102 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



ACID ANHYDRIDES. 



If acid chlorides be allowed to act upon the sodium salt of a fatty 

 acid an acid anhydride is formed : 



CH 3 CO . Cl + NaOOC . CH 3 = NaCl + CH 3 . CO O OC . CH 3 . 

 The constitution of these compounds is analogous to the ethers ; 

 two radicles are united by an oxygen atom. Mixed anhydrides can 

 be prepared by using different acyl chlorides and different sodium 

 salts of fatty acids : 



CH 3 CH 3 , CH 3 CO, CH 3 CO, 



)o ^o )o \>. 



CH 3 / C 2 H 5 / CH 3 C(K CH,CH 2 CO/ 



Ethers. Anhydrides. 



Preparation of Acetic Anhydride. 



40 gm. of fused sodium acetate are placed in a retort which is connected 

 to a condenssr and receiver and fitted with a dropping tunnel. 30 gm. 

 of acetyl chloride are run in slowly and the contents of the flask are kept cold 

 by immersion in cold water. The contents of the retort are well stirred and 

 distilled. Acetic anhydride, which boils at 139, passes over between 130 

 and 140. 



Properties. 



The anhydrides are liquids possessing a pungent smell, but do not 

 fume in the air. 



Reactions. 



The reactions of the anhydrides are the same as the acyl 

 chlorides. 



They are decomposed by water giving the constituent acid. 



They yield esters with alcohols. 



They yield amides with ammonia. 



Like the acyl chlorides they are also used for acylating alcohols 

 and compounds containing amino (NH 2 ) groups. The compound is 

 boiled under an air condenser with the anhydride for some hours and 

 poured into water. The acyl derivative is generally insoluble and is 

 recrystallised from a suitable solvent 



