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Reactions with Radical Intermediates

Reactions of Alkenes

Addition of HX to alkenes will occur by a free radical mechanism in the presence of peroxides. Unlike the electrophilic addition of HX to alkenes, the free radical mechanism occurs with anti-Markovnikov regiospecificity. Be sure before you answer a question on the MCAT whether the mechanism is electrophilic addition or free radical addition!

In the free radical mechanism, as opposed to the electrophilic addition mechanism, the first alteration of the alkene occurs by heterolytic addition of halogen atom to yield the halo alkyl radical. If this free radical addition is occurring upon an asymmetrically substituted alkene, the more substituted radical will be favored to form. This means that halogen will have added to the less substituted of the two carbons, leading ultimately to the anti-Markovnikov product.

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