Allyl free radical relative stability

How can we account for the unusual reactivity of conjugated dienes In our discussion of halogenation of the simple alkenes (Sec. 3.27), we found that not only orientation but also relative reactivity was related to the stability of the free radical formed in the first step. On this basis alone, we might expect addition to a conjugated diene, which yields a stable allyl free radical, to occur faster than addition to a simple alkene. 

The relative rates of the two reactions depend chiefly upon the act s. Stabilization of the incipient allyl free radical lowers the energy level of the transition state stabilization of the diene lowers the energy of the reactants. Whether the net aot is larger or smaller than for addition to a simple alkene depends upon which is stabilized more (see Fig. 8.9). 

The greater stability of allylic radicals relative to their alkyl counterparts suggests that free-radical halogenation of alkenes should be both feasible and regioselective for the allylic position. Although, as we have already seen, the typical reaction of alkenes with halogens at room temperature and below is electrophilic addition to the double bond. 

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