Hint:  Acyl group is more bulky than alkyl group

(A) Acylation of amines gives a monosubstituted product whereas alkylation of amines gives a polysubstituted product.
This is true.

Reason:

• In alkylation, after one alkyl group enters, the amine formed is usually still reactive, so further alkylation can continue.
• Therefore, alkylation often gives a mixture of 1°, 2°, 3° amines and even quaternary salts.
• In acylation, after one acyl group enters, an amide is formed, and amides are much less reactive toward further substitution on nitrogen.

Hence, assertion (A) is true.

(R) Acyl group sterically hinders the approach of further acyl groups.
This statement is generally taken as true at a basic level, but it is not the main reason.

The real main reason is:

• After acylation, the product is an amide
• In amide, the lone pair on nitrogen gets delocalized by resonance with the carbonyl group
• So nitrogen becomes much less nucleophilic
• Therefore, it does not undergo further acylation easily

Hence, reason (R) is true, but it is not the correct explanation of (A).

 

Acylation of amine gives a monosubstituted product whereas alkylation of amino gives polysubstituted product because acylation in amino takes place at N-atom and alkylation takes place at o and p position.