Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions. (I) Ethanal, (II) Propanal, (III) Propanone, (IV) Butanone.

(IV) < (III) < (II) < (I)

Reactivity decreases as the size and number of alkyl groups around C=O bond increases. Thus, reactivity increases in the order: butanone (IV) < propanone (III) < propanal (II) < ethanal(I).