ANHYDRIDES OF ORGANIC ACIDS 235 



existence of thiosulphurous anhydride S 2 0, and, if this is so, 

 we should then have the following schemes to represent the 

 course of the reactions with sulphur chloride and thionyl 

 chloride respectively : 



Sulphur Chloride 



2C 6 H 6 . COOAg+S 2 Cl 2 = (C 9 H 5 . COO) 2 S 2 +2AgCl, 

 (C 6 H 5 .COO) 2 S 2 = (C 6 H 6 .CO) 2 0+S 2 0, 

 2S 2 = S0 2 +3S. 



Thionyl Chloride 



2C 6 H 5 .COOAg+SOCl 2 = (C 6 H 5 COO) 2 SO+2AgCl, 

 (C 6 H 5 . COO) 2 SO = (C 8 H 5 . CO) 2 0+SO 2 . 



These methods of anhydride formation would thus be 

 classified with that in which an unstable mixed anhydride is 

 first formed by the action of benzoyl chloride on a salt of an 

 organic acid. 



The analogous behaviour of thionyl chloride and sulphur 

 chloride may be further developed. If the reaction between 

 thionyl chloride and silver benzoate be carried out, as already 

 described in presence of ether, but with equimolecular pro- 

 portions of the reagents, benzoyl chloride is obtained. With 

 sulphur chloride the further reaction does not take place so 

 readily, but it may be brought about by boiling benzoic 

 anhydride with the chloride. Sulphur dioxide is then evolved 

 and sulphur is liberated. The reactions may be represented 

 by the equations : 



(C 6 H 5 .CO) 2 0+S 2 C1, = 2C 6 H 5 .CO.C1+S 2 0, 

 2S 2 = SO 2 +3S. 



(C 6 H 5 . CO) 2 0+SOC1 2 = 2C 6 H 5 . CO . C1+S0 2 . 



This manner of viewing sulphur chloride as a chloro- 

 anhydride the parent acid of which is extremely unstable, 

 allows of some of its reactions being expressed very simply. 



