Polarity of carbonyl group in benzaldehyde is reduced due to resonance,, making it less reactive in nucleophilic addition reactions. but there is no such resonance effect in propanal and therefore, the polarity of carbonyl group in propanal is more than in benzaldehyde. So, propanal is more reactive than benzaldehyde.
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Manu Ramachandran answered this
gud answer dmanavi. A bit more briefly, resonance effect of benzaldehyde due to its sp2 hybrid character makes it almost unreactive to nucleophilic addition reactions.
