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

Why Benzaldehyde is Less Reactive than Propanal in Nucleophilic Addition Reactions

Benzaldehyde is less reactive in nucleophilic addition reactions than propanal due to the presence of a resonance-stabilized benzene ring attached to its carbonyl group. 

In benzaldehyde, the carbonyl carbon is less electrophilic because of resonance interaction between the carbonyl group and the aromatic ring:

\[ \text{Ph-CHO} \leftrightarrow \text{Ph-C}^{\delta+}\text{=O}^{\delta-} \]

This delocalization reduces the partial positive charge on the carbonyl carbon, making it less susceptible to nucleophilic attack.

In contrast, propanal (\( \text{CH}_3\text{CH}_2\text{CHO} \)) has no such resonance stabilization. The electron-withdrawing nature of the carbonyl group is unopposed, so the carbonyl carbon remains strongly electrophilic and readily undergoes nucleophilic addition.

Conclusion:

Benzaldehyde: Reactivity reduced due to resonance with benzene ring.
Propanal: More reactive as no resonance stabilization exists.