Question

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

Hint:

Resonance effects in aromatic aldehydes like benzaldehyde reduce the electrophilicity of the carbonyl carbon, making them less reactive in nucleophilic addition reactions.

Answer 1

Benzaldehyde would be less reactive in nucleophilic addition reactions compared to propanal. This is because in benzaldehyde, the carbonyl group is attached to a benzene ring, which provides some resonance stabilization. The lone pairs of electrons on the oxygen of the carbonyl group can delocalize into the benzene ring, reducing the partial positive charge on the carbonyl carbon. This makes the carbonyl carbon less electrophilic and less reactive to nucleophilic attack. On the other hand, in propanal, the carbonyl group is attached to a simple alkyl chain, and there is no resonance effect to stabilize the carbonyl carbon. Therefore, the carbonyl carbon in propanal is more electrophilic and more reactive to nucleophilic addition.