Radicals, anti-Markovnikov concentration

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

Although HI addition to alkenes and alkynes is faster than that of the other hydrohalides and free radical anti-Maikovnikov additions are not a problem, this reaction has received less attention than the others.173 The hydroiodination of alkenes is most commonly run using concentrated HI in water or acetic acid at or below room temperature. While the early literature suggests that simple terminal alkenes afford small amounts of anti-Markovnikov products, only Markovnikov products have been reported in the more recent literature (equations 125-129).67 176-179... 

Numerous attempts to obtain the Markovnikov adduct by varying the reaction conditions, including its realization in concentrated HC1, had failed. Moreover, in a competitive reaction of a mixture of 1-heptene and styrene only the anti-Markovnikov adducts were formed for both olefins and, surprisingly, 1-heptene was found to be more reactive than styrene. This is also in agreement with the concept of two mechanisms. Here, 1-heptene assists in the formation of GeCl3 radicals and styrene acts as a radical trap, forming selectively only the anti-Markovnikov adduct. 

---