Question

how bromination is more selective

Answer 1

This can be explained by Hammond's postulate which states that the transition state of a reaction resembles either the reactants or the products, to whichever it is closer in energy.

In an exothermic reaction, the transition state is closer to the reactants than to the products in energy. On the other hand endothermic reactions have product like transition state.

Both chlorination and bromination follows radical mechanism but abstraction of a hydrogen atom by Br⋅ is endothermic while abstraction of a hydrogen atom by Cl⋅ is exothermic.

The transition states for the endothermic bromination have a larger energy difference than those for exothermic chlorination, even though the energy difference of the products is the same in both reactants.

Answer 2

A chain reaction mechanism for the chlorination of methane has been described. Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.




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