Alkenes, radical halogenation reaction with peroxides

Free-radical reactions of tervalent phosphorus acids have been covered to some extent in Section II (addition of PX3 to alkenes, equation 15-17) and in Section IV (oxidations with molecular oxygen). Several other reactions occur via radicals, e.g. reactions with peroxides, certain disulphides and certain halogen compounds. However, these reactions are the subject of chapters by Bentrude and Danko wski in Vol. 1 of this series and will not be covered here. 

Even with HBr addition the reaction chains tend to be rather short— much shorter than those in halogen addition—and more than a trace of peroxide is thus needed to provide sufficient initiator radicals for preparative purposes up to 0 01 mol peroxide per mol of alkene. In practical terms, however, there may already be sufficient peroxide... 

The reaction of an alkene with HBr and peroxide gives the bromide product with the bromine on the less substituted carbon in an anti-Markovnikov addition reaction. This reaction work wells with HBr, but not with HCl or HI because the bromine radical reacts in a selective manner. Differences in the reactivity of halogen radicals are addressed in Chapter 11, Section 11.9. 

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