Amidates and Their Halogenation The Hofmann Rearrangement

In amides, hydrogens on both the carbon and the nitrogen atoms next to the carbonyl group are acidic. Removal of the NH hydrogen, which has a p/fj of abont 22, with base leads to an amidate ion. The CH proton is less acidic, with a pAfa of abont 30 (Section 20-1) therefore, deprotonation of the a-carbon, leading to an amide enolate, is more difficult. 

Practically speaking, therefore, a proton may be removed from the a-carbon only with tertiary amides, in which the nitrogen lacks attached hydrogen atoms. 

The amidate ion formed by deprotonation of a primary amide is a synthetically useful nucleophile. This section focuses on one of its reactions, the Hofmann rearrangement. 

Exercise 20-21 The pXa of 1,2-benzenedicarboximide (phthalimide. A) is 8.3, considerably lower than the of  

In the presence of base, primary amides undergo a special halogenation reaction, the Hofmann rearrangement. In it, the carbonyl group is expelled from the molecule to give a primary amine with one carbon fewer in the chain. 

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