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

Reactivity Comparison: Benzaldehyde vs. Propanal in Nucleophilic Addition

Benzaldehyde exhibits reduced reactivity in nucleophilic addition reactions compared to propanal. This difference in reactivity is attributed to the resonance stabilization imparted by the benzene ring attached to benzaldehyde's carbonyl group.

The carbonyl carbon in benzaldehyde is rendered less electrophilic due to resonance interaction with the aromatic ring:

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

This electron delocalization diminishes the partial positive charge on the carbonyl carbon, thereby decreasing its susceptibility to nucleophilic attack.

Conversely, propanal (\( \text{CH}_3\text{CH}_2\text{CHO} \)) lacks this resonance stabilization. The carbonyl group's electron-withdrawing effect is unimpeded, maintaining a highly electrophilic carbonyl carbon that readily participates in nucleophilic addition.

Summary:

Benzaldehyde: Reactivity is diminished by resonance with the benzene ring.
Propanal: Exhibits higher reactivity due to the absence of resonance stabilization.