19.21 : 胺到烯烃：霍夫曼消除

Amines are converted to alkenes through E2 eliminations, although not directly. This is because elimination reactions require a good leaving group, but the amino group is not. 

Amines are indirectly converted to alkenes by transforming the amino group into a good leaving group.

To do this, the amine is exhaustively alkylated to a quaternary ammonium salt. 

Then, the halide salt is converted to the hydroxide salt, which serves as the base to facilitate the elimination process.

Under thermal conditions, a concerted E2 process involving deprotonation, formation of a double bond, and elimination of the neutral amine gives the alkene. This is the Hofmann elimination.

Unlike regular E2 eliminations which give more-substituted alkenes as the major product, Hofmann eliminations produce less-substituted alkenes as the major product, also known as the Hofmann product.

Hofmann elimination occurs in an anti-periplanar fashion where the β proton and the tertiary amine group are oriented in opposite directions.