Encyclopedia
In
chemistry, an
alcohol is any
organic compound in which a hydroxyl
group is bound to a
carbon atom of an
alkyl or substituted alkyl group. The general formula for a simple acyclic alcohol is
CnH2n+1OH.
In general usage,
alcohol refers almost always to
ethanol, also known as
grain alcohol, a strongly-smelling, colorless, volatile liquid formed by the fermentation of sugars. It also often refers to any beverage that contains ethanol . This sense underlies the term alcoholism . Other forms of alcohol are usually described with a clarifying adjective, as in
isopropyl alcohol or by the suffix
-ol, as in
isopropanol.
Structure
The
functional group of an alcohol is a hydroxyl group bonded to an sp³ hybridized carbon. It can therefore be regarded as a derivative of
water, with an
alkyl group replacing one of the hydrogens. If an aryl group is present rather than an alkyl, the compound is generally called a
phenol rather than an alcohol. Also, if the hydroxyl group is bonded to one of the sp² hybridized carbons of an alkenyl group, the compound is referred to as an
enol. The oxygen in an alcohol has a bond angle of around 109° , and two nonbonded electron pairs. The O-H bond in methanol is around 96 pico
metres long.
Primary, secondary, and tertiary alcohols
There are three major subsets of alcohols- 'primary' , 'secondary' and 'tertiary' , based upon the number of carbons the C-OH carbon is bonded to.
Methanol is the simplest 'primary' alcohol. The simplest secondary alcohol is
isopropanol , and a simple tertiary alcohol is
tert-butanol .
-
The
phenols with parent compound
phenol have a hydroxyl group just like alcohols but differ sufficiently in properties to warrant a separate treatment.
Methanol & ethanol
The simplest and most commonly used alcohols are
methanol and
ethanol , which have the structures shown above.
Methanol was formerly obtained by the distillation of wood, and was called "wood alcohol." It is now a cheap commodity, chemical produced by the high pressure reaction of
carbon monoxide with
hydrogen. In common usage, "alcohol" often refers simply to ethanol or "grain alcohol." Methylated spirits , also called "surgical spirits," is a form of ethanol rendered undrinkable by the addition of methanol. Aside from its major use in alcoholic beverages, ethanol is also used as an industrial solvent and raw material.
Uses
Alcohols are in wide use in industry and science as reagents, solvents, and
fuels. Ethanol and methanol can be made to burn more cleanly than
gasoline or
diesel. Because of its low toxicity and ability to dissolve non-polar substances, ethanol is often used as a solvent in medical drugs,
perfumes, and vegetable essences such as
vanilla. In organic synthesis, alcohols frequently serve as versatile intermediates. In the kitchen, alcoholic beverages are added to dishes not only for their inherent flavors, but also because the alcohol dissolves flavor compounds that water cannot.
Ethanol is commonly used in beverages to promote flavor, reduce social inhibitions, or induce a euphoric intoxication commonly known as "drunkenness" or "being drunk." The consumption of ethanol is illegal in some countries, mainly
Islamic.
Ethanol is a
drug, with potential for overdose or toxic poisoning if taken in excessive quantities. Alcoholism, the physiological or psychological dependency on ethanol, is one of the most common drug addictions in the world. Upon cessation or decrease of use, the physiological dependency can lead to physical withdrawal symptoms, such as restlessness, trouble sleeping, "the shakes," or even death. For the full article on this topic, see effects of alcohol on the body.
Ethanol for consumption has been regulated by taxation. Those who manufacture it for other purposes often avoid this expense by "denaturing" it in a manner that renders it unfit for drinking. A common way to do this is by the addition of
denatonium benzoate. "SD-40" and "SD Alcohol" sometimes followed by "40-B" are designations that were established by the United States'
Bureau of Alcohol, Tobacco, and Firearms for this formulation.
Ethanol is often used as an antiseptic, to disinfect the skin before injections are given, often along with iodine. Ethanol-based soaps are now becoming commonplace within restaurants and are particularly convenient as they do not require drying due to the volatility of the molecule.
Sources
Many alcohols can be created by fermentation of
fruits or
grains with
yeast, but only ethanol is commercially produced this way — chiefly for
fuel and
drink. Other alcohols are generally produced by synthetic routes from
natural gas,
petroleum, or
coal feed stocks; for example, via acid catalyzed hydration of
alkenes. For more details see Preparation of alcohols.
Nomenclature
Systematic names
In the
IUPAC system, the name of the alkane chain loses the terminal "e" and adds "ol", e.g. "methanol" and "ethanol". When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the "ol":
propan-1-ol for CH
3CH
2CH
2OH,
propan-2-ol for CH
3CHCH
3. Sometimes, the position number is written before the IUPAC name: 1-propanol and 2-propanol. If a higher priority group is present , then it is necessary to use the prefix "hydroxy", for example: 1-hydroxy-2-propanone .
Some examples of simple alcohols and how to name them:
Common names for alcohols usually take the name of the corresponding
alkyl group and add the word "alcohol", e.g. methyl alcohol, ethyl alcohol or
tert-butyl alcohol. Propyl alcohol may be
n-propyl alcohol or isopropyl alcohol depending on whether the hydroxyl group is bonded to the 1st or 2nd carbon on the propane chain. Isopropyl alcohol is also occasionally called
sec-propyl alcohol.
As mentioned above alcohols are classified as primary , secondary or tertiary , and common names often indicate this in the alkyl group prefix. For example
3COH is a tertiary alcohol is commonly known as
tert-butyl alcohol. This would be named 2-methylpropan-2-ol under IUPAC rules, indicating a propane chain with methyl and hydroxyl groups both attached to the middle carbon.
An alcohol with two hydroxyl groups is commonly called a "glycol", e.g. HO-CH
2-CH
2-OH is
ethylene glycol. The IUPAC name is ethane-1,2-diol, "diol" indicating two hydroxyl groups, and 1,2 indicating their bonding positions. Geminal glycols , such as ethane-1,1-diol, are generally unstable. For three or four groups, "triol" and "tetraol" are used.
Etymology
The word "alcohol" almost certainly comes from the
Arabic language ; however, the precise origin is unclear. It was introduced into
Europe, together with the art of
distillation and the substance itself, around the
12th century by various European authors who translated and popularized the discoveries of
Islamic alchemists .
A popular theory, found in many dictionaries, is that it comes from ????? =
ALKHL =
al-kuhul, originally the name of very finely powdered
antimony sulfide
Sb2S3 used as an
antiseptic and
eyeliner. The powder is prepared by sublimation of the natural mineral
stibnite in a closed vessel. According to this theory, the meaning of
alkuhul would have been first extended to distilled substances in general, and then narrowed to ethanol. This conjectured etymology has been circulating in England since 1672 at least .
However, this derivation is suspicious since the current Arabic name for alcohol, ?????? =
ALKHWL =
al???, does not derive from
al-kuhul. The
Qur'an in verse 37:47 uses the word ????? =
ALGhWL =
al-ghawl — properly meaning "spirit" or "
demon" — with the sense "the thing that gives the wine its headiness". The word
al-ghawl also originated the
English word "ghoul", and the name of the star
Algol. This derivation would, of course, be consistent with the use of "spirit" or "spirit of wine" as synonymous of "alcohol" in most Western languages.
According to the second theory, the popular etymology and the spelling "alcohol" would not be due to generalization of the meaning of
ALKHL, but rather to Western alchemists and authors confusing the two words
ALKHL and
ALGhWL, which have indeed been transliterated in many different and overlapping ways.
Physical and chemical properties
The hydroxyl group generally makes the alcohol molecule
polar. Those groups can form
hydrogen bonds to one another and to other compounds. Two opposing solubility trends in alcohols are: the tendency of the polar OH to promote solubility in water, and of the carbon chain to resist it. Thus, methanol, ethanol, and propanol are miscible in water because the hydroxyl group wins out over the short carbon chain.
Butanol, with a four-carbon chain, is moderately soluble because of a balance between the two trends. Alcohols of five or more carbons are effectively insoluble because of the hydrocarbon chain's dominance.
Because of
hydrogen bonding, alcohols tend to have higher boiling points than comparable
hydrocarbons and
ethers. All simple alcohols are miscible in organic solvents. This hydrogen bonding means that alcohols can be used as protic solvents.
The lone pairs of electrons on the oxygen of the hydroxyl group also makes alcohols nucleophiles.
Alcohols, like water, can show either acidic or basic properties at the O-H group. With a pK
a of around 16-19 they are generally slightly weaker acids than
water, but they are still able to react with strong bases such as
sodium hydride or reactive metals such as
sodium. The salts that result are called
alkoxides, with the general formula
RO
- M+.
Meanwhile the oxygen atom has lone pairs of nonbonded electrons that render it weakly basic in the presence of strong acids such as
sulfuric acid. For example, with methanol:
Alcohols can also undergo
oxidation to give
aldehydes,
ketones or
carboxylic acids, or they can be dehydrated to
alkenes. They can react to form
ester compounds, and they can undergo
nucleophilic substitution reactions. For more details see the reactions of alcohols section below.
Toxicity
Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages.
Ethanol in the form of
alcoholic beverages has been consumed by humans since pre-historic times, for a variety of hygienic, dietary, medicinal, religious, and recreational reasons. While infrequent consumption of ethanol in small quantities may be harmless or even beneficial, larger doses result in a state known as
drunkenness or intoxication and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use has medical repercussions. Alcohol has also been known to be a catalyst for reckless behaviors that may have undesirable results, such as accidents, fighting, and unprotected sex.
Other alcohols are substantially more poisonous than ethanol, partly because they take much longer to be metabolized, and often their metabolism produces even more toxic substances. Methanol, or
wood alcohol, for instance, is oxidized by
alcohol dehydrogenase enzymes in the liver to the poisonous
formaldehyde, which can cause blindness or death.
An effective treatment to prevent formaldehyde toxicity after methanol ingestion is to administer ethanol. Alcohol dehydrogenase has a higher affinity for ethanol, thus preventing methanol from binding and acting as a substrate. Any remaining methanol will then have time to be excreted through the kidneys. Remaining formaldehyde will be converted to
formic acid and excreted.
Preparation of alcohols
Laboratory
Several methods exist for the preparation of alcohols in the laboratory.
...
or
ketones are
reduced with
sodium borohydride or
lithium aluminium hydride.
The formation of a secondary alcohol via reduction and hydration is shown:
Industrial
Industrially alcohols are produced in several ways:
- By fermentation using glucose produced from sugar from the hydrolysis of starch, in the presence of yeast and temperature of less than 37°C to produce ethanol. For instance the conversion of invertase to glucose and fructose or the conversion of glucose to zymase and ethanol.
- By direct hydration using ethene or other alkenes from cracking of fractions of distilled crude oil. Uses a catalyst of phosphoric acid
| align="center" colspan="2" bgcolor="#ffffff" | |
...
under high temperature and pressure.
- Methanol is producted from water gas: It is manufactured from synthesis gas, where carbon monoxide and 2 equivalents of hydrogen gas are combined to produce methanol using a copper, zinc oxide and aluminium oxide catalyst at 250°C and a pressure of 50-100 atm.
Reactions of alcohols
Deprotonation
Alcohols can behave as weak acids, undergoing deprotonation. The deprotonation reaction to produce an alkoxide
salt is either performed with a strong base such as
sodium hydride or
n-butyllithium, or with sodium or potassium metal.
- 2 R-OH + 2 NaH ? 2 R-O-Na+ + H2
| Triple point [i] || 13.8033 K, 7.042 kPa
...
?
- 2 R-OH + 2Na ? 2R-O-Na + H2
- E.g. 2 CH3CH2-OH + 2 Na ? 2 CH3-CH2-O-Na + H2
Water is similar in pK
a to many alcohols, so with
sodium hydroxide there is an equilibrium set up which usually lies to the left:
- R-OH + NaOH <=> R-O-Na+ + H2O
It should be noted, though, that the bases used to deprotonate alcohols are strong themselves. The bases used and the alkoxides created are both highly moisture sensitive chemical reagents.
The acidity of alcohols is also affected by the overall stability of the alkoxide ion. Electron-withdrawing groups attached to the carbon containing the hydroxyl group will serve to stabilize the alkoxide when formed, thus resulting in greater acidity. On the other hand, the presence of electron-donating group will result in a less stable alkoxide ion formed. This will result in a scenario whereby the unstable alkoxide ion formed will tend to accept a proton to reform the original alcohol.
With
alkyl halides alkoxides give rise to
ethers in the Williamson ether synthesis.
Nucleophilic substitution
The OH group is not a good leaving group in
nucleophilic substitution reactions, so neutral alcohols do not react in such reactions. However if the oxygen is first protonated to give R-OH
2+, the leaving group is much more stable, and nucleophilic substitution can take place. For instance, tertiary alcohols react with
hydrochloric acid to produce tertiary
alkyl halides, where the hydroxyl group is replaced by a chlorine atom. If primary or secondary alcohols are to be reacted with
hydrochloric acid, an activator such as
zinc chloride is needed. Alternatively the conversion may be performed directly using
thionyl chloride.
[1]Alcohols may likewise be converted to alkyl bromides using
hydrobromic acid or
phosphorus tribromide, for example:
- 3 R-OH + PBr3 ? 3 RBr + H3PO3
In the
Barton-McCombie deoxygenation an alcohol is deoxygenated to an
alkane with
tributyltin hydride or a trimethylborane-water complex in a radical substitution reaction.
Dehydration
Alcohols are themselves nucleophilic, so R-OH
2+ can react with ROH to produce
ethers and water in a dehydration reaction, although this reaction is rarely used except in the manufacture of
diethyl ether.
More useful is the E1
elimination reaction of alcohols to produce
alkenes. The reaction generally obeys Zaitsev's Rule, which states that the most stable alkene is formed. Tertiary alcohols eliminate easily at just above room temperature, but primary alcohols require a higher temperature.
This is a diagram of acid catalysed dehydration of ethanol to produce
ethene:
A more controlled elimination reaction is the
Chugaev elimination with carbon disulfide and iodomethane.
Esterification
To form an
ester from an alcohol and a
carboxylic acid the reaction, known as Fischer esterification, is usually performed at
reflux with a
catalyst of concentrated
sulfuric acid:
- R-OH + R'-COOH ? R'-COOR + H2O
In order to drive the equilibrium to the right and produce a good yield of ester, water is usually removed, either by an excess of H
2SO
4 or by using a Dean-Stark apparatus. Esters may also be prepared by reaction of the alcohol with an
acid chloride in the presence of a base such as
pyridine.
Other types of ester are prepared similarly- for example
tosyl esters are made by reaction of the alcohol with p-
toluenesulfonyl chloride in pyridine.
Oxidation
Primary alcohols generally give
aldehydes or
carboxylic acids upon oxidation, while secondary alcohols give
ketones. Traditionally strong
oxidants such as the
dichromate ion or
potassium permanganate are used, under acidic conditions, for example:
- 3 CH3-CH-CH3 + K2Cr2O7 + 4 H2SO4 ? 3 CH3-C-CH3
...
+ Cr
23 + K
2SO
4 + 7
H2OFrequently in
aldehyde preparations these reagents cause a problem of over-oxidation to the
carboxylic acid. To avoid this, other reagents such as
PCC,
Dess-Martin periodinane,
2-Iodoxybenzoic acid,
TPAP or methods such as
Swern oxidation and
Corey-Kim oxidation are now preferred.
Alcohols with a methyl group attached to the alcohol carbon can also undergo a
haloform reaction in the presence of the
halogen and a base such as sodium hydroxide.
Tertiary alcohols resist oxidation, but can be oxidised by reagents such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
See also
References
External links
- Aimed at high school students
- Interesting information about alcohols.
