Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer.

Explanation

Due to weak +-effect of the ethyl group, the positive charge on the carbon atom of the carbonyl group in propanal is reduced to small extent. In contrast, in benzaldehyde, the positive charge on the carbon atom of the carbonyl group is reduced to a greater extent due to stronger +R-effect of the benzene ring. As a result, the carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group in propanal and hence benzaldehyde is less reactive than propanal in nucleophilic addition reactions.