Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions.
(i) Ethanal, Propanal, Propanone, Butanone.
(ii) Benzaldehyde, p-Tolualdehyde, p-Nitrobenzaldehyde, Acetophenone.
Hint: Consider steric effect and electronic effect.

Explanation

(a) The increasing order of reactivity of the carbonyl compounds towards nucleophilic addition reactions is :
`"butanone" lt "propanone" lt "propanal" lt "ethanal"`
The reactivity is based upon two factors. These are : steric factors and electronic factors. For details, consult section 13.8.
(b) The increasing order of reactivity is :
`"acetophenone" lt "p-tolualdehyde" lt "benzaldehyde" lt "p-nitrobenzaldehyde"`
Explanation : Acetophenone being a ketone is the least reactive towards nucleophilic addition. All other are aldeydes. Among them, p-tolualdehyde is less reactive than benzaldehyde because `CH_(3)` group present at the para position w.r.t.-CHO group will increase the electron density on the carbonyl carbon atom due to hyper conjugation effect. As a result, the nucleophile attack occurs to lesser extent as compared to benzaldehde.

In p-nitrobenzaldehyde, the nitro group has an opposing effect. It is electron withdrawing in nature due to -I effect as well as-R effect. The electron density on the carbonyl carbon atom decreases and this favours the nucleophile attack.