Assertion (A): Free radical bromination of alkanes is much more selective than chlorination. Reason (R): The hydrogen abstraction by $Br X^{∙}$ is endothermic, so its transition state is late and resembles the alkyl radical.

ABoth A and R are true but R is not the correct explanation of A.
BA is true but R is false.
CA is false but R is true.
DBoth A and R are true and R is the correct explanation of A.

Answer: D

Bromination is indeed far more selective (tertiary : primary of about 1600 : 1 against about 5 : 1 for chlorination). Abstraction by

Br X^{∙}

is endothermic, so by the Hammond postulate the transition state is late and carries much radical character. The difference in radical stability is then felt strongly, and R correctly explains A.

Practice General Organic Chemistry III: Reaction Intermediates More on Hammond postulate and reactivity-selectivity

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Assertion (A): Free radical bromination of alkanes is much more selective than chlorination. Reason (R): The hydrogen abstraction by BrX∙ is endothermic, so its transition state is late and resembles the alkyl radical.

Answer: D

More questions on Hammond postulate and reactivity-selectivity

Assertion (A): Free radical bromination of alkanes is much more selective than chlorination. Reason (R): The hydrogen abstraction by $Br X^{∙}$ is endothermic, so its transition state is late and resembles the alkyl radical.