Which carbocations can rearrange by a 1,2-shift to a more stable carbocation?

A $CH X_{3} - CH X^{+} - CH X_{2} CH X_{3}$
B $(CH X_{3}) X_{3} C - CH X^{+} - CH X_{3}$
C $(CH X_{3}) X_{3} C X^{+}$
D $(CH X_{3}) X_{2} CH - CH X^{+} - CH X_{3}$

Answer: B, D

The first cation can shift a methyl, and the second a hydride, to give a tertiary cation. The tert-butyl cation is already tertiary. The sec-butyl cation could only shift to another secondary ion, or to a primary one, so there is no gain.

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Which carbocations can rearrange by a 1,2-shift to a more stable carbocation?

Answer: B, D

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