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

Ethanal, Propanal, Propanone, Butanone.

Hint: Consider steric effect and electronic effect.

The +I effect increases in the order as:

Ethanal<Propanal<Propanone<Butanone.

Therefore the electron density of the carbonyl carbon increases as the +I effect due to the alkyl group increases. As a result, the chances of attack by a nucleophile (which are rich in electron and carries a negative charge, not necessary usually have a lone pair of an electron) decreases because the carbonyl carbon has electron density on it. Hence the increasing order of the reactivities of given compounds in nucleophilic addition reaction is:

Ethanal > Propanal> Propanone > Butanone.