ACID OR ACYL CHLORIDES. 



The fatty acids, like the alcohols, contain a hydroxyl group. Phos- 

 phorus pentachloride and phosphorus trichloride act upon the acids 

 forming the acid or acyl chloride : 



CH 3 . COOH + PC1 5 = CH 3 . CO . Cl + POC1 3 + HC1 

 3 CH 3 . COOH + 2 PC1 3 = 3CH 3 . CO . Cl + P 2 O 3 + 3 HC1. 



Preparation of Acetyl Chloride. 



A distilling flask is fitted with a tap funnel and connected with a condenser 

 and receiver. If the preparation be not carried out in a fume cupboard, the 

 receiver should be connected with a tower containing soda lime to absorb 

 hydrochloric acid. 



25 gm. of glacial acetic acid are placed in the distilling flask and 20 gm. 

 of phosphorus trichloride are slowly dropped upon it through the tap funnel. 

 The flask is warmed upon a water-bath at 40-50 until the hydrochloric 

 acid evolution has ceased ; the contents of the flask are then distilled from 

 a water-bath. Acetyl chloride, which boils at 55, passes over. 



Properties of Acyl Chlorides. 



Formyl chloride is not known. Acetyl chloride is a liquid, other 

 acyl chlorides are liquids or solids. They fume in moist air and under- 

 go decomposition into hydrochloric acid and the acid from which they % 

 are derived. 



Reactions of Acyl Chlorides. 



Acyl chlorides are decomposed by water giving the acid and hydro- 

 chloric acid : 



CH 3 COC1 + H 2 O = HCl + CHgCOOH. 



They react with alcohols giving esters (p. 71). 



They react with ammonia giving amides (p. 129). 



The acyl chlorides, though decomposed by water, are sometimes 

 only decomposed slowly and can be used in aqueous or alkaline solu- 

 tion for preparing esters or for preparing acyl derivatives of amines. 

 The process is known as acetylation, or acylation, or arylation if aro- 

 matic acid chlorides be used. An alkaline solution of the alcohol or 

 of an amine is shaken with the acyl chloride. The ester or acyl de- 

 rivative is generally insoluble and can be filtered off and purified by 

 crystallisation. 



