Thiocarboxylic acids are organosulfur compounds related to carboxylic acids by replacement of one of the two oxygen atoms with a sulfur atom. There are two possible tautomers, RC(S)OH and RC(O)SH, which are called the "carbothioic O-acid" and "carbothioic S-acid" forms, respectively. The carbothioic S-acid tautomer is far more common.
In the laboratory, the most common thiocarboxylic acid is thioacetic acid. A naturally occurring thiocarboxylic acid is 2,6-pyridinedicarbothioic acid, which functions as a siderophore. Thiocarboxylic acids are more acidic than the corresponding carboxylic acids.
Thiocarboxylic acids are typically prepared 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
Dithiocarboxylic acids, with the formula RCS2H, are also known. They are less common than the monothio derivatives. Such compounds are commonly prepared by the reaction of carbon disulfide with a Grignard reagent:
- RMgX + CS2 → RCS2MgX
- RCS2MgX + HCl → RCS2H + MgXCl
This reaction is comparable to the formation of carboxylic acids using a Grignard and carbon dioxide.
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.
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