Thio Acid

any of a group of sulfur analogues of oxy acids, in whose molecules oxygen is replaced by sulfur.

Inorganic thio acids are unstable and usually cannot be isolated in the free state. Salts, ethers, and anhydrides of such thio acids, however, are sufficiently stable. Examples are Na₂S₂O₃ among the salts (thio salts), As (SC₆H₅)₃ among the ethers, and Sb₂S₃ among the anhydrides.

Among organic thio acids, a distinction is made between singly substituted acids—the thiolic acids (a) and thionic acids (b)—and the dithio acids (c):

The thiolic and thionic acids exist as tautomeric mixtures in which the thiolic form is strongly predominant; derivatives of both forms are known.

Thio acids, especially the simplest ones, have a strong, unpleasant odor. Compared to the corresponding carboxylic acids, thio compounds are stronger acids, are less soluble in water, and boil at lower temperatures. Organic thio acids are produced mainly by the reaction of carboxylic acids with phosphorus penta-sulfide (1) or by the reaction of carboxylic acid derivatives with hydrogen sulfide (2):

The amides of thionic acids, RC(S)NR₂, are used in the synthesis of heterocyclic compounds; α-ethylisonicotinoylthioamide, or ethionamide, is an antituberculotic. The esters of dithiocarbonic acid, xanthates, are used in the production of rayon and as herbicides.