Question

Why can ammonia act as a base?

Answer 1

Because the nitrogen can be quaternized.

Explanation:

We represent the acid-base behaviour of ammonia in water by the following rxn:

`H_2O(l) + :NH_3(aq) rarr ""^+NH_4 + ""^(-)OH`

In `"ammonium"` one of the `N-H` bonds is donative, or coordinate covalent. Of course, by symmetry, all of the `N-H` bonds are equivalent. The coordinate covalency of the 4th `N-H` is a convenient little fiction.

The nitrogen in ammonium is now coordinatively saturated, and can no longer expand its valence shell.