ANHYDRIDES OF ORGANIC ACIDS 227 



With excess of sodium benzoate the benzoyl chloride first 

 formed yields benzoic anhydride. 



Technical processes based on the above reactions have 

 been patented by H. Kessler (D. R. P. 132605 ; Abstr., 1903, 

 i. 309) and by T. Goldschmidt (Eng. Pat. 25433; J. Soc. 

 Chem. Ind., 1910, 112, 592). Kessler specifies the use of sul- 

 phur dichloride SC1 2> while Goldschmidt causes excess of 

 chlorine to react with the dry sodium salt in presence of 

 sulphur. The acid chloride is thus formed, and the mixture 

 on being heated gives the anhydride. 



Carius (Annalen, 1858, 106, 291) investigated the reaction 

 between equimolecular proportions of sodium chloride and 

 sulphur monochloride S 2 C1 2 , both in the presence and in the 

 absence of solvent, and found that although the reaction is 

 not altogether smooth, the initial and final stages are repre- 

 sented essentially by the equation : 



2C 6 H 6 . COONa+2S 2 Cl 2 = 2C 6 H 5 . CO . Cl-f 2NaCl+S0 2 +3S 



Gerhardt proposed another method for the preparation of 

 organic anhydrides, in which the mechanism of the reaction 

 is somewhat different from that described above for cases 

 in which inorganic chlorides are used. This method is based 

 on the observation that an organic acid chloride, such as 

 benzoyl chloride, gives a mixed anhydride on reaction with 

 a metallic salt of a different organic acid, according to the 

 equation : 



CH 3 . COONa+C 6 H 5 . CO . Cl = CH 3 . CO . . CO . C 6 H 6 +NaCl. 



Such mixed anhydrides are unstable, so that on being heated 

 they decompose with formation of the simple anhydrides : 



2CH 3 . CO . . CO . C 6 H 5 = (CH 3 . CO) 2 O+(C 6 H 6 . C0) 2 0. 



Mixed anhydrides can also be prepared by heating excess of 

 an anhydride such as acetic anhydride with another acid 

 (Autehrieth, Ber., 1887, 20, 3188). 



