Reactions and characterisation of primary aromatic amides

Primary aromatic amides are crystalline solids with definite melting points. Upon boiling with 10-20 per cent, sodium or potassium hydroxide solution, they are hydrolysed with the evolution of ammonia (vapour turns red litmus paper blue and mercurous nitrate paper black) and the formation of the alkali metal salt of the acid  

The acid is liberated upon acidification. Hydrolysis may also be effected (but less readily and usually not quite so satisfactorily) by boiling with dilute hydrochloric acid (1 1) or 20 per cent, sulphuric acid  

The hydrolysis by alkali is illustrated by the following experimental details for benzamide. Place 3 g. of benzamide and 50 ml. of 10 per cent, sodium hydroxide solution in a 150 ml. conical or round-bottomed flask equipped with a reflux condenser. Boil the mixture gently for 30 minutes ammonia is freely evolved. Detach the condenser and continue the boiling in the open flask for 3-4 minutes to expel the residual ammonia. Cool the solution in ice, and add concentrated hydrochloric acid until the mixture is strongly acidic benzoic acid separates immediately. Leave the mixture in ice until cold, Alter at the pump, wash with a little cold water and drain well. Recrystallise the benzoic acid from hot water. Determine the m.p., and confirm its identity by a mixed m.p. test. 

The characterisation of a primary aromatic amide is based upon its own m.p. and the identification of the acid (see Section IV,175) produced on hydrolysis. A crystalline derivative may be prepared directly with xanthhydrol (for experimental details, see Section 111,110,1). 

The melting points of a few selected primary aromatic amides (together with those of the xanthylamides, where known) are collected in Table IV,191. A more detailed list will be found in the column headed Amides in Table IV,175 (Arotmtic Carboxylic Acids). 

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