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. Reactivity of Benzaldehyde vs. Propanal in Nucleophilic Addition Reactions:
Benzaldehyde would be less reactive in nucleophilic addition reactions than propanal.

Reason:
The reactivity of an aldehyde in nucleophilic addition reactions depends on the electron density on the carbonyl carbon. The carbonyl carbon is electrophilic and attracts nucleophiles for the addition reaction. The electron-donating or electron-withdrawing effects of substituents on the carbonyl group affect the electrophilicity of the carbonyl carbon.

Explanation:
In the case of benzaldehyde, the carbonyl group is attached to a phenyl group (C₆H₅), which is an electron-withdrawing group through resonance. This resonance effect decreases the electron density on the carbonyl carbon, making it slightly less electrophilic and less susceptible to attack by nucleophiles. As a result, benzaldehyde is less reactive in nucleophilic addition reactions compared to propanal.

On the other hand, propanal (an aliphatic aldehyde) does not have such an electron-withdrawing group attached to the carbonyl carbon. This makes the carbonyl carbon in propanal more electrophilic, and thus more reactive towards nucleophiles.

Final Conclusion:
Benzaldehyde is less reactive than propanal in nucleophilic addition reactions due to the electron-withdrawing resonance effect of the phenyl group attached to the carbonyl carbon.