Dithiocarboxylic acid

Dithiocarboxylic acids are organosulfur compounds with the formula RCS₂H. They are the dithia analogues of carboxylic acids. A closely related and better studied family of compounds are the monothiocarboxylic acids, with the formula RCOSH.^[1]

Dithiocarboxylic acids are about 3x more acidic than the monothiocarboxylic acids. Thus, for dithiobenzoic acid pK_a = 1.92. Contrariwise, they do not exhibit hydrogen bonding.^[2]

Such compounds are commonly prepared by the reaction of carbon disulfide with a Grignard reagent:^[3]

RMgX + CS₂ → RCS₂MgX

RCS₂MgX + HCl → RCS₂H + MgXCl

This reaction is comparable to the formation of carboxylic acids using a Grignard reagent and carbon dioxide.

Nitrogen derivatives substitute onto dithioic acids similar to the monothioic and carboxylic acids. Thus amines ammonolyze to thioamides or amidines, hydrazines to thio-hydrazides, and hydroxylamines to thio-hydroxamic acids or nitriles.^{[2]: 749–750} Likewise, unsaturated (i.e., allylic, vinylic, propargylic, or acetylenic) Grignard reagents attack dithioesters with an allylic shift much like a traditional Grignard reaction. In other cases, the carbonyl polarity is reversed: thus saturated Grignards alkylate the sulfur atoms to form a dithioacetal carbanion. Ketonic dithioesters react at the dithioester first, before the resulting carbanion attacks the ketone to form a cyclic alcohol.^[4]: 1047

Aryldithiocarboxylic acids, e.g., dithiobenzoic acid, chlorinate to give the thioacyl chlorides.^{[citation needed]} Oxygen transfer from e.g. air^[2]: 750 to a dithiocarboxylate ester gives a sulfine-like S-oxide that rearranges to an acyl disulfide.^[5] Dithiocarboxylate salts readily S-alkylate to give dithiocarboxylate esters:^[6]

RCS₂Na + R'Cl → RCS₂R' + NaCl

Dithioketene acetals (i.e., dithioate enol dithioethers) are nucleophilic at the sulfur atoms, not the α carbon.^[2]: 757