Dissociative mechanism reaction coordinate diagram

Since the pyridinyl radical is soluble in n-hexane, and BrCHjBr has a dipole moment of l.OD, the initial state is not very polar and the transition state can thus not be very polar. A free-energy versus reaction coordinate diagram (Fig. 24a) illustrates the point. The rate-limiting step must be the transfer of a bromine atom from the halocarbon to the pyridinyl radical, yielding a bromomethyl radical and one or two bromodihydropyridines. The latter dissociate to the pyridinium bromide, while the former combines with a second pyridinyl radical to form two bromo-methyldihydropyridines. The mechanism is shown in Fig. 25 for the reaction with BrCH Cl. 

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