The reaction of
phenylamine with water
This is where it is
possible to tell that phenylamine is a much weaker base than ammonia and the
aliphatic amines like methylamine and ethylamine.
Phenylamine reacts reversibly
with water to give phenylammonium ions and hydroxide ions.
The position of
equilibrium lies well to the left of the corresponding ammonia or aliphatic
amine equilibria - which means that not many hydroxide ions are formed in the
solution.
The effect of this is
that the pH of a solution of phenylamine will be quite a bit lower than a
solution of ammonia or one of the aliphatic amines of the same concentration.
For example, a 0.1 M phenylamine solution has a pH of about 9 compared to a pH
of about 11 for 0.1 M ammonia solution.
Why is phenylamine such
a weak base?
Amines are bases because
they pick up hydrogen ions on the lone pair on the nitrogen atom. In
phenylamine, the attractiveness of the lone pair is lessened because of the way
it interacts with the ring electrons.
The lone pair on the
nitrogen touches the delocalised ring electrons . . .
. . . and becomes
delocalised with them:
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