Alkanes and radicals 4: Additionreactions of radicals

Alkanes and radicals

Addition of radicals on an alkene

Look at the two reactions below, the first reaction is in normal circumstances, which will result in a normal Markovnikov reaction (with the addition of a protic acid HX to an alkene, the acid hydrogen (H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents). In the second reaction, peroxide is used as condition, this will result in an anti-Markovnikov reaction. The peroxide will generate Br radicals in small amounts (this is the initiation for this anti-reaction).

Let's see the peroxide mechanism in detail. As stated above, an alkyl peroxide is a radical initiator. The electrophile will add on the sp2 carbon which carries the most hydrogens (where also the radical with most alkyl groups will be formed). The termination reactions will have several other outcomes. In the major outcome, the radical will not be rearranged (which oftens occurs however with carbocations = 'carbocation shift').

Although, the radical addition on an alkene in combination with the peroxide effect can only happen with HBr, why? Because both propagation steps from the HBr process are exotherm, in contrary to HCl & HI. With these compounds, the normal electrophile addition occurs via the carbocation intermediate.

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