Thiocarboxylic acids are organosulfur compounds related to carboxylic acids by replacement of one of the oxygen atoms with a sulfur atom. Two tautomers are possible: a thione form (RC(S)OH) and a thiol form (RC(O)SH). These are sometimes also referred to as "carbothioic O-acid" and "carbothioic S-acid" respectively. Of these the thiol form is most common (e.g. thioacetic acid).
Thiocarboxylic acids are typically prepared by salt metathesis from the acid chloride, as in the following conversion of benzoyl chloride to thiobenzoic acid using potassium hydrosulfide according to the following idealized equation:
- C6H5C(O)Cl + KSH → C6H5C(O)SH + KCl
Thiocarboxylic acids are more acidic than the analogous carboxylic acids. Thus at neutral pH, the acids are fully ionized. Salts of their conjugate bases, (e.g. potassium thioacetate), serve as reagents for installing thiol groups via the displacement of alkyl halides to give the thioester, which in turn are susceptible to hydrolysis.
Thiocarboxylic acids react with various nitrogen functional groups, such as organic azide, nitro, and isocyanate compounds, to give amides under mild conditions. This method avoids needing a highly nucleophilic aniline or other amine to initiate an amide-forming acyl substitution, but requires synthesis and handling of the unstable thiocarboxylic acid. Unlike the Schmidt reaction or other nucleophilic-attack pathways, the reaction with an aryl or alkyl azide begins with a [3+2] cycloaddition; the resulting heterocycle expels N2 and the sulfur atom to give the monosubstituted amide.
- RMgX + CS2 → RCS2MgX
- RCS2MgX + HCl → RCS2H + MgXCl
This reaction is comparable to the formation of carboxylic acids using a Grignard reagent and carbon dioxide.
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