Why can ammonia act as a base?

1 Answer
anor277
Mar 26, 2017

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.

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