Thiol Thiol Still camera Category="Functional groups"Category="Thiols"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.

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.

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), 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 wieght. 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 to natural gas, which is naturally odorless, after the deadly 1937 New London School explosion. Thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and Yeast.

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 noncovalent bonds between different peptide chains. Sulfhydryl groups in the Active site of an Enzyme can form noncovalent bonds with the enzyme's substrate as well, contributing to catalytic activity.

Nomenclature

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

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

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