Chemoselectivity of Radical Brominations

Let us go back to radical brominations (cf. Section 1.7.3). The bromination of alkyl aromatics takes place completely regioselectively only the benzylic position is brominated. The intermediates are the most stable radicals that are available from alkyl aromatics, namely, benzylic radicals. Refluxing ortho-xylene reacts with 2 equiv. of bromine to give one monosubstitution 

With Wohl-Ziegler brominations unsymmetric allyl radicals can basically react with the bromine atom at any of their nonequivalent ends. The product in which the bromine is localized on the less alkylated C atom and in which the higher alkylated C=C bond is present will form preferentially. 

Side Note 1.2. The Rate of the Wohl-Ziegler Bromination Is More Dependent on a Polar Effect than on Product Development Control 

The rate analysis of radical reactions will only be complete if electronic effects are also considered. In the transition state of the hydrogen transfer from the substrate to a bromine atom, partial charges develop during the course of the reaction. This is reflected in the transition states H and I in terms of a polarization S C.H.Br150. A partial positive charge 

So what have we learned here Both C and D lead to the same intermediate E upon hydrogen abstraction, but H is much higher in energy than I, allowing a new pathway to complete for C to the exclusion of H. Product development control does not work here and it should be known that, like many of our models in organic chemistry, it has its limits. 

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