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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g3051

Select the transformation(s) in which a 1,2-shift to a more stable cation is energetically favourable.

A2,2-dimethylpropyl cation to 2-methylbutan-2-yl cation (methyl shift)
B2-methylpropan-2-yl cation to 2-methylpropyl cation (hydride shift)
C3-methylbutan-2-yl cation to 2-methylbutan-2-yl cation (hydride shift)
D1-phenylpropan-2-yl cation to 1-phenylpropyl cation (hydride shift)

Answer: A, C, D

The first goes

1^{\circ} \to 3^{\circ}

, the second

2^{\circ} \to 3^{\circ}

, and the fourth

2^{\circ} \to

benzylic

2^{\circ}

(resonance stabilised). The third converts a tertiary cation into a primary one, which is strongly unfavourable.

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