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JEE Advanced levelSingle correct optionDifficulty 3 of 3o2034

4-Methylpent-1-ene, , reacts with HBr in the dark. Which is the major product?

  1. A1-bromo-4-methylpentane
  2. B2-bromo-4-methylpentane
  3. C2-bromo-2-methylpentane
  4. D3-bromo-2-methylpentane

Answer: B

Protonation gives the secondary cation at C-2. A 1,2-hydride shift from C-3 would only give another secondary cation (which would lead to 3-bromo-2-methylpentane), so no stabilising shift occurs; the tertiary C-4 is a 1,3 relationship away and a 1,3-shift does not take place. The cation is trapped directly, giving 2-bromo-4-methylpentane.
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