Radical Additions and Substitutions with Alkenes
Radical Bromination in Allylic Position
The addition of halogens to alkenes at room temperature with a high halogen concentration yields vicinal dihalogenides. However, at a low halogen concentration and high temperature, the mechanism of a radical chain reaction is favoured. As a result, a radical substitution occurs, yielding predominantly halogenation in allylic or benzylic position, respectively.
Radical brominations are normally carried out with a suspension of N-bromosuccinimide (NBS) in carbon tetrachloride. During this reaction, NBS serves as a continous source of very small amounts of bromine, as it is insoluble in CCl4. Therefore, the conditions of this reaction are ideal for a bromination in allylic position.
The chain initiation occurs by the reaction between NBS and hydrogen bromide, which yields bromine. Afterwards, bromine is homolytically cleaved into bromine radicals by the action of light. In the first chain propagation reaction, a bromine radical abstracts an allylic hydrogen atom from cyclohexene, yielding hydrogen bromide and an allylic cyclohexenyl radical. The allyl radical then reacts with the bromine that was previously formed from NBS and hydrogen bromide. This results in a new bromine radical, as well as in the product of the allylic bromination, 1-bromocyclohex-2-ene. The bromine radical is thus recovered for the first step of the chain propagation, while the hydrogen bromide formed in the first propagation step is continously used in order to supply the second propagation step with bromine through the reaction with NBS. With this, the cycle of the chain reaction is then completed. As a result of the reaction mechanism, NBS must be applied in stoichiometric amounts, since it is both a catalyst and the actual source of bromine, as well.
The progression of the reaction can easily be observed. Both the starting product N-bromosuccinimide and the product succinimide are insoluble in the solvent carbon tetrachloride. However, N-bromosuccinimide has a higher density than CCl4 does. Therefore, it stays at the bottom of the reaction vessel, while succinimide, whose density is lower, floats on the solvent's surface.
Why does bromination under these conditions only occur in allylic position? In order to answer this question, refer to the dissociation energies.
Cyclohexene contains three types of hydrogen atoms: the alkylic, allylic, and vinylic hydrogens. The C-H bond in allylic position most easily cleaved homolytically, since the allyl radical is resonance-stabilized. The allylic cyclohexenyl radical is about 40 kJ/mol more stable than the alkylic cyclohexyl radical is.
The selectivity of the allylic halogenation depends not only on the stability of the intermediately formed radical, but also on the halogen's reactivity. Usually, brominations are more selective than chlorinations, due to the fact that the chlorine radical is more reactive than the bromine radical.