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

1. Nucleophilic Addition Reactivity: Benzaldehyde vs. Propanal
Propanal exhibits greater reactivity in nucleophilic addition reactions than benzaldehyde.

Rationale:
The carbonyl carbon's electrophilicity dictates aldehyde reactivity in nucleophilic addition. Electron-donating or withdrawing substituents influence this electrophilicity.

Detailed Analysis:
Benzaldehyde features a phenyl group attached to its carbonyl carbon. This phenyl group acts as an electron-withdrawing substituent via resonance, reducing electron density on the carbonyl carbon. This diminished electrophilicity makes benzaldehyde less prone to nucleophilic attack, resulting in lower reactivity compared to propanal.

Conversely, propanal, an aliphatic aldehyde, lacks such an electron-withdrawing group. Its carbonyl carbon is therefore more electrophilic and readily undergoes nucleophilic addition.

Summary:
The electron-withdrawing resonance effect of the phenyl group in benzaldehyde renders it less reactive than propanal in nucleophilic addition reactions.