Alkenylbenzenes conjugated, stability

A carbon atom that is directly attached to a benzene ring is called a benzylic carbon (analogous to the allylic carbon of C=C—C). A phenyl group (C H —) is an even better conjugating substituent than a vinyl group (H2C=CH—), and benzylic carbocations and radicals are more highly stabilized than their allylic counterparts. The double bond of an alkenylbenzene is stabilized to about the same extent as that of a conjugated diene. 

Addition to conjugated alkenylbenzenes orientation. Stability of the benzyl cation... 

The fact is that conjugated alkenylbenzenes are much more reactive than simple alkenes toward both ionic and free-radical addition. Here again—as in most cases of this sort—resonance stabilization of the transition state leading to a carbonium ion or free radical is more important than resonance stabilization of the reactant. We must realize, however, that this is not always true. 

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