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thiol, also called mercaptan,  any of a class of organic chemical compounds similar to the alcohols and phenols but containing a sulfur atom in place of the oxygen atom. Thiols are among the odorous principles in the scent of skunks and of freshly chopped onions; their presence in petroleum and natural gas is objectionable because they have disagreeable odours, interfere with catalysts used in refining processes, and produce sulfur dioxide upon combustion.

Thiols show many reactions like those of the hydroxyl compounds, such as formation of thioesters and thioethers (sulfides). Toward oxidation, however, they differ profoundly from alcohols: whereas oxidation of an alcohol usually leads to a product in which the oxidation state of a carbon atom has been changed, oxidation of a thiol affects the sulfur atom. Mild oxidants convert thiols to disulfides, and more vigorous reagents result in the formation of sulfonic acids. Aliphatic thiols are commonly prepared from alkyl halides and sodium hydrosulfide or from olefins and hydrogen sulfide. Aromatic thiols can be made from the amino compounds by way of the diazonium salts. A thiol generally has a much lower boiling point than the hydroxyl compound of like structure; for example, methanethiol boils at 6° C (43° F), methanol at 65° C (149° F). The lower aliphatic thiols are useful as warning odorants in fuel gases and are somewhat toxic.

Thiol nomenclature is relatively similar to the nomenclature of alcohols. However, instead of adding the suffix -ol as in the case of alcohols, thiols are usually named by adding the suffix -thiol. In addition, thiols prepared from alkanes retain the -e in the name of the alkane—for example, butanethiol and ethanethiol as opposed to the alcohols butanol and ethanol. The −SH group of a thiol is known as a mercapto group, and therefore the prefix mercapto- may be included in the names of certain compounds—for example, 2-mercaptoethanol or 2-mercaptobenzothiazole. When a thiol is named relative to a corresponding oxygen compound, the prefix thiol- is used—for example, thiophenol.

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