Arrange the following compounds in an increasing order of their reactivity in nucleophilic addition reactions: ethanol, propanal, propanone, butanone.

Positive inductive effect (+I groups) increases the electron density on carbonyl carbon which leads to the generation of carbocation which favours nucleophilic addition (so more the +I group more will be the ability of formation of carbocation so more is the ease to under go nucleophilic addition reaction).So, from ethanal to butanone, +I effect increase due to bulky carbon chain so the order of nucleophilicity is Ethanal > propanal > propanone > Butanone.