Thiol

In organic chemistry, a thiol is a compound that contains the functional group composed of a sulfur atom and a hydrogen atom (-SH). This functional group is referred to either as a thiol group or a sulfhydryl group. More traditionally, thiols have been referred to as mercaptans. In general, the deprotonated form - $\mathrm{S^{-}}$ (called a thiolate) is more chemically reactive than the protonated thiol form -SH.

Chemistry

The thiol group is the sulfur analog of the hydroxyl group (-OH) found in alcohols. Since sulfur and oxygen belong to the same periodic table group, they share some similar chemical bonding properties. The chemistry of thiols is thus related to the chemistry of alcohols: thiols form thioethers, thioacetals and thioesters, which are analogous to ethers, acetals, and esters. Furthermore, a thiol group can react with an alkene to create a thioether. (In fact, biochemically, thiol groups may react with vinyl groups to form a thioether linkage.)

The sulfur atom of a thiol is quite nucleophilic, rather more so than the oxygen atom of an alcohol. The thiol group is fairly acidic with a usual pK_a around 10 to 11. In the presence of a base, a thiolate anion is formed which is a very powerful nucleophile. The group and its corresponding anion are readily oxidized by reagents such as bromine to give an organic disulfide (R-S-S-R), or by more powerful reagents such as sodium hypochlorite to yield sulfonic acids (RSO₃H).

Because of the small electronegativity difference between sulfur and hydrogen, an S-H bond is practically nonpolar covalent. Thiols show little association by hydrogen bonding. They have lower boiling points and are less soluble in water and other polar solvents than alcohols of similar molecular weight.

Many thiols are colorless liquids having an odor resembling that of garlic. The odor of thiols is often strong and repulsive, particularly for those of low molecular weight. Thiols bind strongly to skin proteins, and are responsible for the intolerable, persistent odor produced by feces, rotting flesh and the spraying of skunks. Natural gas distributors began adding various forms of pungent thiols, usually ethanethiol, to natural gas, which is naturally odorless, after the deadly 1937 New London School explosion in Texas, United States. Thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and yeast. However, not all thiols have unpleasant odors. For example, grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of grapefruit.

Biological importance

As the functional group of the amino acid cysteine, 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 folding, an oxidation reaction can create a cystine unit with a disulfide bond (-S-S-). Disulfide bonds can contribute to a protein's tertiary structure if the cysteines are part of the same peptide chain, or contribute to the quaternary structure of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. The heavy and light chains of antibodies are held together by disulfide bridges. Also, the kinks in curly hair are a product of cystine formation. Permanents take advantage of the oxidizability of cysteine residues. The chemicals used in hair straightening are reductants that reduce cystine disulfide bridges to free 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 of an enzyme can form noncovalent bonds with the enzyme's substrate as well, contributing to catalytic activity. Active site cysteine residues are the functional unit in cysteine proteases.

Nomenclature

When a thiol group is a substituent on an alkane, there are several ways of naming the resulting thiol:

The preferred method (used by the IUPAC) is to add the suffix -thiol to the name of the alkane. Example: CH₃SH would be methanethiol.
An older method, the word mercaptan replaces alcohol in the name of the equivalent alcohol compound. Example: CH₃SH would be methyl mercaptan.
As a prefix, the term mercapto- is used. Example: mercaptopurine.

Etymology

The term mercaptan comes from the Latin mercurius captans, meaning 'laying hold of mercury,' because the –SH group binds tightly to the element mercury.

Examples of thiols

Methanethiol - CH₃SH
Ethanethiol - C₂H₅SH
Coenzyme A
Lipoamide
Glutathione
Cysteine
Dithiothreitol/dithioerythritol (an epimeric pair)

Manufacturers

Asemblon, Inc, also builds custom chemical compounds

External links

Mercaptan, by The Columbia Electronic Encyclopedia
What is Mercaptan?, by Columbia Gas of Pennsylvania and Maryland
What Is the Worst Smelling Chemical?, by About Chemistry

Thiols | Functional groups | Organosulfur compounds

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