Arrange the following in increasing order of their reactivity towards…

The correct answer is Option (1) → A<B<D<C

\( \ce{OH^{-}} \) attacks the carbonyl carbon in a nucleophilic addition. The more positive and the less crowded the carbonyl carbon, the faster the attack. Two factors decide the order:

• Electronic: alkyl groups release electrons (+I) and reduce the positive charge on the carbonyl carbon. A phenyl group attached to C=O reduces it even more by resonance with the ring. So each group in place of H, and especially each phenyl, lowers the reactivity.

• Steric: larger groups hinder the approach of the nucleophile. Aldehydes have one group and one H; ketones have two groups.

Applying this to the four compounds:

    (C) Ethanal, \( \ce{CH3CHO} \): aldehyde, one small group, most reactive
    (D) Acetone, \( \ce{CH3COCH3} \): ketone, two methyl groups
    (B) Acetophenone, \( \ce{C6H5COCH3} \): one phenyl and one methyl
    (A) Benzophenone, \( \ce{C6H5COC6H5} \): two phenyl groups, least reactive

Shortcut: benzophenone (A) is the least reactive and ethanal (C) the most reactive. Only Option (1) starts with A and ends with C.

NCERT (2026-27 edition):
Reactivity – “Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.”

Nucleophilic Addition Reactions – “… The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance as shown below and hence it is less reactive than propanal.”