Free Radical Halogenation
Because of the relative stability of alkyl radical intermediates, selectivity in free radical halogenation favors tertiary over secondary over primary carbon radicals. Bromination, though, is more selective than chlorination, because the proton extraction step is more endothermic in bromination than chlorination. This follows from Hammondís postulate, which governs the correlation between proximity in energy and proximity in structure among transition states and intermediates. Halogenation is the classic illustration of Hammondís postulate. Because the activated complex prior to formation of the alkyl radical intermediate must have more radical character for bromination compared to chlorination, the effect of substitution in stabilizing radicals plays a greater roll with bromination leading to a higher degree of regioselectivity.