Explain why aniline is not as basic as ammonia.

Amines are bases because the lone pair of electrons on the nitrogen atom can accept a hydrogen ion - in other words, for exactly the same reason that ammonia is a base.
With phenylamine (Aniline), the only difference is that it is a much weaker base than ammonia.
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. That means that the lone pair is no longer fully available to combine with hydrogen ions. Delocalisation makes molecules more stable and electrons