Mechanism for the monochlorination of methane

The reaction of a chlorine radical with an alkane to form a primary, secondary, or tertiary radical is exothermic, the transition states resemble the reactants more than the products. The reactants all have approximately the same energy, so there will be only a small difference in the activation energies for removal of a hydrogen atom from a primary, secondary, or tertiary carbon. In contrast, the reaction of a bromine radical with an alkane is endothermic, so the transition states resemble the products more than the reactants. In other words, because a bromine radical is relatively unreactive, it is very selective about which hydrogen atom it abstracts. In contrast, the much more reactive chlorine radical is much less selective. Reactivity-selectivity principle which states that the greater the reactivity of a species, the less selective it will be.