19.21:
Amines to Alkenes: Hofmann Elimination
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.
19.21:
Amines to Alkenes: Hofmann Elimination
Alkenes can be obtained from amines via an E2 elimination. The amine is first converted into a good leaving group, such as a quaternary ammonium salt. This is accomplished by treating the amine with an excess of alkyl halide, which results in a halide salt. Next, the halide salt is transformed into a hydroxide salt that functions as a base to enable elimination.
Under thermal conditions, the hydroxide can abstract a proton from the β carbon; this generates an alkene with the simultaneous release of a neutral amine molecule. This elimination process is called the Hofmann elimination. For the elimination to occur, the β proton and tertiary amine group must be positioned opposite or anti-periplanar to each other. This reaction yields the less substituted alkene as the major product, also known as the Hofmann product.