11.14: Preparation and Reactions of Thiols
Thiols are prepared using the hydrosulfide anion as a nucleophile in a nucleophilic substitution reaction with alkyl halides. For instance, bromobutane reacts with sodium hydrosulfide to give butanethiol.
This reaction fails because the thiol product can undergo a second nucleophilic substitution reaction in the presence of an excess alkyl halide to generate a sulfide as a by-product.
This limitation can be overcome by using thiourea as the nucleophile. The reaction first produces an alkyl isothiourea salt as an intermediate, which forms thiol as a final product upon hydrolysis with an aqueous base.
Thiols can readily oxidize to disulfides, sulfinic acid, and sulfonic acid. The oxidation of thiols to disulfides can even occur in the presence of atmospheric air. Thus, the high susceptibility of thiols to undergo air oxidation necessitates the storage of thiols in an inert atmosphere. Oxidation of thiols to disulfides can also be accomplished using reagents like molecular bromine or iodine in the presence of a base. Disulfides, however, can be easily reduced back to thiols by treatment with reducing agents such as HCl in the presence of zinc. Notably, oxidation of thiols to disulfides is a redox reaction. The interconversion between thiols and disulfides is ascribed to the bond strength of the S–S bond, which is approximately half the strength of other covalent bonds.