question_answer

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

A)  III < II < I < IV   

B)  II < I < III < IV

C)  IV < III < II < I    

D)  I < II < III < IV

Correct Answer: C

Solution :

                 Carbonyl group is polar   because of  the presence of partial positive charge over carbon and partial negative charge at oxygen. Presence of electron releasing substituent increases the electron density at carbon, thus, reduces the magnitude of charge and hence, the polar nature. Thus, it can be said that as the number of alkyl group or size of alkyl group increases, reactivity decreases. Thus, the order of reactivity is \[\underset{(IV)}{\mathop{Butanone}}\,<\underset{(III)}{\mathop{propanone}}\,<\underset{(II)}{\mathop{propanal}}\,<\underset{(I)}{\mathop{ethanal}}\,\] (Remember ! aldehydes are more reactive as compared to ketones.)