Question

The increasing order of the rate of HCN addition to compounds A - D is
A. HCHO B. CH3COCH3
C. PhCOCH3 D. PhCOPh.

(a) A < B < C < D

(b) D < B < C < A

(c) D < C < B < A

(d) C < D < B < A.

Answer 1

(c) : Addition of HCN to carbonyl compounds is a characteristic nucleophilic addition reaction of carbonyl compounds. Order of reactivity:

The lower reactivity of ketones over aldehydes is due to +I - effect of the alkyl (R) group and steric hindrance. As the size of the alkyl group increases, the reactivity of the ketones further decreases.

The aromatic aldehydes and ketones are less reactive than their aliphatic analogous. This is due to the +R - effect of the benzene ring.

PhCHO > PhCOCH3 > PhCOPh

From the above information, it is clear that increasing order of the rate of HCN addition to compounds HCHO, CH3COCH3, PhCOCH3 and PhCOPh is

PhCOPh < PhCOCH3 < CH3COCH3 < HCHO.