`NH_3` and its derivative do not show nucleophillic addition reactions with aldehydes and ketone in high acidic medium. Justify.

Explanation

Reaction of aldehydes and ketones with ammonia derivatives involves following steps.

As shown, above the protonations of carbonyl oxygen makes the the carbonyl carbon more susceptible to nucleophilic attack by ammonia derivative. Thus we ca expect that the reaction will be favoured be highly acidic medium i.e., lower pH. However, the medium should not be highly acidic. This is because if the medium is highly acidic, then the ammonia derivatives being basic, will form their respectively salts
`overset(..)(N)H_(2) - Z + H^(+) rarr overset(+)(N)H_(3) - Z`
These salts of ammonia derivatives cannot act as nucleophiles. It is due to the absence of lone pair and the charge they carry. Therefore at very low pH (i.e. highly acidic) no reaction takes place. Thus to carry out these reactions, we have to control the acidic strength i.e., the pH of the reaction medium. The optimum pH for the reaction medium is 3-4.