loo Biological Chemistry. 



Part VIII. Some Derivatives of Fatty Acids. 



(a) ACID CHLORIDES. 



It has been already stated that carboxylic acids, when 

 treated with phosphorus pentachloride, react according to 

 the equation 



R.CO-OH + PC1 5 = POC1 3 + K-CO-Cl + HC1 



Phosphorus trichloride can also be employed, the reaction 

 being represented by the equation 



3R CO-OH + PC1 3 = 3R.CO-C1 + P 2 3 + 3HC1 

 The substance obtained from acetic acid, CH 3 COC1, is 

 known as acetyl chloride ; that from propionic acid, pro- 

 pionyl chloride, the termination yl being generally em- 

 ployed in designating substance of this class. The group 

 (CH 3 CO) is known as the acetyl group. 



These acid chlorides readily react with other substances 

 containing a hydroxyl group, as, for example, water or 

 alcohols, the chlorine being removed from the molecule 

 in the form of hydrochloric acid. The reactions may be 

 illustrated by the two following equations 



CH 3 -COC1 + H,0 =CH 3 -COOH + HC1 

 CH 3 -COC1 + C 2 H 5 OH = CH 3 -COOC 2 H 5 + HC1 



In the former case, the original acid is regenerated, and 

 in the latter, an ester is formed. Both of these reactions 

 are types of general ones, and the second is often employed 

 for the preparation of esters. 



Acetyl chloride is a liquid which boils at 55, and 

 has, like other members of the series, a very pungent 

 odour. It forms fumes when exposed to moist air. 



(6) ACID ANHYDRIDES. 



The carboxylic acids are capable of forming anhydrides 

 of the general formula 



