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JEE Advanced levelSingle correct optionDifficulty 3 of 3o2044

Methylenecyclobutane is treated with HBr in the dark. The major product is

  1. A1-bromo-2-methylcyclobutane
  2. Bbromocyclopentane
  3. C1-bromo-1-methylcyclobutane
  4. D(bromomethyl)cyclobutane

Answer: C

Protonation of the exocyclic gives the tertiary 1-methylcyclobutyl cation directly. It is already tertiary, so no shift or ring expansion is needed, and bromide captures it. The product is 1-bromo-1-methylcyclobutane.
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