Radical Additions and Substitutions with Alkenes
Radical Addition of Hydrogen Bromide to Alkenes
The addition of HBr to alkenes proved to be a perplexing issue to chemists for quite a while. The result of this reaction varied from experiment to experiment, as it could not be controlled. For instance, the reaction between HBr and 1-butene that has just been distilled usually yields the Markovnikov product 2-bromobutane. Astonishingly, if 1-butene is exposed to air prior to the experiment, the addition of HBr predominantly results in the anti-Markovnikov product 1-bromobutane. Why is it that the experiments all had different results? This dispute was answered by Kharasch in the third decade of the 20th century. If the alkene is exposed to air, peroxides are formed by the reaction between the alkene and oxygen. These peroxides are radical formers, as their RO-OR bond can be easily homolytically cleaved. If HBr is then added to the alkene that contains a small amount of peroxides, the radical chain reaction largely exceeds the ionic mechanism, since the reaction rate of the radical chain reaction is much higher. The formation of radicals must be prevented in order to obtain an ionic mechanism, which results in the Markovnikov product. Therefore, freshly distilled alkene, which contains virtually no peroxides and radicals, must be applied in order to obtain mainly the Markovnikov product in the addition of HBr. The addition often yields product mixtures when the peroxides have not been carefully removed.
Mechanism of the radical HBr addition to 1-butene (alkenes)
The radical chain reaction is initiated by the endothermic, homolytical cleavage of the O-O peroxide bond, which yields alkoxy radicals. These alkoxy radicals form bromine radicals by abstracting hydrogen from hydrogen bromide. This step is an exothermic reaction, as the bond energy released by the formation of the O-H bond is comparatively high in relation to that of the H-Br bond.
In the first step of the chain propagation the bromine radical is added to the π bond of 1-butene, yielding a bromoalkyl radical. The attack of the bromine radical to 1-butene follows a regioselective course, which yields the more stable secondary alkyl radical.
In contrast, in the ionic mechanism of the HBr addition to 1-butene, a proton is first added to the double bond, which results in the more stable secondary carbenium ion.
Thus, in the radical mechanism the more stable, secondary alkyl radical is formed by the addition of a bromine radical to the double bond, while in the ionic mechanism the more stable, secondary carbenium ion that does not yet contain a bromine atom is formed by adding a proton to the double bond. Therefore, the radical mechanism yields the anti-Markovnikov product 1-bromobutane. Concurrently, the ionic mechanism yields the Markovnikov product 2-bromobutane.