Question

Arrange the following compounds in an increasing order of the reactivity in nucleophilic addition reactions:

ethanol, propanal, propanone, butanone.

Answer 1

An increasing order of the reactivity in nucleophilic addition reactions: $Butanone<Propanone<Propanal<Ethanal$

For maximum reactivity towards nucleophiles, the carbonyl carbon atom should be positive as possible and not sterically hindered by adjacent groups. The ketones have two alkyl groups on carbonyl carbon while aldehydes have one alkyl group only. Each alkyl group being electron-releasing decreases the positivity of carbon atom and on account of its bulk offers steric hindrance to approaching reagents. For both these reasons, the nucleophilic attack on the carbonyl carbon of ketone is retarded.