JEE Main 2025 Apr 4 Shift 2, Chemistry Q5: Resonance & Mesomeric Effect

Compare each pair on the dominant electronic effect.

(A) A carbocation with an attached $-OMe$ is stabilised by lone-pair back donation from oxygen (resonance / $+M$). A $-Me$ group offers only weak hyperconjugation/$+I$. So the $OMe$-substituted cation is more stable. $✓$

(B) For the carbanion, an ortho-$N{O}_2$ pulls negative charge away by $-M$ and $-I$, stabilising it strongly. For the carbocation, an ortho-$N{O}_2$ is destabilising ($-I$ withdraws from a positive centre). Hence the first ion (carbanion with $N{O}_2$) is far more stable than the second. $✓$

(C) The first cation is a primary $-C{H}_2^{+}$ on a saturated ring. The second is the same group on a ring bearing a $C=C$, giving allylic resonance. The allylic (second) cation is more stable, so the first is not more stable. $\times$

(D) Both are tertiary cations. The second has an $OMe$ group capable of $+M$ back donation, stabilising it. So the first is less stable than the second. $\times$

$\therefore$ Only (A) and (B) have the first ion more stable.