From Perthiocyanic Acid

Synthesis from Thiocyanic Add (Isoperthiocyanic and Perthiocyanic Acids)... 

Wohler143 first described the formation, from concentrated aqueous thiocyanic acid, of a compound, C2H2N2S3 later named isoperthiocyanic acid, and now formulated as 3-imino-5-thiono-l,2,4-dithia-zolidine (144). Solutions of isoperthiocyanic acid in alkali deposit sulfur this redissolves presently giving a liquid containing144-147 the salt of an isomeric acid, later named perthiocyanic acid, for which the 3,5-dimercapto-l,2,4-thiadiazole structure is now accepted (see Section III, J, 1). This acid, isolated from its barium salt by treatment with mineral acid and ether extraction,144,145 readily reverts to isoperthiocyanic acid,144 but fairly stable specimens are obtainable under carefully controlled conditions.148 148... 

PhCH2), obtained in their turn by treatment of 3-alkyl(or aralkyl) thio-5-chloro-l,2,4-thiadiazoles (344 R = Me or PhCH2) with ammonium sulfide.91 Additional support in favor of structure 340 for perthiocyanic acid is the alternative production of its disodium salt (343) from 5-chloro-3-mercapto-l,2,4-thiadiazole (344 R = Na) and sodium sulfide.151... 

In 1821 Wohler discovered that a solid deposited from concentrated aqueous solutions of thiocyanic acid. The solid, which was called isoperthiocyanic acid (3-imino-5-mercapto-1,2,4-dithiazole) (361), formed a new product perthiocyanic acid (3,5-dimercapto-l,2,4-thiadiazole) (18) when treated with alkali and then acid. On storage perthiocyanic acid (18) readily reverted to isoperthiocyanic acid (361) (65AHC(5)119). The mechanisms of these interconversions are still not known with certainty but the transformations outlined in Scheme 130 are suggested. Wohler proposed the initial formation of a dimer of thiocyanic acid for which structure (359) appears resonable. Addition of the imine function of (359) to the nitrile function of HSCN would produce the trimer (360) which could readily eliminate hydrogen cyanide to produce isoperthiocyanic acid (361). 

Hantzsch s first important research was the synthesis of pyridine from acetoacetic ester and aldehyde ammonia, a general method of synthesis of pyridine derivatives. Coumarone, discovered by Fittig and Ebert (see p. 768), was (with some derivatives) synthesised by Hantzsch, who called it the fur-furane of the naphthalene series . He synthesised thiazole, and benzene, pyridine, and thiophen derivatives from derivatives of pentamethylene, and pyrrole, investigated tetrazoles, and perthiocyanic acid C2N2S3H2 (discovered by Wohler), giving it a cyclic structure." ... 

The dipotassium sail of 3,5-dimercapto-l,2,4-thiadiazole ( perthiocyanic acid ) (36) is most conveniently prepared by refluxing a solution of (31) in methanol with sulphur. The salt is readily chlorinated to the stable bis(sulphenylchloride) (37). The observations supplement and further clarify previous work on the synthesis of 1,2,4-thiadiazoles from thiocyanic acid (see Volume 1, p. 448 and ref. 15). 

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