Question

Arrange the following compounds in increasing order of their reactivity in nucleophilic addition.

(i) Ethanal, Propanal, Propanone, Butanone,

(ii) $Benzaladehyde,p-Tolualdehyde,p-Ntrobenzaldehyde,Acetophenone.$

Answer 1

(i) The $+I$ effect of the alkyl group increases in the order

Ethanal $<$ Propanal $<$ Propanone $<$ Butanone

The electron density at the carbonyl carbon increases with the increase in the $+I$ effect. As a result, the chances of attack by a nucleophile decrease. Hence the increasing order of the reactivities of the given carbonyl compounds in nucleophilic addition reaction is:

(Refer to Image)

(ii) The $+I$ effect is more in ketone than in aldehyde. Hence, acetophenone is the least reactive in nucleophilic addition reactions. Among aldehydes, the $+I$ effect is the highest in $p-tolualdehyde$ because of the presence of electron donating $-C{H}_3$ group & the lowest in $p-nitrobenzaldehyde$ because of the presence of the electron withdrawing $-N{O}_2$ group. Hence, the increasing order of the reactivities of the given compound is

Acetophenone $<$ p-tolualdehyde $<$ Benzaldehyde $<$ p-nitrobenzaldehyde.