Thiol

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Thiol

In organic chemistry

Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
, a thiol is a compound that contains the functional group composed of a sulfur

Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
atom and a hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
atom (-SH). Being the sulfur analogue of an alcohol

Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
group (-OH), this functional group is referred to either as a thiol group or a sulfhydryl group. More traditionally, thiols are often referred to as mercaptans.

a thiol group is a substituent

Substituent

In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl is used when naming organic compounds that contain a substituent....
on an alkane

Alkane

Alkanes, also known as paraffins, are chemical compounds that consist only of the elements carbon and hydrogen , wherein these atoms are linked together exclusively by single bonds without any cyclic structure ....
, there are several ways of naming the resulting thiol:

term mercaptan comes from the Latin

Latin

Latin is an Italic language, historically spoken in Latium and Ancient Rome. Through the Military history of the Roman Empire, Latin spread throughout the Mediterranean and a large part of Europe....
mercurium captans, meaning 'laying hold of mercury,' because the –SH group binds tightly to the element mercury

Mercury (element)

Mercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure....
.

thiols are colorless liquid

Liquid

Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material....
s having an odor

Odor

An odor or odour is a volatilized chemical compound, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction....
resembling that of garlic

Garlic

Allium sativum L., commonly known as garlic, is a species in the onion family Alliaceae. Its close relatives include the onion, shallot, leek, and chive....
.

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Encyclopedia

In organic chemistry

Organic chemistry

Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
atom and a hydrogen

Hydrogen

Alcohol

Nomenclature

When a thiol group is a substituent

Substituent

Alkane

The preferred method (used by the IUPAC) is to add the suffix -thiol to the name of the alkane. The method is nearly identical to naming an alcohol
Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
. Example: CH₃SH would be methanethiol
Methanethiol

Methanethiol is a colorless gas with a smell like rotten cabbage. It is a natural substance found in the blood, brain, and other animal as well as plant tissues....
.
An older method, the word mercaptan replaces alcohol in the name of the equivalent alcohol compound. Example: CH₃SH would be methyl mercaptan
Methanethiol

Methanethiol is a colorless gas with a smell like rotten cabbage. It is a natural substance found in the blood, brain, and other animal as well as plant tissues....
. (CH₃OH would be methyl alcohol
Methanol

Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a chemical compound with chemical formula carbonhydrogen3oxygenhydrogen ....
)
As a prefix, the terms sulfanyl or mercapto are used. Example: mercaptopurine
Mercaptopurine

Mercaptopurine is an immunosuppressive drug.It is a thiopurine....
.

Etymology

The term mercaptan comes from the Latin

Latin

Mercury (element)

Physical properties

Odor

Many thiols are colorless liquid

Liquid

Odor

An odor or odour is a volatilized chemical compound, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction....
resembling that of garlic

Garlic

Allium sativum L., commonly known as garlic, is a species in the onion family Alliaceae. Its close relatives include the onion, shallot, leek, and chive....
. The odor of thiols is often strong and repulsive, particularly for those of low molecular weight. Thiols bind strongly to skin

Skin

The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s....
protein

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s. Natural gas

Natural gas

Natural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills....
distributors began adding various forms of pungent thiols, originally ethanethiol

Ethanethiol

Ethanethiol is the organic compound with the formula CH3CH2SH. It consists of an ethyl group, CH3CH2, attached to a thiol group, SH....
, to natural gas

Natural gas

Natural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills....
, which is naturally odorless, after the deadly 1937 New London School explosion

New London School explosion

The New London School explosion occurred on March 18, 1937, when a natural gas leak caused an explosion, destroying the New London School of the city of New London, Texas....
in New London, Texas

New London, Texas

New London is a city in Rusk County, Texas, Texas, United States. The population was 987 at the 2000 United States Census.On March 18, 1937, the New London School explosion killed in excess of three hundred people ....
. Most gas odorants utilized in the world contain mixtures of mercaptans and sulfides, with t-butyl mercaptan as the main odor constituent. Thiols are also responsible for a class of wine fault

Wine fault

A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage....
s caused by an unintended reaction between sulfur and yeast

Yeast

Yeasts are eukaryote microorganisms classified in the Kingdom fungus, with about 1,500 species currently described; they dominate fungal diversity in the oceans....
and the "skunky" odor of beer which has been exposed to ultraviolet light. However, not all thiols have unpleasant odors. For example, grapefruit mercaptan

Grapefruit mercaptan

Grapefruit mercaptan is the common name for a natural organic compound found in grapefruit. It is a terpene that contains a thiol functional group....
, a monoterpenoid

Terpene

Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium....
thiol, is responsible for the characteristic scent of grapefruit

Grapefruit

The grapefruit is a subtropics citrus tree grown for its bitter fruit which was originally named the "forbidden fruit" of Barbados.These evergreen trees are usually found at around 5-6 m tall, although they can reach 13-15 m ....
. It should be noted that this effect is present only at low concentrations. The pure mercaptan has an unpleasant odor.

Boiling points and solubility

Due to the small electronegativity

Electronegativity

Electronegativity, symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond....
difference between sulfur and hydrogen, an S-H bond is practically nonpolar covalent. Therefore, the S-H bond in the thiols have a lower dipole moment

Dipole moment

Dipole moment refers to the quality of a system to behave like a dipole. Dipole moment is the measured polarity of a polar covalent bond. It is defined as the product magnitude of charge on the atoms and the distance between the two bonded atoms....
as compared to the alcohol's O-H bond. Thiols show little association by hydrogen bond

Hydrogen bond

A hydrogen bond is the attractive force between one electronegative atom and a hydrogen covalently bonded to another electronegative atom. It results from a dipole-dipole force with a hydrogen atom bonded to nitrogen, oxygen or fluorine ....
ing, with both water molecules and among themselves. Hence, they have lower boiling point

Boiling point

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
s and are less soluble in water and other polar solvents

Solvent

A solvent is a liquid or gas that dissolves a solid, liquid, or gaseous solute, resulting in a solution.The most common solvent in everyday life is water....
than alcohols of similar molecular weight. Thiols are as soluble and have similar boiling points to isomeric sulfides.

Chemical properties

Synthesis

The methods used in making thiols are analogous to those used to make alcohols and ethers. The reactions are quicker and higher yielding because sulfur anions are better nucleophiles than oxygen atoms.

Thiols are formed when a halogenoalkane is heated with a solution of sodium hydrosulfide

Sodium hydrosulfide

Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half -neutralization of hydrogen sulfide with a sodium-derived base....

CH₃CH₂Br + NaSH heated in ethanol(aq) ? CH₃CH₂SH + NaBr

In addition, disulfides can be readily reduced by reducing agents such as lithium aluminium hydride in dry ether to form two thiols.

R-S-S-R' ? R-SH + R'-SH

Reactions

The thiol group is the sulfur analog of the hydroxyl group (-OH) found in alcohol

Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
s. Since sulfur

Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
and oxygen

Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
belong to the same periodic table group

Periodic table group

In chemistry, a group is a vertical column in the periodic table of the chemical elements. The name family is derived from the fact that the elements share similar characteristics and traits, just as members of any human family would....
, they share some similar chemical bonding

Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
properties. Like alcohol, in general, the deprotonated form RS^- (called a thiolate) is more chemically reactive than the protonated thiol form RSH

The chemistry of thiols is thus related to the chemistry of alcohols: thiols form thioether

Thioether

A thioether is a functional group in organic chemistry that has the structure R1-S-R2 as shown on right. Like many other sulfur-containing compounds, Volatile organic compound thioethers characteristically have foul odors....
s, thioacetal

Thioacetal

Thioacetals are the sulfur analog of acetals. They are prepared in a similar way to acetals: by reacting a thiol with an aldehyde:Dithioacetals are prepared similarly to thioacetals, which are intermediates:...
s and thioester

Thioester

Thioesters are compounds resulting from the bonding of sulfur with an acyl group with the general formula R-S-CO-R. They are the product of esterification between a carboxylic acid and a thiol ....
s, which are analogous to ethers, acetal

Acetal

An acetal is a molecule with two single bonded oxygens attached to the same carbon atom.Traditional usages distinguish ketal from acetal . Current accepted terminology classifies ketals as a subset of acetals....
s, and esters. Furthermore, a thiol group can react with an alkene

Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
to create a thioether. (In fact, biochemically, thiol groups may react with vinyl

Vinyl

A vinyl compound is any organic compound that contains a vinyl group , −CarbonHydrogenCovalent bondCH2. These are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group....
groups to form a thioether linkage.)

Acidity

The sulfur atom of a thiol is quite nucleophilic

Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
, rather more so than the oxygen atom of an alcohol. The thiol group is fairly acidic with a usual pK_a

PKA

PKA or pKa may be:* Protein kinase A, a cAMP activated protein kinase* pKa, the symbol for Acid dissociation constant...
around 10 to 11. In the presence of a base

Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
, a thiolate anion is formed which is a very powerful nucleophile. The group and its corresponding anion are readily oxidized

Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
by reagents such as bromine

Bromine

Bromine , , meaning "stench " ), is a chemical element with the symbol Br and atomic number 35. A halogen element, bromine is a reddish-brown Volatility liquid at Standard conditions for temperature and pressure that is intermediate in reactivity between chlorine and iodine....
to give an organic disulfide

Disulfide

In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. The disulfide anion is S22-....
(R-S-S-R).

2R-SH + Br₂ ? R-S-S-R + 2HBr

Oxidation by more powerful reagents such as sodium hypochlorite

Sodium hypochlorite

Sodium hypochlorite is a chemical compound with the chemical formula NaClO. Sodium hypochlorite solution, commonly known as bleach, is frequently used as a disinfectant or a bleaching agent....
or hydrogen peroxide

Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
yield sulfonic acid

Sulfonic acid

Sulfonic acid usually refers to a member of the class of organic acids with the general formula R-S2-OH, where R is usually a hydrocarbon side chain....
s (RSO₃H).

R-SH + 3H₂O₂ ? RSO₃H + 3H₂O

Biological importance

As the functional group of the amino acid

Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
cysteine

Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, the thiol group plays an important role in biological systems. When the thiol groups of two cysteine residues (as in monomers or constituent units) are brought near each other in the course of protein

Protein

Cystine

Cystine is the amino acid dimer formed when a pair of cysteine molecules are joined by a disulfide bond. It is described by the formula 2....
unit with a disulfide bond

Disulfide bond

In chemistry, a disulfide bond is a single covalent bond derived from the coupling of thiol groups. The linkage is also called an SS-bond or disulfide bridge....
(-S-S-). Disulfide bonds can contribute to a protein's tertiary structure

Tertiary structure

In biochemistry and chemistry, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates....
if the cysteines are part of the same peptide

Peptide

Peptides are short polymers formed from the linking, in a defined order, of a-amino acids. The link between one amino acid residue and the next is known as an amide chemical bond or a peptide bond....
chain, or contribute to the quaternary structure

Quaternary structure

In biochemistry, quaternary structure is the arrangement of multiple protein folding protein molecules in a multi-subunit complex....
of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. The heavy

Heavy chain

The immunoglobulin heavy chain is the large polypeptide subunit of an antibody.A typical antibody is composed of two immunoglobulin heavy chains and two Ig light chains....
and light chain

Light chain

A light chain is the small polypeptide subunit of a protein complex such as a motor protein or an antibody .More specifically, it can refer to:...
s of antibodies are held together by bridges. Also, the kinks in curly hair

Hair

Hair is a protein filament that epidermal growth from hair follicle deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats....
are a product of multiple cystine formations. Permanents (perms) take advantage of the oxidizability of cysteine residues. The chemicals used in hair straightening

Hair straightening

Hair straightening is a hair styling technique which involves the flattening and straightening of hair in order to give it a smooth, streamlined and 'sleek' appearance....
are reductants that reduce cystine disulfide bridges to cysteine sulfhydryl groups, while chemicals used in hair curling are oxidants that oxidize cysteine sulfhydryl groups to form cystine disulfide bridges. Sulfhydryl groups in the active site

Active site

The active site of an enzyme contains the catalysis and binding sites. The structure and chemical properties of the active site allow the recognition and binding of the substrate ....
of an enzyme

Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
can form noncovalent bonds with the enzyme's substrate

Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalysis chemical reactions involving the substrate. The substrate binds with the enzyme active site, and an enzyme-substrate complex is formed....
as well, contributing to catalytic activity

Catalysis

Catalysis is the process in which the reaction rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst....
. Active site cysteine residues are the functional unit in cysteine protease

Cysteine protease

Proteases are enzymes that degrade protein. Cysteine proteases have a common catalytic mechanism that involves a nucleophile cysteine thiol in a catalytic triad....
s.

Cysteine residues may also bind with heavy metal ions (Pb²⁺, Hg²⁺, Ag²) because of the high affinity between the soft sulfide and the soft metal (see hard and soft acids and bases). This can deform and inactivate the protein, and is one mechanism of heavy metal poisoning.

Examples of thiols

Methanethiol
Methanethiol

Methanethiol is a colorless gas with a smell like rotten cabbage. It is a natural substance found in the blood, brain, and other animal as well as plant tissues....
- CH₃SH
Ethanethiol
Ethanethiol

Ethanethiol is the organic compound with the formula CH3CH2SH. It consists of an ethyl group, CH3CH2, attached to a thiol group, SH....
- C₂H₅SH
Propanethiol - C₃H₇SH
Coenzyme A
Coenzyme A

Coenzyme A is a coenzyme, notable for its role in the Fatty acid metabolism#Synthesis and Fatty acid metabolism#.CE.B2-Oxidation of fatty acids, and the oxidation of pyruvic acid in the citric acid cycle....
Lipoamide
Lipoamide

Lipoamide is a trivial name for 6,8-dithiooctanoic amide. It is lipoic acid's functional form where the carboxyl group is attached to protein by an amide linkage to an amino acid....
Glutathione
Glutathione

Glutathione is a tripeptide. It contains an unusual peptide linkage between the amino acid of cysteine and the carboxyl group of the glutamate side chain....
Cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
2-Mercaptoethanol
2-Mercaptoethanol

2-Mercaptoethanol is the chemical compound with the chemical formula HOCH2CH2SH. It is a hybrid of ethylene glycol, HOCH2CH2OH, and 1,2-Ethanedithiol, HSCH2CH2SH....
Dithiothreitol
Dithiothreitol

Dithiothreitol is the common name for a small-molecule redox reagent known as Cleland's reagent. DTT's formula is C4H10O2S2 and the molecular structure of its reduced form is shown at the right; its oxidized form is a disulfide bond 6-membered ring ....
/dithioerythritol
Dithioerythritol

Dithioerythritol is a sulfur containing sugar derived from the corresponding 4-carbon monosaccharide erythrose. It is an epimer of dithiothreitol ....
(an epimer
Epimer

In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another, unlike enantiomers which are non-superposable mirror images of one another....
ic pair)
2-Mercaptoindole
2-Mercaptoindole

2-Mercaptoindole is a bicyclic heterocycle containing a thiol group. It is a popular ligand and building block for more complex structures....
transglutaminase
Transglutaminase

Transglutaminases are a family of enzymes that catalyze the formation of a covalent bond between a free amine group and the gamma-carboxamid group of protein- or peptide-bound glutamine....

External links

by D. Witt, R. Klajn, P. Barski, B.A. Grzybowski at Northwestern University.
, by The Columbia Electronic Encyclopedia
, by Columbia Gas of Pennsylvania and Maryland
, by About Chemistry

Thiol

Nomenclature

Etymology

Physical properties

Odor

Boiling points and solubility

Chemical properties

Synthesis

Reactions

Acidity

Biological importance

Examples of thiols

See also

External links