|| Chapter 22: Amines
Reaction type: Elimination
- 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
- Ag2O / H2O
reacts giving the quaternary ammonium hydroxide, silver iodide precipitates.
- 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.
- The outcome is dictated by steric
effects of the large leaving group and the alkyl chain (more
- 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.
MECHANISM OF THE HOFMANN
|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.