Reactions of free acids with nitrogen compounds

Ammonia and primary and secondary amines can be acylated by heating them with carboxylic acids. The removal of water occurs within the framework of an equilibrium that affords worthwhile amounts of amide only at temperatures above 100° (usually at 180-200°). So one of the starting materials (acid or amine) is usually taken in excess and the water formed is removed continuously from the reaction mixture. For formylation only, aqueous acid can be used. Yields are mostly good. 

Heptanamide, for instance, was obtained in 70-80% yield by heating urea (1 mole) with heptanoic acid (0.5 mole) at 170-180° for 4 h.685 

Urea may be replaced by an amide of a more volatile acid, e.g., formamide was used to afford the amide of oleic acid.686 

TV-Substituted carboxamides can be obtained analogously, e.g., the di-methylamides of simple aliphatic acids by passing dimethylamine into the hot acid.683,687 For the preparation of benzanilide molar amounts of aniline and benzoic acid are heated at 180-190°, yields being 84%.688 The urea method can also be applied for TV-substituted amides, if appropriately substituted ureas or thioureas are used.689,690 The following technique has proved useful for preparation of a variety of carboxanilides  

This procedure affords TV-phenylsuccinimide from succinic acid. 

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