Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions.
Benzaldehyde , p-Toualdehyde, p-Nitrobenzaldehyde , Acetophenone.

Explanation

Order of reactivity: acetophenone `lt` p-tolualdehyde `lt` benzaldehyde `lt` p-nitrobenzaldehyde. Due to hyperconjugation, methyl group in p-tolualdehyde is electron releasing because of which the positive charge on carbonyl carbon decreases. Due to resonance effect (electron withdrawing effect of nitro group) in p-nitrobenzaldehyde, the positive charge on carbonyl carbon increases. Greater the positive charge on carbonyl carbon, more is the reactivity. Further, it may be noted that ketones are less reactive than aldehydes.