Question

Which of the following is most reactive towards the addition reaction of hydrogen cyanide to form corresponding cyanohydrin?
(A) Acetone
(B) Formaldehyde
(C) Acetaldehyde
(D) Diethyl Ketone

Answer 1

To check the reactivity of the given compounds you need to check for the reactivity of the carbonyl group to decide which compound will react with the $C{N^ - }$ nucleophile to form Cyanohydrin rapidly. The compound whose electrophilic character will be the highest among the others will be the most reactive towards the addition reaction of hydrogen cyanide.

Complete step by step answer:
has a presence of two alkyl groups in its structure. So this alkyl group will exhibit $+ I$ (inductive) effect and will decrease the electrophilic character of the carbonyl C-atom. Lower the electrophilic character, lower will be the reactivity of the carbonyl C-atom towards Addition reaction.
Formaldehyde has no alkyl group present in its structure therefore, the carbonyl C-atom will react with the $C{N^ - }$ nucleophile to form Cyanohydrin rapidly.
Acetaldehyde has a presence of one alkyl group in its structure. So this alkyl group will exhibit $+ I$ (inductive) effect and will decrease the electrophilic character of the carbonyl C-atom. As the electrophilic character will decrease, lower will be the reactivity of the carbonyl C-atom towards Addition reaction.
Diethyl Ketone also has two alkyl groups present which will exhibit $+ I$ (inductive) effect and will decrease the electrophilic character of the carbonyl C-atom which will further lower the electrophilic character and the reactivity of carbonyl C-atom towards addition reaction.
Hence, out of all the given compounds formaldehyde will have the highest rate of reactivity towards the addition reaction of hydrogen cyanide.

Therefore, Option (B) is Correct.

Note:
The reaction of Hydrogen cyanide with formaldehyde to form cyanohydrin is a classic example of base catalysed nucleophilic addition reaction. In the first step, the cyanide ion acts as a nucleophile and forms a bond with the electrophilic carbonyl carbon and in the second step, the alkoxide ion is protonated by HCN which regenerates the cyanide ion.