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Answer:

The correct answer is due to its reactivity with alkanes.

Explanation:

The reactivity of the different halogens varies considerably. The relative reactivity rate of bromine is 7 * 10^-11. Due to this bromine behavior, the reaction of brominated alkanes is slow and requires high levels of UV irradiation, therefore it is not possible to use pure bromine in free radical reactions with alkanes.

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Pure bromine can not be used in free-radical reactions with alkenes because it favors the ionic mechanism rather than the free radical mechanism.

Alkenes react with bromine molecule to form addition products. Pure bromine refers to bromine molecule(Br2). Addition is one of the reactions that are characteristic of alkenes owing to the presence of the double bond in the molecule.

If we intend to carry out free- radical bromination of alkenes, we do not use pure bromine because pure bromine molecule favors the ionic mechanism in which a cyclic brominium ion is formed rather than taking the free radical pathway. NBS is a commonly used reagent in the bromination of alkenes.

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