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.
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.
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