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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1147

The 3-methylbutan-2-yl cation, $(CH X_{3}) X_{2} CH - CH X^{+} - CH X_{3}$ , rearranges by a 1,2-hydride shift to a more stable carbocation. The number of alpha C-H bonds changes from

A4 in the initial cation to 9 in the rearranged cation
B4 in the initial cation to 8 in the rearranged cation
C3 in the initial cation to 8 in the rearranged cation
D4 in the initial cation to 6 in the rearranged cation

Answer: B

Initial cation: alpha carbons are

CH X_{3}

(3 H) and

CH (CH X_{3}) X_{2}

(1 H), total 4. The hydride from the methine carbon moves to the cationic carbon, giving

(CH X_{3}) X_{2} C X^{+} - CH X_{2} CH X_{3}

(tertiary). Its alpha H count is

2 \times 3 + 2 = 8

.

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