Question

Which is more reactive, Benzaldehyde or propanal?

Answer 1

Hint: Let us first observe the structures of both benzaldehyde and propanal. Benzaldehyde has an aromatic group in the molecule whereas propanal only has an aliphatic carbon chain.
Benzaldehyde has the following structure:

Propanal has the following structure:

Complete answer:
The reactivity of Propanal is more than Benzaldehyde.
Reactivity of the compound that has an aldehyde group depends on the electrophilicity/the polarity of the carbonyl group. The carbonyl carbon should have partial positive charge and the oxygen should have a partial negative charge. Depending on the polarisation we can define the reactivity of the molecule. The better extent of polarisation means that the molecule is better reactive.
In the case of benzaldehyde, since the carbonyl carbon is attached to a benzene ring there is a reduction in the polarity. This is due to the fact that benzene undergoes resonance and the positive charge will be spread throughout the molecule. This makes it less reactive to nucleophilic reactions.

In the case of propanal there is no such resonance effect that comes into play. Due to this the polarity in the carbonyl group is usually unaffected. Thus the carbonyl carbon of benzaldehyde is less electrophilic than carbonyl carbon present in propanal.
This is the reason why Propanal is more reactive than Benzaldehyde.

Note:
Benzaldehyde is less reactive than propanal. This can also be interpreted as Benzaldehyde is more stable than propanal. All compounds that have an aromatic character are always more stable than the groups having aliphatic character. This is because resonance always stabilises a molecule.