230 THE PREPARATION OF 



extracted by means of ether and can be freed from traces of 

 sulphur by recrystallisation. 



An even simpler way to prepare acid anhydrides is afforded 

 by the employment of thionyl chloride in the same manner. 

 If a solution of this reagent in ether is shaken with an organic 

 silver salt, immediate reaction occurs, but usually no inter- 

 mediate compound can be isolated, so that after filtration 

 from the silver chloride the anhydride can be crystallised in 

 a pure condition from the filtrate : 



2C 6 H 5 .COOAg+SOCl 2 = (C 6 H 5 .CO) 2 0+S0 2 +2AgCl. 



The yield of anhydride by this method is nearly quanti- 

 tative, even although only small quantities are prepared. 

 Thus, in one experiment, 2*1 grams of crude but nearly pure 

 benzoic anhydride were obtained instead of the calculated 

 quantity, 2'3 grams. The method is applicable to the prepara- 

 tion of anhydrides of fatty and aromatic acids, both of small 

 and large molecular weights, to the preparation of anhydrides 

 of halogen and nitro acids and of some dibasic acids. 



The use of a solvent is an essential feature of the new 

 methods, and it is important that the solvent be carefully 

 dried if successful results are to be obtained. 



The low temperature at which these reactions occur, and the 

 smooth manner in which they proceed, suggested their applica- 

 tion to the preparation of normal anhydrides of hydroxy- 

 acids, a type of compound which cannot be obtained by the 

 methods hitherto in use, since by the employment of chlorides 

 of phosphorus chlorination occurs, while, by the influence of 

 heat, anhydro-acids and similar compounds are produced. 



It was found, however, that with sulphur chloride and 

 salts of glycollic, mandelic and malic acids the reaction is 

 abnormally slow, while with salts of the hydroxybenzoic acids 

 it is irregular, so that it is improbable that the normal anhy- 

 drides of the hydroxy-acids can be prepared by the use of 

 this reagent. 



Experiments with thionyl chloride gave more promising 



