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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3144

(R)-3-Methylbutan-2-ol, , is heated with concentrated aqueous HBr. The major substitution product is

  1. Aracemic 2-bromo-3-methylbutane, formed through the secondary carbocation without rearrangement
  2. B2-bromo-2-methylbutane, an achiral compound formed after a 1,2-hydride shift
  3. C(S)-2-bromo-3-methylbutane, formed by SN2 inversion
  4. D(R)-2-bromo-3-methylbutane, formed with retention

Answer: B

A secondary alcohol in strong HBr (a poor SN2 setting, with a hindered beta branch) ionises to the secondary cation . A 1,2-hydride shift from the neighbouring tertiary C-H gives the more stable tertiary cation , which captures bromide to give 2-bromo-2-methylbutane. This has no stereocentre, so the original configuration is lost and the product is optically inactive.
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