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JEE Main levelSingle correct optionDifficulty 2 of 3o3023

1-Bromobicyclo[2.2.1]heptane (bromine on a bridgehead carbon) is almost inert to solvolysis in aqueous ethanol, although it is a tertiary bromide. The main reason is that

  1. Abromide is a poor leaving group from a bridgehead carbon
  2. Bthe bridgehead carbocation cannot become planar in the rigid bicyclic skeleton, so it is highly strained
  3. Cthe cation would be primary
  4. Dwater cannot approach the bridgehead carbon from the back side in the slow step

Answer: B

An SN1 reaction needs the cationic carbon to flatten to sp2 geometry. In the bicyclic framework the bridgehead carbon cannot do this without severe angle strain, so the cation is formed extremely slowly. The leaving group ability of bromide is unchanged and the carbon is tertiary, not primary; back-side hindrance is irrelevant to an SN1 pathway.
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