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Reactions of Alkanes

Let us reprise our discussion of Hammond's postulate in the context of free radical halogenation. Hammond's postulate is a favorite MCAT concept. Free radical halogenation can yield a mixture of primary, secondary, and tertiary alkyl halides, but the degree of regioselectivity is greater for bromination compared to chlorination. Bromination is more selective for the more highly substituted products than chlorination. Explaining this difference in regioselectivity is the quintessential application of Hammond's postulate. Hammond's postulate reasons that if an activated complex and an intermediate derived from it (or vice versa) are similar in energy, then they are similar in structure. Because hydrogen atom removal is more endothermic in bromination than in chlorination, the energy of transition state for hydrogen atom extraction is relatively nearer to the radical intermediate in energy in bromination, as compared to the initial alkyl bromide, than is the case with chlorination, where the transition state energy is only slightly nearer to the intermediate. Hammond's postulate tells us that because the transition state in bromination is very close to the alkyl radical in energy, this transition state derives more of its character from the alkyl radical than would be the case with chlorination (the structure of the transition state in chlorination has more of a contribution from the structure of the reagents). As with carbocations, the stability of carbon radicals increases with greater substitution. Substituent stabilization can play a greater roll, to summarize, in determining the distribution of product in bromination than chlorination because the alkyl radical has more influence over the transition state in bromination.

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