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JEE Advanced levelSingle correct optionDifficulty 3 of 3g3161

Bond energies in $kcal mo l^{- 1}$ : primary C-H 101, tertiary C-H 96, H-Cl 103, H-Br 88. Of the following hydrogen abstractions, the transition state expected to be the latest (most product-like) is that for

A $Cl X^{∙}$ abstracting a primary hydrogen
B $Br X^{∙}$ abstracting a tertiary hydrogen
C $Br X^{∙}$ abstracting a primary hydrogen
D $Cl X^{∙}$ abstracting a tertiary hydrogen

Answer: C

Δ H

= C-H energy - H-X energy.

Br X^{∙}

+ primary:

101 - 88 = + 13

.

Br X^{∙}

+ tertiary:

96 - 88 = + 8

.

Cl X^{∙}

+ primary:

101 - 103 = - 2

.

Cl X^{∙}

+ tertiary:

96 - 103 = - 7

. The most endothermic step (+13) has the latest transition state.

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