Radical free energy versus reaction

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. 

Fig. 24. Free energy versus reaction coordinate diagrams for the reaction of a pyridinyl radical with halocarbons in solvents of low and high polarity.

In Table is the data for polymer composition versus reaction temperature in the free radical polymerization of 1,3-butadiene (8A). An Arrhenius plot for this data is given in Figure 2 (8B), which shows that a reaction temperature of approximately 105OK would be needed to form a polymer containing 95% trans-1, units, which are the units formed with the lowest activation energy presumably for steric reasons. 

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