Thiol

Overview
In organic chemistry, a thiol is a compound that contains the functional group composed of a sulfur atom and a hydrogen atom (-SH). Being the sulfur analogue of an alcohol 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.

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. The method is nearly identical to naming an alcohol. 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. (CH3OH would be methyl alcohol)
 * 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.

Odor
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 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 New London, Texas. 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.

Boiling points and solubility
Due to the small electronegativity 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 as compared to the alcohol's O-H bond. Thiols show little association by hydrogen bonding, with both water molecules and among themselves. Hence, they have lower boiling points and are less soluble in water and other polar solvents than alcohols of similar molecular weight. Thiols are as soluble and have similar boiling points to isomeric sulfides.

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


 * 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 alcohols. Since sulfur and oxygen belong to the same periodic table group, they share some similar chemical bonding 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 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.)

Acidity
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 pKa 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).


 * 2R-SH + Br2 → R-S-S-R + 2HBr

Oxidation by more powerful reagents such as sodium hypochlorite or hydrogen peroxide yield sulfonic acids (RSO3H).


 * 2R-SH + 2H2O2 → RSO3H + 2H2O

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.

Examples of thiols

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