Which of the following reagents convert the alkene shown mainly into a product that arises through a carbocation rearrangement (hydride shift)?

A $HBr$ (no peroxide)
B $H X_{3} O X^{+}$
C(i) $Hg (OAc) X_{2}$ , $H X_{2} O$ ; (ii) $NaBH X_{4}$
D(i) $BH X_{3}$ in THF; (ii) $H X_{2} O X_{2}$ , $NaOH$

Answer: A, B

The alkene is 3-methylbut-1-ene.

HBr

and

H X_{3} O X^{+}

both generate a free secondary carbocation, which undergoes a hydride shift to the tertiary cation (2-bromo-2-methylbutane, 2-methylbutan-2-ol). Oxymercuration-demercuration (3-methylbutan-2-ol) and hydroboration-oxidation (3-methylbutan-1-ol) do not involve free carbocations and give no rearrangement.

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Which of the following reagents convert the alkene shown mainly into a product that arises through a carbocation rearrangement (hydride shift)?

Answer: A, B

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