An MCAT favorite, Hammond's postulate provides the conceptual basis to analyze the structure of transition states based on the structures of reagents and products (or intermediates in multistep reactions). The postulate stipulates that transition state structure will resemble the form to which it is most close in energy. The increased regioselectivity in halogenation with bromination over chlorination is the example most of us learn in 1st year organic chemistry of the interpretive power of Hammond's postulate.

The first step in free radical halogenation is hydrogen removal, producing an alkyl radical intermediate. Because hydrogen atom removal in bromination is more endothermic than with chlorination, Hammond's postulate tells us that the transition state with bromination will have more alkyl radical character. The reason for this is that the more endothermic pathway places the transition state closer in energy to the alkyl radical intermediate. Hammond's postulate takes the proximity in energy and makes the connection to state that the transition state in bromination must have greater alkyl radical character.

For the transition state in bromination to have more alkyl radical character means that substitution effects that stabilize alkyl radicals will play more of a role in determining regioselectivity in bromination compared to chlorination. Because greater substitution decreases the energy of alkyl radicals, and because the transition state in bromination has more radical character than with chlorination, bromination will be more selective. A greater portion of the product leads to tertiary alkyl halides with bromination.

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