The Addition of Radicals to an Alkene

The addition of HBr to 1-butene forms 2-bromobutane, because the electrophile (H ) adds to the sp carbon bonded to the most hydrogens (Section 6.4). If, however, you want to synthesize 1-bromobutane, then you need to find a way to make bromine an electrophile so it, instead of H, will add to the sp carbon bonded to the most hydrogens. 

The electrophile adds to the carbon that is bonded to the most hydrogens. 

If a peroxide (ROOR) is added to the reaction mixture, the product of the addition reaction will be the desired 1-bromobutane. Thus, the peroxide changes the mechanism of the reaction in a way that causes a bromine radical to be the electrophile. 

The following mechanism for the addition of HBr to an alkene in the presence of a peroxide shows that it is a radical chain reaction with characteristic initiation, propagation, and termination steps  

The weak O—O bond of the peroxide readily breaks homolytically in the presence of light or heat to form alkoxy radicals. This is an initiahon step because it creates radicals. 

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