Question

What's wrong with this reaction? -OH + NH3 --> H2O + -NH2

Answer 1

Well, at least it is balanced with respect to mass and charge...

Explanation:

i.e. `HO^(-) + NH_3(aq) rarr H_2O(l) + NH_2^(-)`

But in aqueous solution, it does not represent solution behaviour. The strongest base that can exist in water is conceived to be hydroxide anion, i.e. `HO^-`. The `"amide ion"`, `NH_2^(-)` is in fact a STRONGER base than hydroxide, and is UNKNOWN in water.... The conjugate acid of amide ion is in fact a weak Bronsted base:

`NH_3(aq) + H_2O(l) rarr NH_4^(+) + HO^-`

On other hand, in a solution of liquid ammonia, which is accessible in the lab, tho you need a tank of ammonia gas, and a means to condense same....the following acid-base equilibrium operates...

`2NH_3(l)rightleftharpoonsNH_2^(-) + NH_4^(+)`

`K_"am"=[NH_2^(-)][NH_4^+]=10^(-30)`...

Ammonia is conceived to be a water-like solvent by reason of the given equilibrium. However, it is a MUCH more basic medium than is water. And thus it could support stronger nucleophiles than can water.