Benzaldehyde is more reactive towards nucleophilic addition reactions than acetone. This is because benzaldehyde has a polar carbonyl group (-C=O) attached to an electron-withdrawing group (-Ph), which makes the carbon atom of the carbonyl group more electrophilic and prone to nucleophilic attack. In contrast, acetone has a polar carbonyl group, but the carbon atom is attached to two electron-donating methyl groups (-CH3), which reduce the electrophilicity of the carbon atom and make it less reactive towards nucleophilic attack.
