NH2- + H2O < = > NH3+ + OH-
Weak base + Weak acid < = > Strong acid + Strong base
The above equilibrium shift from right to left.
Therefore, OH- is a strong base than NH2-
then OH- is poor leaving group, nucleophilic substituation is impossible!
Only when carbonyl compound is protonated, H2O is better leaving group and substitution will occur in higher possiblility theoretically!