Question

Arrange the following in the order of their increasing reactivity towards\[HCN\]:
\[C{H_3}CHO,C{H_3}COC{H_3},HCHO,{C_2}{H_5}COC{H_3}\]

Answer 1

Hint: The given compounds are all carbonyl compounds that have a double bond between carbon and oxygen. Thus, this makes it susceptible to nucleophilic attack at the carbon because the oxygen attached to carbon withdraws the electron density towards itself making carbon electron deficient in nature thus a nucleophile can attack it easily.

Complete step-by-step answer:Addition of \[HCN\] is a nucleophilic addition reaction in which the anionic part of \[HCN\] that is cyanide ion attacks the carbonyl compounds which are electron deficient in nature.
Thus, more the electron deficiency of carbonyl compound more will be the reactivity of \[HCN\]. Thus, groups attached to carbonyl compounds that are electron withdrawing in nature favours the attack of nucleophile. Ketones are less reactive towards the nucleophilic attack. This is because the alkyl groups attached to ketones in addition to the aldehydes are electron releasing in nature and this decreases the electron deficiency of the carbonyl compounds.

Formaldehyde having formula $HCHO$ shows highest nucleophilic addition reaction followed by acetaldehyde having chemical formula\[C{H_3}CHO\] then \[C{H_3}COC{H_3}\].
Ethyl group is more electron releasing in nature thus making\[{C_2}{H_5}COC{H_3}\] least susceptible to the attack of cyanide ion.
Thus, using the above explanations, the correct order of their increasing reactivity towards\[HCN\]is \[{C_2}{H_5}COC{H_3} < C{H_3}COC{H_3} < C{H_3}CHO < HCHO\]

Note: Reactivity of carbonyl compound toward nucleophilic addition further increase in the presence of electron withdrawing groups. Due to this reason, the acid chloride has reactivity towards nucleophilic addition reaction.