Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Justify your answer.
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.
Benzaldehyde: Reactivity reduced due to resonance with benzene ring.
Propanal: More reactive as no resonance stabilization exists.
Explain with the help of chemical reactions: (I) Acetone is treated with semicarbazide.