8.1: Regioselectividad de las adiciones electrofílicas - Efecto peróxido
In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
In the first initiation step, an oxygen–oxygen bond in the radical initiator undergoes homolytic cleavage.
The di-tert-butyl peroxide is an excellent free-radical initiator as the homolysis of the O–O bond requires just 159 kJ mol–1 (38 kcal mol–1) of energy.
The second initiation step involves the exothermic (ΔH = –70 kJ mol–1) abstraction of hydrogen from HBr by the tert-butoxy radical. The abstraction of bromine, however, is thermodynamically unfavorable (ΔH = 163 kJ mol–1).
In propagation steps, a bromine radical reacts with an alkene to generate an alkyl radical.
The regioselective addition of bromine at the less substituted carbon in the presence of peroxide can be understood from the transition states. The transition state shows that the formation of the more substituted radical involves an attack by a bromine radical at the less substituted (and less hindered) carbon atom, which is lower in energy than the transition state for the less substituted radical. Another reason is the stability exhibited by the more substituted radicals owing to the hyperconjugation and inductive effect.
The reaction is terminated when radicals combine to yield non-radical products.
While the peroxide-mediated addition of HI to an alkene does not occur because the first propagation step is endothermic, the reaction with HCl does not proceed as the second propagation step is endothermic.
In the addition of hydrogen bromide to an alkene, the bromine radicals can attack the less substituted vinylic carbon from either face to an equal extent. Hence, when an alkene is stereogenic, a racemic mixture of products is obtained.