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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3151

1-Bromobicyclo[2.2.1]heptane (bromine at a bridgehead) is almost inert both to in acetone and to boiling aqueous ethanol, whereas reacts readily in the second medium. The best explanation is

  1. Abromide is too poor a leaving group on a secondary carbon
  2. Bthe bromide is primary and therefore never ionises
  3. Cthe SN1 reaction is possible, but the solvent is not polar enough
  4. Dbackside attack at the bridgehead carbon is blocked by the cage, and the bridgehead carbocation cannot become planar

Answer: D

SN2 needs the nucleophile to approach opposite to the leaving group, but at a bridgehead that direction points into the ring system. SN1 needs a planar (sp2) cation, which the rigid bicyclic skeleton cannot reach without prohibitive strain, so ionisation is also very slow. The tertiary tert-butyl bromide has no such restrictions and ionises readily.
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