Arrange the following carbonyl compounds in the increasing order of their reactivity in nucleophilic addition reactions :
` (i)` Ethanal, propanal, propanone, butane
(ii ) Benzaldehyde, p-tolualdehyde, p-nitrobenzaldehyde , acetophenone

Explanation

As we move from `"ethanal"to"Propanal"to"propanal"to"butanone"`, the +I-effect of the alkyl group increase. As a result, electron density on the carbon atom of the carbonyl group grogressively increases and hence attack by the nucleophili become slower and slower. thus, the reactivity increases in the reverse order, i.e., butanoneltpropanoneltpropanallt ethanal.
(ii) Acetophenone is a ketone, while all others are aldehydes, therefore, it is the least reactive. in p-tolualdehyde, there is a `CH_(3)` group at the p-position w.r.t. to the carbonyl group, which increases the electron density on the carbon of the carbonyl group by hyperconjugation effect thereby making it less reactive than benzaldehyde.
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On the other hand, in p-nitrobenzaldehyde, the `NO_(2)` group is a powerful electron-withdrawing group. It withdraws electrons, both by inductive and resonance effect thereby decreasing the electron-density on the carbon atom of the carbonyl group. this facilitates the attack of the nucleophile and hence makes it more reactive than benzaldehyde.
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Therefore, the overall ractivity increases in the order:
acetophenoneltp-tolualdehydeltbanzaldehydeltp-nitrobenzaldehyde.