
- C4: 2 isomers: n-butane & isobutane
Isobutane, also known as methylpropane, is an isomer of butane. It is the simplest alkane with a tertiary carbon. Concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers,...
- C5: 3 isomers: pentane
Pentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
, isopentaneIsopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and...
, neopentaneNeopentane, also called dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is an extremely flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, or when compressed to a higher...
- C6: 5 isomers: hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
, 2-Methylpentane2-Methylpentane is a branched-chain alkane with the molecular formula C6H14. It is a structural isomer of hexane composed of a methyl group bonded to the second carbon atom in a pentane chain. It is of similar structure to one of its isomers, 3-methylpentane, which has the methyl group located on...
, 3-Methylpentane3-Methylpentane is a branched-chain alkane with the molecular formula C6H14. It is a structural isomer of hexane composed of a methyl group bonded to the third carbon atom in a pentane chain. It is of similar structure to the isomeric 2-methylpentane, which has the methyl group located on the...
, 2,3-Dimethylbutane2,3-Dimethylbutane, also known as diisopropyl, is an isomer of hexane. It has the chemical formula 2CHCH2 It is a colorless liquid which boils at 57.9 °C....
& 2,2-Dimethylbutane2,2-Dimethylbutane, trivially known as neohexane, is an organic compound with formula C6H14 or 3-C-CH2-CH3. It is therefore an alkane, indeed the most compact and branched of the hexane isomers — the only one with a quaternary carbon and a butane backbone.-See also:*methylbutane...
- C12: 355 isomers
- C32: 27,711,253,769 isomers
- C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
Branched alkanes can be chiral:
3-methylhexane3-Methylhexane is a branched hydrocarbon. It is one of the isomers of heptane....
and its higher
homologuesIn chemistry, homology refers to the appearance of homologues. A homologue is a compound belonging to a series of compounds differing from each other by a repeating unit, such as a methylene group, a peptide residue, etcetera....
are
chiralA chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
due to their stereogenic center at carbon atom number 3. Chiral alkanes are of certain importance in biochemistry, as they occur as sidechains in
chlorophyllChlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
and
tocopherolTocopherols are a class of chemical compounds of which many have vitamin E activity. It is a series of organic compounds consisting of various methylated phenols...
(
vitamin EVitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...
). Chiral alkanes can be resolved into their enantiomers by enantioselective
chromatographyChromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
.
In addition to these isomers, the chain of carbon atoms may form one or more loops. Such compounds are called
cycloalkaneCycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...
s.
Nomenclature
The IUPAC nomenclature (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane".
August Wilhelm von HofmannAugust Wilhelm von Hofmann was a German chemist.-Biography:Hofmann was born at Gießen, Grand Duchy of Hesse. Not intending originally to devote himself to physical science, he first took up the study of law and philology at Göttingen. But he then turned to chemistry, and studied under Justus von...
suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine (or -yne), -one, -une, for the hydrocarbons. The first three name hydrocarbons with single, double and triple bonds; "-one" represents a
ketoneIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
; "-ol" represents an alcohol or OH group; "-oxy-" means an
etherEthers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
and refers to oxygen between two carbons, so that methoxy-methane is the IUPAC name for dimethyl ether.
It is difficult or impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets. Numbers in the name, referring to which carbon a group is attached to, should be as low as possible, so that 1- is implied and usually omitted from names of organic compounds with only one side-group; "1-" is implied in nitro-octane. Symmetric compounds will have two ways of arriving at the same name.
Linear alkanes
Straight-chain alkanes are sometimes indicated by the prefix
n- (for
normal) where a non-linear
isomerIn chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g.,
n-hexaneHexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
or 2- or 3-methylpentane.
The members of the series (in terms of number of carbon atoms) are named as follows:
- methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, CH4 - one carbon and four hydrogen
- ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
, C2H6 - two carbon and six hydrogen
- propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
, C3H8 - three carbon and 8 hydrogen
- butane
Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
, C4H10 - four carbon and 10 hydrogen
- pentane
Pentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
, C5H12 - five carbon and 12 hydrogen
- hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
, C6H14 - six carbon and 14 hydrogen
These names were
derivedIn etymology, back-formation is the process of creating a new lexeme, usually by removing actual or supposed affixes. The resulting neologism is called a back-formation, a term coined by James Murray in 1889...
from
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
,
etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
,
propionic acidPropanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...
and
butyric acidButyric acid , also known under the systematic name butanoic acid, is a carboxylic acid with the structural formula CH3CH2CH2-COOH. Salts and esters of butyric acid are known as butyrates or butanoates...
, respectively. Alkanes with five or more carbon atoms are named by adding the
suffixAn affix is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes...
-ane to the appropriate
numerical multiplierThe numerical multiplier in IUPAC nomenclature indicates how many particular atoms or functional groups are attached at a particular point in a molecule...
prefix with elision of any terminal vowel (
-a or
-o) from the basic numerical term. Hence,
pentanePentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
, C
5H
12;
hexaneHexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
, C
6H
14;
heptanen-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...
, C
7H
16;
octaneOctane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH36CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain...
, C
8H
18; etc. The prefix is generally Greek, with the exceptions of nonane which has a Latin prefix, and undecane and tridecane which have mixed-language prefixes. For a more complete list, see
List of alkanes.
Branched alkanes
Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for example
n-pentanePentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
,
isopentaneIsopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and...
, and
neopentaneNeopentane, also called dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is an extremely flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, or when compressed to a higher...
.
IUPAC naming conventions can be used to produce a systematic name.
The key steps in the naming of more complicated branched alkanes are as follows:
- Identify the longest continuous chain of carbon atoms
- Name this longest root chain using standard naming rules
- Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl"
- Number the root chain so that sum of the numbers assigned to each side group will be as low as possible
- Number and name the side chains before the name of the root chain
- If there are multiple side chains of the same type, use prefixes such as "di-" and "tri-" to indicate it as such, and number each one.
- Add side chain names in alphabetical (disregarding "di-" etc. prefixes) order in front of the name of the root chain
Comparison of nomenclatures for three isomers of C5H12
| Common name |
n-pentane |
isopentane |
neopentane |
| IUPAC name |
pentane |
2-methylbutane |
2,2-dimethylpropane |
| Structure |
 |
 |
 |
Cyclic alkanes
So-called cyclic alkanes are, in the technical sense,
not alkanes, but cycloalkanes. They are hydrocarbons just like alkanes, but contain one or more rings.
Simple cycloalkanes have a prefix "cyclo-" to distinguish them from alkanes. Cycloalkanes are named as per their acyclic counterparts with respect to the number of carbon atoms, e.g.,
cyclopentaneCyclopentane is a highly flammable alicyclic hydrocarbon with chemical formula 510 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occurs as a colorless liquid with a petrol-like odor. Its melting point is −94 °C...
(C
5H
10) is a cycloalkane with 5 carbon atoms just like
pentanePentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
(C
5H
12), but they are joined up in a five-membered ring. In a similar manner,
propanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
and
cyclopropaneCyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms...
,
butaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
and
cyclobutaneCyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
, etc.
Substituted cycloalkanes are named similar to substituted alkanes — the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by Cahn-Ingold-Prelog rules.
Trivial names
The trivial (non-
systematicA chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry ....
) name for alkanes is "
paraffinIn chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
s." Together, alkanes are known as the
paraffin series. Trivial names for compounds are usually historical artifacts. They were coined before the development of systematic names, and have been retained due to familiar usage in industry. Cycloalkanes are also called naphthenes.
It is almost certain that the term
paraffinIn chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
stems from the petrochemical industry. Branched-chain alkanes are called
isoparaffins. The use of the term "paraffin" is a general term and often does not distinguish between a pure compounds and mixtures of isomers with the same
chemical formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
(i.e., like a chemical
anagramAn anagram is a type of word play, the result of rearranging the letters of a word or phrase to produce a new word or phrase, using all the original letters exactly once; e.g., orchestra = carthorse, A decimal point = I'm a dot in place, Tom Marvolo Riddle = I am Lord Voldemort. Someone who...
), e.g.,
pentanePentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
and
isopentaneIsopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and...
.
Examples
The following trivial names are retained in the IUPAC system:
- isobutane
Isobutane, also known as methylpropane, is an isomer of butane. It is the simplest alkane with a tertiary carbon. Concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers,...
for 2-methylpropane
- isopentane
Isopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and...
for 2-methylbutane
- neopentane
Neopentane, also called dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is an extremely flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, or when compressed to a higher...
for 2,2-dimethylpropane.
Table of alkanes
| Alkane |
Formula |
Boiling point [°C] |
Melting point [°C] |
Density [g·cm3] (at 20°C) |
| Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
|
CH4 |
-162 |
-183 |
gas |
EthaneEthane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
|
C2H6 |
-89 |
-172 |
gas |
PropanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
|
C3H8 |
-42 |
-188 |
gas |
ButaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
|
C4H10 |
0 |
-138 |
gas |
PentanePentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
|
C5H12 |
36 |
-130 |
0.626(liquid) |
| Hexane Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
|
C6H14 |
69 |
-95 |
0.659(liquid) |
Heptanen-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...
|
C7H16 |
98 |
-91 |
0.684(liquid) |
| Octane Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH36CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain...
|
C8H18 |
126 |
-57 |
0.703(liquid) |
| Nonane Nonane is a linear alkane hydrocarbon with the chemical formula C9H20.Nonane has 35 structural isomers. Tripropylene is a mixture of three specific isomers of nonane.Its substituent form is nonyl. Its cycloalkane counterpart is cyclononane, ....
|
C9H20 |
151 |
-54 |
0.718(liquid) |
| Decane Decane is an alkane hydrocarbon with the chemical formula CH38CH3.75 structural isomers of decane exist, all of which are flammable liquids. Decane is one of the components of gasoline . Like other alkanes, it is nonpolar and therefore will not dissolve in polar liquids such as water...
|
C10H22 |
174 |
-30 |
0.730(liquid) |
| Undecane Undecane is a liquid alkane hydrocarbon with the chemical formula CH39CH3. It is used as a mild sex attractant for various types of moths and cockroaches, and an alert signal for a variety of ants. It has 159 isomers....
|
C11H24 |
196 |
-26 |
0.740(liquid) |
| Dodecane Dodecane is a liquid alkane hydrocarbon with the chemical formula CH310CH3 , an oily liquid of the paraffin series. It has 355 isomers....
|
C12H26 |
216 |
-10 |
0.749(liquid) |
| Icosane |
C20H42 |
343 |
37 |
solid |
| Triacontane |
C30H62 |
450 |
66 |
solid |
| Tetracontane |
C40H82 |
525 |
82 |
solid |
| Pentacontane |
C50H102 |
575 |
91 |
solid |
| Hexacontane |
C60H122 |
625 |
100 |
solid |
Boiling point
Alkanes experience inter-molecular
van der Waals forceIn physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
s. Stronger inter-molecular van der Waals forces give rise to greater boiling points of alkanes.
There are two determinants for the strength of the van der Waals forces:
- the number of electrons surrounding the molecule, which increases with the alkane's molecular weight
- the surface area of the molecule
Under standard conditions, from CH
4 to C
4H
10 alkanes are gaseous; from C
5H
12 to C
17H
36 they are liquids; and after C
18H
38 they are solids. As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has almost a linear relationship with the size (molecular weight) of the molecule. As a rule of thumb, the boiling point rises 20 - 30 °C for each carbon added to the chain; this rule applies to other homologous series.
A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules. For example, compare
isobutaneIsobutane, also known as methylpropane, is an isomer of butane. It is the simplest alkane with a tertiary carbon. Concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers,...
(2-methylpropane) and n-butane (butane), which boil at -12 and 0 °C, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 and 58 °C, respectively. For the latter case, two molecules 2,3-dimethylbutane can "lock" into each other better than the cross-shaped 2,2-dimethylbutane, hence the greater van der Waals forces.
On the other hand, cycloalkanes tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact.
Melting point
The
melting pointThe melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
s of the alkanes follow a similar trend to
boiling pointsBoiling Points is a prank reality television show, much like the format used on Candid Camera. It is broadcast on MTV in the United States. In each half-hour episode, annoying situations are set up and deliberately inflicted on one or more young adults who are unaware that they are being tested...
for the same reason as outlined above. That is, (all other things being equal) the larger the molecule the higher the melting point. There is one significant difference between boiling points and melting points. Solids have more rigid and fixed structure than liquids. This rigid structure requires energy to break down. Thus the better put together solid structures will require more energy to break apart. For alkanes, this can be seen from the graph above (i.e., the blue line). The odd-numbered alkanes have a lower trend in melting points than even numbered alkanes. This is because even numbered alkanes pack well in the solid phase, forming a well-organized structure, which requires more energy to break apart. The odd-number alkanes pack less well and so the "looser" organized solid packing structure requires less energy to break apart.
The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to packing well in the solid phase: This is particularly true for isoalkanes (2-methyl isomers), which often have melting points higher than those of the linear analogues.
Conductivity and Solubility
Alkanes do not conduct
electricityElectricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
, nor are they substantially polarized by an
electric fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. For this reason they do not form
hydrogen bondA hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s and are insoluble in polar solvents such as water. Since the hydrogen bonds between individual water molecules are aligned away from an alkane molecule, the coexistence of an alkane and water leads to an increase in molecular order (a reduction in
entropyEntropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
). As there is no significant bonding between water molecules and alkane molecules, the
second law of thermodynamicsThe second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...
suggests that this reduction in entropy should be minimized by minimizing the contact between alkane and water: Alkanes are said to be hydrophobic in that they repel water.
Their solubility in nonpolar solvents is relatively good, a property that is called lipophilicity. Different alkanes are, for example, miscible in all proportions among themselves.
The density of the alkanes usually increases with increasing number of carbon atoms, but remains less than that of water. Hence, alkanes form the upper layer in an alkane-water mixture.
Molecular geometry
The molecular structure of the alkanes directly affects their physical and chemical characteristics. It is derived from the
electron configurationIn atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
of
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, which has four
valence electronIn chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
s. The carbon atoms in alkanes are always sp
3 hybridized, that is to say that the valence electrons are said to be in four equivalent orbitals derived from the combination of the 2s orbital and the three 2p orbitals. These orbitals, which have identical energies, are arranged spatially in the form of a
tetrahedronIn geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
, the angle of cos
−1(−⅓) ≈ 109.47° between them.
Bond lengths and bond angles
An alkane molecule has only C – H and C – C single bonds. The former result from the overlap of a sp³-orbital of carbon with the 1s-orbital of a hydrogen; the latter by the overlap of two sp³-orbitals on different carbon atoms. The
bond length- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
s amount to 1.09×10
−10 m for a C – H bond and 1.54×10
−10 m for a C – C bond.
The spatial arrangement of the bonds is similar to that of the four sp³-orbitals—they are tetrahedrally arranged, with an angle of 109.47° between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not correspond with the reality.
Conformation
The structural formula and the bond angles are not usually sufficient to completely describe the geometry of a molecule. There is a further
degree of freedomA degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...
for each carbon – carbon bond: the torsion angle between the atoms or groups bound to the atoms at each end of the bond. The spatial arrangement described by the torsion angles of the molecule is known as its
conformationIn chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...
.
EthaneEthane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
forms the simplest case for studying the conformation of alkanes, as there is only one C – C bond. If one looks down the axis of the C – C bond, one will see the so-called
Newman projectionA Newman projection, useful in alkane stereochemistry, visualizes chemical conformations of a carbon-carbon chemical bond from front to back, with the front carbon represented by a dot and the back carbon as a circle . The front carbon atom is called proximal, while the back atom is called distal...
. The hydrogen atoms on both the front and rear carbon atoms have an angle of 120° between them, resulting from the projection of the base of the tetrahedron onto a flat plane. However, the torsion angle between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the rear carbon can vary freely between 0° and 360°. This is a consequence of the free rotation about a carbon – carbon single bond. Despite this apparent freedom, only two limiting conformations are important:
eclipsedIn chemistry an eclipsed conformation is a conformation in which two substituents X and Y on adjacent atoms A, B are in closest proximity, implying that the torsion angle X-A-B-Y is 0°. Such a conformation exists in any open chain single chemical bond connecting two sp3 hybridised atoms, and is...
conformation and staggered conformation.
The two conformations, also known as rotamers, differ in energy: The staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation (the least stable).
This difference in energy between the two conformations, known as the torsion energy, is low compared to the thermal energy of an ethane molecule at ambient temperature. There is constant rotation about the C-C bond. The time taken for an ethane molecule to pass from one staggered conformation to the next, equivalent to the rotation of one CH
3-group by 120° relative to the other, is of the order of 10
−11 seconds.
The case of
higher alkanesHigher alkanes are often defined as alkanes having nine or more carbon atoms. Nonane is the lightest alkane to have a flash point above 25 °C, and so not to be classified as dangerously flammable....
is more complex but based on similar principles, with the antiperiplanar conformation always being the most favored around each carbon-carbon bond. For this reason, alkanes are usually shown in a zigzag arrangement in diagrams or in models. The actual structure will always differ somewhat from these idealized forms, as the differences in energy between the conformations are small compared to the thermal energy of the molecules: Alkane molecules have no fixed structural form, whatever the models may suggest.
Spectroscopic properties
Virtually all organic compounds contain carbon – carbon and carbon – hydrogen bonds, and so show some of the features of alkanes in their spectra. Alkanes are notable for having no other groups, and therefore for the
absence of other characteristic spectroscopic features.
Infrared spectroscopy
The carbon–hydrogen stretching mode gives a strong absorption between 2850 and 2960
cm−1In the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...
, while the carbon–carbon stretching mode absorbs between 800 and 1300 cm
−1. The carbon–hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450 cm
−1 and 1375 cm
−1, while methylene groups show bands at 1465 cm
−1 and 1450 cm
−1. Carbon chains with more than four carbon atoms show a weak absorption at around 725 cm
−1.
NMR spectroscopy
The proton resonances of alkanes are usually found at
δHIn nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
= 0.5 – 1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon: δ
C = 8 – 30 (primary, methyl, -CH
3), 15 – 55 (secondary, methylene, -CH
2-), 20 – 60 (tertiary, methyne, C-H) and quaternary. The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack of
Nuclear Overhauser effectThe Nuclear Overhauser Effect is the transfer of nuclear spin polarization from one nuclear spin population to another via cross-relaxation. It is a common phenomenon observed by nuclear magnetic resonance spectroscopy. The theoretical basis for the NOE was described and experimentally verified...
and the long
relaxation timeIn the physical sciences, relaxation usually means the return of a perturbed system into equilibrium.Each relaxation process can be characterized by a relaxation time τ...
, and can be missed in weak samples, or sample that have not been run for a sufficiently long time.
Mass spectrometry
Alkanes have a high
ionization energyThe ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
, and the molecular ion is usually weak. The fragmentation pattern can be difficult to interpret, but, in the case of branched chain alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resulting free radicals. The fragment resulting from the loss of a single methyl group (M−15) is often absent, and other fragment are often spaced by intervals of fourteen mass units, corresponding to sequential loss of CH
2-groups.
Chemical properties
In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no
functional groupIn organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
s.
They react only very poorly with ionic or other polar substances. The
acid dissociation constantAn acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
(pK
a) values of all alkanes are above 60, hence they are practically inert to acids and bases (see: carbon acids). This inertness is the source of the term
paraffins (with the meaning here of "lacking affinity"). In crude oil the alkane molecules have remained chemically unchanged for millions of years.
However redox reactions of alkanes, in particular with oxygen and the halogens, are possible as the carbon atoms are in a strongly reduced condition; in the case of methane, the lowest possible oxidation state for carbon (−4) is reached. Reaction with oxygen (
if an amount of the least is enough to meet the reaction stoichometry) leads to combustion without any smoke; with halogens, substitution. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes in (See: carbon-hydrogen bond activation).
Free radicals, molecules with unpaired electrons, play a large role in most reactions of alkanes, such as cracking and reformation where long-chain alkanes are converted into shorter-chain alkanes and straight-chain alkanes into branched-chain isomers.
In highly branched alkanes, the bond angle may differ significantly from the optimal value (109.5°) in order to allow the different groups sufficient space. This causes a tension in the molecule, known as steric hindrance, and can substantially increase the reactivity.
Reactions with oxygen (combustion reaction)
All alkanes react with
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in a
combustionCombustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is:
- CnH2n+2 + (1.5n+0.5)O2 → (n+1)H2O + nCO2
In the absence of sufficient oxygen,
carbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
or even
sootSoot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
can be formed, as shown below:
- CnH(2n+2) + nO2
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
→ (n+1)H2O + nCOCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
For example
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
:
- 2CH4 + 3O2 → 2CO + 4H2O
- CH4 + 1.5O2 → CO + 2H2O
See the alkane heat of formation table for detailed data.
The
standard enthalpy change of combustionThe standard enthalpy of combustion is the enthalpy change when one mole of a substance completely reacts with oxygen under standard thermodynamic conditions...
, Δ
cHo, for alkanes increases by about 650 kJ/mol per CH
2 group. Branched-chain alkanes have lower values of Δ
cHo than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable.
Reactions with halogens
Alkanes react with
halogenThe halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
s in a so-called
free radical halogenation reaction. The hydrogen atoms of the alkane are progressively replaced by halogen atoms. Free-radicals are the reactive species that participate in the reaction, which usually leads to a mixture of products. The reaction is highly
exothermicAn exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
, and can lead to an explosion.
These reactions are an important industrial route to halogenated hydrocarbons. There are three steps:
- Initiation the halogen radicals form by homolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...
. Usually, energy in the form of heat or light is required.
- Chain reaction or Propagation then takes place—the halogen radical abstracts a hydrogen from the alkane to give an alkyl radical. This reacts further.
- Chain termination where step the radicals recombine.
Experiments have shown that all halogenation produces a mixture of all possible isomers, indicating that all hydrogen atoms are susceptible to reaction. The mixture produced, however, is not a statistical mixture: Secondary and tertiary hydrogen atoms are preferentially replaced due to the greater stability of secondary and tertiary free-radicals. An example can be seen in the monobromination of propane: [In the Figure below, the Statistical Distribution should be 25% and 75%]
Cracking
Cracking breaks larger molecules into smaller ones. This can be done with a thermal or catalytic method. The thermal cracking process follows a homolytic mechanism with formation of free-radicals. The catalytic cracking process involves the presence of
acidAn acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
catalysts (usually solid acids such as silica-alumina and
zeoliteZeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s), which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s of opposite charges, usually a carbocation and the very unstable
hydrideIn chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
anion. Carbon-localized free-radicals and cations are both highly unstable and undergo processes of chain rearrangement, C-C scission in position
betaBeta scission is an important reaction in the chemistry of thermal cracking of hydrocarbons and the formation of free radicals. Free radicals are formed upon splitting the carbon-carbon bond. Free radicals are extremely reactive and short-lived...
(i.e., cracking) and
intra-Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...
and intermolecular hydrogen transfer or
hydrideIn chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
transfer. In both types of processes, the corresponding
reactive intermediateIn chemistry a reactive intermediate is a short-lived, high energy, highly reactive molecule. When generated in a chemical reaction it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation...
s (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.
Isomerization and reformation
Isomerization and reformation are processes in which straight-chain alkanes are heated in the presence of a
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
catalyst. In isomerization, the alkanes become branched-chain isomers. In reformation, the alkanes become
cycloalkaneCycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...
s or
aromatic hydrocarbonAn aromatic hydrocarbon or arene is a hydrocarbon with alternating double and single bonds between carbon atoms. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent...
s, giving off hydrogen as a by-product. Both of these processes raise the octane number of the substance.
Other reactions
Alkanes will react with
steamSteam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
in the presence of a
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
catalyst to give
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. Alkanes can be chlorosulfonated and
nitratedNitration is a general chemical process for the introduction of a nitro group into a chemical compound. The dominant application of nitration is for the production of nitrobenzene, the precursor to methylene diphenyl diisocyanate...
, although both reactions require special conditions. The
fermentationFermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
of alkanes to
carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s is of some technical importance. In the
Reed reactionThe Reed reaction is a chemical reaction that utilizes light to oxidize hydrocarbons to sulfonyl chlorides. The reaction performs via the free radicals. First, the light makes a molecule of chlorine dissociate homolytically, then an chlorine atom produced attacks the hydrocarbon chain to form...
,
sulfur dioxideSulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
,
chlorineChlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
and
lightPhotochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...
convert hydrocarbons to
sulfonyl chloridesSulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...
.
Occurrence of alkanes in the Universe
Alkanes form a small portion of the atmospheres of the outer gas planets such as
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
(0.1% methane, 0.0002% ethane),
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
(0.2% methane, 0.0005% ethane),
UranusUranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
(1.99% methane, 0.00025% ethane) and
NeptuneNeptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
(1.5% methane, 1.5 ppm ethane).
TitanTitan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
(1.6% methane), a satellite of Saturn, was examined by the
Huygens probe, which indicate that Titan's atmosphere periodically rains liquid methane onto the moon's surface. Also on Titan, a methane-spewing volcano was spotted and this volcanism is believed to be a significant source of the methane in the atmosphere. There also appear to be Methane/Ethane lakes near the north polar regions of Titan, as discovered by Cassini's radar imaging.
MethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and
ethaneEthane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
have also been detected in the tail of the comet Hyakutake. Chemical analysis showed that the abundances of ethane and methane were roughly equal, which is thought to imply that its ices formed in interstellar space, away from the Sun, which would have evaporated these volatile molecules. Alkanes have also been detected in
meteoriteA meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s such as
carbonaceous chondriteCarbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
s.
Occurrence of alkanes on Earth
Traces of methane gas (about 0.0001% or 1 ppm) occur in the Earth's atmosphere, produced primarily by organisms such as
ArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, found for example in the gut of cows.
The most important commercial sources for alkanes are
natural gasNatural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and
oilPetroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
. Natural gas contains primarily methane and ethane, with some
propanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
and
butaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
: oil is a mixture of liquid alkanes and other hydrocarbons. These hydrocarbons were formed when dead marine animals and plants (zooplankton and phytoplankton) died and sank to the bottom of ancient seas and were covered with sediments in an anoxic environment and converted over many millions of years at high temperatures and high pressure to their current form. Natural gas resulted thereby for example from the following reaction:
- C6H12O6 → 3CH4 + 3CO2
These hydrocarbons collected in porous rocks, located beneath an impermeable cap rock and so are trapped. These deposits, e.g., oil fields, have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons is the basis for what is known as the
energy crisisAn energy crisis is any great bottleneck in the supply of energy resources to an economy. In popular literature though, it often refers to one of the energy sources used at a certain time and place, particularly those that supply national electricity grids or serve as fuel for vehicles...
.
Solid alkanes are known as
tarTar is modified pitch produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest...
s and are formed when more volatile alkanes such as gases and oil
evaporateEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
from hydrocarbon deposits. One of the largest natural deposits of solid alkanes is in the
asphaltAsphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
lake known as the
Pitch LakeThe Pitch Lake is the largest natural deposit of asphalt in the world, located at La Brea in southwest Trinidad, within the Siparia Regional Corporation. The lake covers about 40 ha and is reported to be 75 m deep....
in
Trinidad and TobagoTrinidad and Tobago officially the Republic of Trinidad and Tobago is an archipelagic state in the southern Caribbean, lying just off the coast of northeastern Venezuela and south of Grenada in the Lesser Antilles...
.
Methane is also present in what is called
biogasBiogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal dung, and kitchen waste can be converted into a gaseous fuel called biogas...
, produced by animals and decaying matter, which is a possible renewable energy source.
Alkanes have a low solubility in water, so the content in the oceans is negligible; however, at high pressures and low temperatures (such as at the bottom of the oceans), methane can co-crystallize with water to form a solid methane hydrate. Although this cannot be commercially exploited at the present time, the amount of combustible energy of the known methane hydrate fields exceeds the energy content of all the natural gas and oil deposits put together;methane extracted from methane hydrate is considered therefore a candidate for future fuels.
Biological occurrence
Although alkanes occur in nature in various way, they do not rank biologically among the essential materials. Cycloalkanes with 14 to 18 carbon atoms occur in
muskMusk is a class of aromatic substances commonly used as base notes in perfumery. They include glandular secretions from animals such as the musk deer, numerous plants emitting similar fragrances, and artificial substances with similar odors. Musk was a name originally given to a substance with a...
, extracted from
deerDeer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...
of the family Moschidae. All further information refers to (acyclic) alkanes.
Bacteria and archaea
Certain types of
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
can metabolize alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains.
On the other hand, certain
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, the
methanogenMethanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria...
s, produce large quantities of
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
by the metabolism of
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
or other oxidized organic compounds. The energy is released by the oxidation of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
:
- CO2 + 4H2 → CH4 + 2H2O
Methanogens are also the producers of marsh gas in wetlands, and release about two billion tonnes of methane per year—the atmospheric content of this gas is produced nearly exclusively by them. The methane output of
cattleCattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
and other
herbivoreHerbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s, which can release up to 150 liters per day, and of
termiteTermites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...
s, is also due to methanogens. They also produce this simplest of all alkanes in the
intestineIn human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s of humans. Methanogenic archaea are, hence, at the end of the
carbon cycleThe carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
, with carbon being released back into the atmosphere after having been fixed by
photosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. It is probable that our current deposits of
natural gasNatural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
were formed in a similar way.
Fungi and plants
Alkanes also play a role, if a minor role, in the biology of the three
eukaryoticA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
groups of organisms:
fungiA fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
, plants and animals. Some specialized yeasts, e.g.,
Candida tropicale,
PichiaPichia is a genus of yeasts in the family Saccharomycetaceae with spherical, elliptical or oblong acuminate cells. Pichia is a teleomorph, and forms during sexual reproduction hat-shaped, hemispherical or round ascospores. The anamorphs of some Pichia species are Candida species...
sp.,
RhodotorulaRhodotorula is a pigmented yeast, part of the Basidiomycota phylum, quite easily identifiable by distinctive orange/red colonies when grown on SDA . This distinctive colour is the result of pigments that the yeast creates to block out certain wavelengths of light that would otherwise be damaging...
sp., can use alkanes as a source of carbon and/or energy. The fungus
Amorphotheca resinae prefers the longer-chain alkanes in
aviation fuelAviation fuel is a specialized type of petroleum-based fuel used to power aircraft. It is generally of a higher quality than fuels used in less critical applications, such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperatures,...
, and can cause serious problems for aircraft in tropical regions.
In plants, the solid long-chain alkanes are found in the
plant cuticlePlant cuticles are a protective waxy covering produced only by the epidermal cells of leaves, young shoots and all other aerial plant organs without periderm...
and
epicuticular waxIn botany, the plant cuticle is covered by epicuticular wax or bloom mainly consistingof straight-chain aliphatic hydrocarbons with a variety of substituted groups, serving to decrease moisture loss and decay...
of many species, but are only rarely major constituents. They protect the plant against water loss, prevent the
leachingIn agriculture, leaching refers to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss.Leaching may also refer to ...
of important minerals by the rain, and protect against bacteria, fungi, and harmful insects. The carbon chains in plant alkanes are usually odd-numbered, between twenty-seven and thirty-three carbon atoms in length and are made by the plants by
decarboxylationDecarboxylation is a chemical reaction that releases carbon dioxide . Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is the first chemical step in photosynthesis, is called carbonation, the addition of CO2 to...
of even-numbered
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s. The exact composition of the layer of wax is not only species-dependent, but changes also with the season and such environmental factors as lighting conditions, temperature or humidity.
Animals
Alkanes are found in animal products, although they are less important than unsaturated hydrocarbons. One example is the shark liver oil, which is approximately 14%
pristanePristane is a natural saturated terpenoid alkane obtained primarily from shark liver oil, from which its name is derived . It is also found in mineral oil and some foods...
(2,6,10,14-tetramethylpentadecane, C
19H
40). Their occurrence is more important in
pheromoneA pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s, chemical messenger materials, on which above all insects are dependent for communication. With some kinds, as the support beetle
Xylotrechus colonus, primarily pentacosane (C
25H
52), 3-methylpentaicosane (C
26H
54) and 9-methylpentaicosane (C
26H
54), they are transferred by body contact. With others like the
tsetse flyTsetse , sometimes spelled tzetze and also known as tik-tik flies, are large biting flies that inhabit much of mid-continental Africa between the Sahara and the Kalahari deserts. They live by feeding on the blood of vertebrate animals and are the primary biological vectors of trypanosomes, which...
Glossina morsitans morsitans, the pheromone contains the four alkanes 2-methylheptadecane (C
18H
38), 17,21-dimethylheptatriacontane (C
39H
80), 15,19-dimethylheptatriacontane (C
39H
80) and 15,19,23-trimethylheptatriacontane (C
40H
82), and acts by smell over longer distances, a useful characteristic for
pest controlPest control refers to the regulation or management of a species defined as a pest, usually because it is perceived to be detrimental to a person's health, the ecology or the economy.-History:...
.
Waggle-dancingWaggle dance is a term used in beekeeping and ethology for a particular figure-eight dance of the honey bee. By performing this dance, successful foragers can share with their hive mates information about the direction and distance to patches of flowers yielding nectar and pollen, to water...
honeybees produce and release two alkanes, tricosane and pentacosane.
Ecological relations
One example, in which both plant and animal alkanes play a role, is the ecological relationship between the sand bee (
Andrena nigroaenea) and the early spider orchid (
Ophrys sphegodesOphrys sphegodes, commonly known as the Early Spider Orchid, is a species of orchid found on alkaline meadows and waste land. It has a distribution that includes western and northern Europe extending to parts of southern England but may also be found as far east as Corfu and possibly also in...
); the latter is dependent for
pollinationPollination is the process by which pollen is transferred in plants, thereby enabling fertilisation and sexual reproduction. Pollen grains transport the male gametes to where the female gamete are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself...
on the former. Sand bees use pheromones in order to identify a mate; in the case of
A. nigroaenea, the females emit a mixture of tricosane (C
23H
48), pentacosane (C
25H
52) and heptacosane (C
27H
56) in the ratio 3:3:1, and males are attracted by specifically this odor. The orchid takes advantage of this mating arrangement to get the male bee to collect and disseminate its pollen; parts of its flower not only resemble the appearance of sand bees, but also produce large quantities of the three alkanes in the same ratio as female sand bees. As a result numerous males are lured to the blooms and attempt to copulate with their imaginary partner: although this endeavor is not crowned with success for the bee, it allows the orchid to transfer its pollen,
which will be dispersed after the departure of the frustrated male to different blooms.
Petroleum refining
As stated earlier, the most important source of alkanes is
natural gasNatural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and crude oil. Alkanes are separated in an
oil refineryAn oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...
by
fractional distillationFractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation...
and processed into many different products.
Fischer-Tropsch
The
Fischer-Tropsch processThe Fischer–Tropsch process is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a petroleum substitute, typically from coal, natural gas, or biomass for use as synthetic...
is a method to synthesize liquid hydrocarbons, including alkanes, from
carbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and hydrogen. This method is used to produce substitutes for petroleum distillates.
Laboratory preparation
There is usually little need for alkanes to be synthesized in the laboratory, since they are usually commercially available. Also, alkanes are generally non-reactive chemically or biologically, and do not undergo functional group interconversions cleanly. When alkanes are produced in the laboratory, it is often a side-product of a reaction. For example, the use of
n-butyllithiumn-Butyllithium is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene...
as a strong
baseFor the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
gives the conjugate acid, n-butane as a side-product:
- C4H9Li + H2O → C4H10 + LiOH
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
However, at times it may be desirable to make a portion of a molecule into an alkane like functionality (alkyl group) using the above or similar methods. For example, an
ethyl groupIn chemistry, an ethyl group is an alkyl substituent derived from ethane . It has the formula -C2H5 and is very often abbreviated -Et.Ethylation is the formation of a compound by introduction of the ethyl functional group, C2H5....
is an alkyl group; when this is attached to a
hydroxyA hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
group, it gives
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
, which is not an alkane. To do so, the best-known methods are
hydrogenationHydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
of
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s:
- RCH=CH2 + H2 → RCH2CH3 (R = alkyl)
Alkanes or alkyl groups can also be prepared directly from alkyl halides in the
Corey-House-Posner-Whitesides reactionThe Corey–Posner, Whitesides–House synthesis is an organic reaction that involves the reaction of a lithium dialkyl cuprate with an alkyl halide to form a new alkane, an organocopper compound and a lithium halide.-Reaction mechanism:This reaction occurs in two steps...
. The
Barton-McCombie deoxygenationThe Barton–McCombie deoxygenation is an organic reaction in which an hydroxy functional group in an organic compound is replaced by a hydride to give an alkyl group . It is named for the British chemists Sir Derek Harold Richard Barton and Stuart W. McCombie.This deoxygenation reaction is a...
removes hydroxyl groups from alcohols e.g.
and the
Clemmensen reductionClemmensen reduction is a chemical reaction described as a reduction of ketones to alkanes using zinc amalgam and hydrochloric acid. This reaction is named after Erik Christian Clemmensen, a Danish chemist....
removes carbonyl groups from aldehydes and ketones to form alkanes or alkyl-substituted compounds e.g.:
Applications
The applications of a certain alkane can be determined quite well according to the number of carbon atoms. The first four alkanes are used mainly for heating and cooking purposes, and in some countries for electricity generation.
MethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and
ethaneEthane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
are the main components of natural gas; they are normally stored as gases under pressure. It is, however, easier to transport them as liquids: This requires both compression and cooling of the gas.
PropanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
and
butaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
can be liquefied at fairly low pressures, and are well known as
liquified petroleum gasLiquefied petroleum gas is a flammable mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer...
(LPG). Propane, for example, is used in the propane gas burner, butane in disposable cigarette lighters. The two alkanes are used as propellants in
aerosol sprayAerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. This is used with a can or bottle that contains a liquid under pressure. When the container's valve is opened, the liquid is forced out of a small hole and emerges as an aerosol or mist...
s.
From
pentanePentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
to
octaneOctane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH36CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain...
the alkanes are reasonably volatile liquids. They are used as fuels in
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
s, as they vaporise easily on entry into the combustion chamber without forming droplets, which would impair the uniformity of the combustion. Branched-chain alkanes are preferred as they are much less prone to premature ignition, which causes
knockingKnocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
, than their straight-chain homologues. This propensity to premature ignition is measured by the
octane ratingOctane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...
of the fuel, where
2,2,4-trimethylpentane2,2,4-Trimethylpentane, also known as isooctane, iso-octane, is an organic compound with the formula 3CCH2CH2. It is one of several isomers of octane . This particular isomer is the standard for 100 point on the octane rating scale...
(
isooctane) has an arbitrary value of 100, and
heptanen-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...
has a value of zero. Apart from their use as fuels, the middle alkanes are also good
solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s for nonpolar substances.
Alkanes from
nonaneNonane is a linear alkane hydrocarbon with the chemical formula C9H20.Nonane has 35 structural isomers. Tripropylene is a mixture of three specific isomers of nonane.Its substituent form is nonyl. Its cycloalkane counterpart is cyclononane, ....
to, for instance,
hexadecaneHexadecane is an alkane hydrocarbon with the chemical formula C16H34. Hexadecane consists of a chain of 16 carbon atoms, with three hydrogen atoms bonded to the two end carbon atoms, and two hydrogens bonded to each of the 14 other carbon atoms.Cetane is often used as a short-hand for cetane...
(an alkane with sixteen carbon atoms) are liquids of higher
viscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, less and less suitable for use in gasoline. They form instead the major part of diesel and
aviation fuelAviation fuel is a specialized type of petroleum-based fuel used to power aircraft. It is generally of a higher quality than fuels used in less critical applications, such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperatures,...
. Diesel fuels are characterized by their
cetane numberCetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality.- Definition :...
, cetane being an old name for hexadecane. However, the higher melting points of these alkanes can cause problems at low temperatures and in polar regions, where the fuel becomes too thick to flow correctly.
Alkanes from hexadecane upwards form the most important components of
fuel oilFuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash...
and lubricating oil. In latter function, they work at the same time as anti-corrosive agents, as their hydrophobic nature means that water cannot reach the metal surface. Many solid alkanes find use as paraffin wax, for example, in
candleA candle is a solid block or cylinder of wax with an embedded wick, which is lit to provide light, and sometimes heat.Today, most candles are made from paraffin. Candles can also be made from beeswax, soy, other plant waxes, and tallow...
s. This should not be confused however with true
waxthumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
, which consists primarily of
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s.
Alkanes with a chain length of approximately 35 or more carbon atoms are found in bitumen, used, for example, in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by
crackingIn petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...
.
Some synthetic polymers such as
polyethylenePolyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
and
polypropylenePolypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
are alkanes with chains containing hundreds of thousands of carbon atoms. These materials are used in innumerable applications, and billions of kilograms of these materials are made and used each year.
Environmental transformations
When released in the environment, alkanes don't undergo rapid biodegradation, because they have no functional groups (like
hydroxylA hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
or
carbonylIn organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
) that are needed by most organisms in order to metabolize the compound.
However, some bacteria can metabolize some alkanes (especially those linear and short), by oxidizing the terminal carbon atom. The product is an
alcoholIn chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, that could be next oxidized to an
aldehydeAn aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
, and finally to a
carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
. The resulting
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
could be metabolized through the
fatty acid degradationFatty acid degradation is the process in which fatty acids are broken down into their metabolites, in the end generating acetyl-CoA, the entry molecule for the citric acid cycle, the main energy supply of animals...
pathway.
Hazards
Methane is explosive when mixed with air (1 – 8% CH
4). Other lower alkanes can also form explosive mixtures with air. The lighter liquid alkanes are highly flammable, although this risk decreases with the length of the carbon chain. Pentane, hexane, heptane, and octane are classed as
dangerous for the environment and
harmful. The straight-chain isomer of hexane is a
neurotoxinA neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...
. Halogen-rich alkanes, like
chloroformChloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
, can be carcinogenic as well.
See also
- Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
- Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
- Cycloalkane
Cycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...
- Basketane
Basketane is a polycyclic alkane with the chemical formula C10H12. The name is taken from its structural similarity to a basket shape. Basketane was first synthesised in 1966, independently by Masamune and Dauben and Whalen.-Further reading:...
Further reading