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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1219

Cation I is 4-(dimethylamino)benzyl cation. Cation II is 4-(dimethylamino)-3,5-dimethylbenzyl cation (the two methyl groups are ortho to the group). Which statement is correct?

  1. AII is more stable than I, because the two extra methyl groups add +I and hyperconjugation that exceed any effect on the nitrogen
  2. BI is less stable than II, because the nitrogen lone pair acts only through its -I effect in both
  3. CI is more stable than II, because in II the ortho methyl groups twist the group out of the ring plane and its +M effect is lost (steric inhibition of resonance)
  4. DI and II have the same stability, because the +M effect of nitrogen does not depend on coplanarity with the ring

Answer: C

In I the nitrogen lone pair lies parallel to the ring p orbitals and donates strongly to the para cationic centre (). In II the bulky methyl groups next to nitrogen force the group out of the plane, so the lone pair no longer overlaps (steric inhibition of resonance). The +M stabilisation is largely lost, so I is more stable.
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