Ch22: Hofmann elimination

Chapter 22: Amines

Hofmann Elimination Reaction type: Elimination

Summary

Like alcohols, amines can undergo elimination reactions.
Quaternary ammonium salts undergo an E2 elimination when heated with silver oxide, Ag₂O, in water.
Amines can readily be converted into quaternary ammonium iodides by treating them with excess methyl iodide.
Ag₂O / H₂O reacts giving the quaternary ammonium hydroxide, silver iodide precipitates.

completely methylate then convert to the ammonium hydroxide salt

When heated the hydroxide induces a base promoted 1,2- or β-elimination giving an amine and alkene.
The regioselectivity is opposite to that predicted by Zaitsevs rule in that it leads to the less highly substituted alkene.
The less highly substituted alkene is sometimes referred to as the Hofmann product.

1,2-elimination of the ammonium salt gives mainly the less highly substituted alkene

The outcome is dictated by steric effects of the large leaving group and the alkyl chain (more details ?)
NH₂^-and NR₂^- are very poor leaving groups (both anionic), but NR₃ is much better (neutral)
Compare this with with -OH and H₂O in the dehydration of alcohols.

Related reactions

MECHANISM OF THE HOFMANN ELIMINATION
The initial steps are an example of the alkylation of an amine by methyl iodide. The mechanism of the elimination step is shown. When heated, the hydroxide removes the more accessible proton, the π bond of the alkene C=C forms and the leaving group, a neutral amine departs.

© Dr. Ian Hunt, Department of Chemistry