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

Options

A. `III lt II lt I lt IV`
B. `II lt I lt III lt IV`
C. `IV lt III lt II lt I`
D. `I lt II lt III lt IV`

Answer

C

Explanation

Reactivity of aldehydes and ketones are mainly governed by two factors:
(i) Inductive effect : The reactivity of the carbonyl group towards the addition reactions depends upon the magnitude of the positive charge on the carbonyl carbon atom. Hence, any substituent that increases the positive charge on the carbonyl carbon must increase its reactivity towards addition reactions. The introduction of negative group (-I effect) increases the reactivity while introduction of alkyl group (+I effect) decreases the reactivity, therefore, greater the number of alkyl groups attached to the carbonyl group hence, lower is its reactivity towards nucleophilic addition reactions.
Thus, the following decreasing order of reactivity is observed :

(ii) Steric effect : In formaldehyde, there is no alkyl group while in all other aldehydes there is one alkyl group, so here the nucleophilic attack is relatively more easy, but in ketones there are two alkyl groups attached to carbonyl group and these cause hindrance to the attacking group. This factor is also called steric hindrance (crowding). In other words, as the hindrance increases, the reactivity decreases accordingly.
Thus, the order of reactivity is : `IV lt III lt II lt I`.