In the following compounds, what is the increasing order of their reactivity towards nucleophilic addition reactions ?
Benzaldehyde, p-Tolualdehyde, p-Nitrobenzaldehyde, Acetophenone
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde
In nucleophilic addition, the nucleophile attacks the electrophilic carbonyl carbon. Two factors decide the reactivity:
Aldehydes are more reactive than ketones: a ketone has two groups on the carbonyl carbon, which crowd the nucleophile (steric reason) and reduce the positive charge on the carbon more (electronic reason).
Ring substituents: an electron withdrawing group (–\( \ce{NO2} \)) makes the carbonyl carbon more positive and the compound more reactive; an electron donating group (–\( \ce{CH3} \)) makes it less positive and less reactive.
With the given compounds:
Acetophenone, \( \ce{C6H5COCH3} \): a ketone, least reactive: first
p-Tolualdehyde, \( \ce{CH3C6H4CHO} \): an aldehyde with an electron donating –\( \ce{CH3} \) group: second
Benzaldehyde, \( \ce{C6H5CHO} \): an aldehyde with no substituent: third
p-Nitrobenzaldehyde, \( \ce{O2NC6H4CHO} \): an aldehyde with an electron withdrawing –\( \ce{NO2} \) group, most reactive: last
Shortcut: The ketone (acetophenone) must come first and p-nitrobenzaldehyde last. Options (2) and (3) do that, and only Option (3) puts p-tolualdehyde (deactivated by –\( \ce{CH3} \)) before benzaldehyde.
NCERT (2026-27 edition):
Reactivity
