Why can ammonia act as a base?

1 Answer
Mar 26, 2017

Answer:

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.