The greater selectivity of bromination compared with chlorination of alkanes is best explained by the fact that

Ahydrogen abstraction by $Br \cdot$ is highly exothermic
Bhydrogen abstraction by $Br \cdot$ is endothermic, with a late transition state resembling the alkyl radical
Cthe Br-Br bond is stronger than the Cl-Cl bond
D $Br \cdot$ is more reactive than $Cl \cdot$

Answer: B

Br \cdot

abstracts H endothermically, so by the Hammond postulate the transition state is late and carries much radical character. Differences in radical stability (

3^{\circ} > 2^{\circ} > 1^{\circ}

) are then strongly reflected in the rates. Abstraction by

Cl \cdot

is nearly thermoneutral or exothermic with an early transition state, so it is less selective.

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The greater selectivity of bromination compared with chlorination of alkanes is best explained by the fact that

Answer: B

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