JEE Main 2026 Jan 21 Shift 2, Chemistry Q12: Resonance & Mesomeric Effect

JEE Main2026Jan 21, Shift 2Chemistry

The correct order of the rate of the reaction for the following reaction with respect to nucleophiles is:

$C H_{3} B r + N u^{⊖} ⟶ C H_{3} N u + B r^{⊖}$

A
$P h O^{-} >^{-} O H > C H_{3} C O O^{-} > C l O_{4}^{-}$
B
$C l O_{4}^{-} > C H_{3} C O O^{-} >^{-} O H > P h O^{-}$
C
$C H_{3} C O O^{-} > P h O^{-} >^{-} O H > C l O_{4}^{-}$
D
$^{-} O H > P h O^{-} > C H_{3} C O O^{-} > C l O_{4}^{-}$

Solution

This is an $S_{N} 2$ on methyl bromide, so nucleophilicity tracks (inversely) with the stability of the conjugate acid’s anion: the less stable the anion, the more reactive it is as a nucleophile.

Stability of the anion: $C l O_{4}^{-}$ (delocalised over 4 O, very stable) > $C H_{3} C O O^{-}$ (delocalised over 2 O) > $P h O^{-}$ (delocalised into the ring) > $O H^{-}$ (charge fully on one O).

Reversing for nucleophilicity: $O H^{-} > P h O^{-} > C H_{3} C O O^{-} > C l O_{4}^{-}$ .

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JEE Main 2026 Jan 21 Shift 2, Chemistry Q12: Resonance & Mesomeric Effect

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