Which reactions involve a skeletal rearrangement (hydride or methyl shift) as the main pathway?

A3,3-dimethylbut-1-ene with $HCl$
B3-methylbut-1-ene with dilute $H X_{2} SO X_{4}$
C3,3-dimethylbut-1-ene with $BH X_{3} \cdot THF$ then $H X_{2} O X_{2} / NaOH$
D3,3-dimethylbut-1-ene with $Hg (OAc) X_{2}, H X_{2} O$ then $NaBH X_{4}$

Answer: A, B

The two ionic additions go through a secondary cation next to a tertiary-forming centre, so shifts occur. Oxymercuration uses a bridged mercurinium ion, and hydroboration is concerted, so neither rearranges.

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Which reactions involve a skeletal rearrangement (hydride or methyl shift) as the main pathway?

Answer: A, B

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