226 THE PREPARATION OF 



Chlorosulphonic acid and chlorosulphonates have also been 

 employed (D. R. P. 146690; Abstr., 1904, i. 282). 



Sulphur tetrachloride, as is well known, does not exist 

 under ordinary conditions, but thionyl chloride, which may 

 be regarded as being related to sulphur tetrachloride in the 

 same manner as phosphorus oxychloride is to phosphorus 

 pentachloride, is a familiar reagent. This compound has been 

 applied to the preparation of acid chlorides by H. Meyer, 

 who prepared a number of acid chlorides by heating the free 

 acid with five or six times its weight of thionyl chloride. 

 He noticed that the anhydrides were formed in some cases 

 (Monatsheft., 1901, 22, 777), and later (Chem. Zeit., 1909, 

 1036) the same author outlines a method for the preparation 

 of the anhydride of the sulphonic acids from thionyl chloride 

 and the free acids of their potassium salts, apparently under 

 similar conditions. 



In the methods considered above, the formation of an 

 anhydride is usually regarded as being due to the primary 

 formation of an acid chloride, which then, on reaction with 

 more anhydrous salt, gives rise to the anhydride and metallic 

 salt. Thus, in the familiar example of phosphorus oxy- 

 chloride, we have the following equations to represent the two 

 stages in the reaction : 



2CH 3 . COONa+POCl 3 - 2CH 3 . CO . Cl+NaP0 3 +NaCl 

 CH 3 . COONa+CH 3 . CO . Cl = (CH 3 . CO) 2 0+NaCl. 



Similar reactions occur when other non-metallic chlorides, 

 such as the chlorides of sulphur, are used. 



The reaction which takes place between excess of the 

 so-called sulphur dichloride and a dry salt of an organic 

 acid was investigated by Heintz, who found that the chloro- 

 anhydride is formed among other products (Jahresb. Chem., 

 1856, 569). Thus, using sodium benzoate, the reaction is 

 represented by the equation : 



4C 6 H 5 .COONa+3SCl 2 = 2S+Na 2 S0 4 +2NaCl+4C 6 H 5 CO . Cl. 



