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:
Note: If you don't understand these diagrams, you will need to read aboutbonding in benzenein order to make sense of them.
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That means that the lone pair is no longer fully available to combine with hydrogen ions. The nitrogen is still the most electronegative atom in the molecule, and so the delocalised electrons will be attracted towards it, but the electron density around the nitrogen is nothing like it is in, say, an ammonia molecule.
The other problem is that if the lone pair is used to join to a hydrogen ion, it is no longer available to contribute to the delocalisation. That means that the delocalisation would have to be disrupted if the phenylamine acts as a base. Delocalisation makes molecules more stable, and so disrupting the delocalisation costs energy and won't happen easily.
Taken together - the lack of intense charge around the nitrogen, and the need to break some delocalisation - means that phenylamine is a very weak base indeed.


Thank You
Ruchi
Askiitians Faculty