Addition reactions of alkenylbenzenes

Alkenylbenzenes are prepared by the various methods described in Chapter 5 for the preparation of alkenes dehydrogenation, dehydration, and dehydrohalogenation. 

Dehydrogenation of alkylbenzenes is not a convenient laboratory method but is used industrially to convert ethylbenzene to styrene. 

Acid-catalyzed dehydration of benzyUc alcohols is a useful route to alkenylbenzenes, as is dehydrohalogenation under E2 conditions. 

Most of the reactions of alkenes that were discussed in Chapter 6 find a parallel in the reactions of alkenylbenzenes. 

Hydrogenation of the side-chain double bond of an alkenylbenzene is much easier than hydrogenation of the aromatic ring and can be achieved with high selectivity, leaving the ring unaffected. 

Practically all of the 1.3 X 10 lb of ethylbenzene produced annually in the United States is converted to styrene. 

Both 1,2-dihydronaphthalene and 1,4-dihydronaphthalene may be selectively hydrogenated to 1,2,3,4-tetrahydronaphthalene. 

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Section 11 16 Addition reactions to alkenylbenzenes occur at the double bond of the alkenyl substituent and the regioselectivity of electrophilic addition is governed by carbocation formation at the benzylic carbon See Table 11 2... 

Addition of an unsymmetrical reagent to a double bond may in general yield two different products. In our discussion of alkenes (Secs. 6.11 and 6.17), we found that usually one of the products predominates, and that we can predict which it will be in a fairly simple way in either electrophilic or free-radical addition, the first step takes place in the way that yields the more stable particle, carbonium ion in one kind of reaction, free radical in the other kind. Does this rule apply to reactions of alkenylbenzenes ... 

The side-chain double bond is more reactive than the aromatic ring toward most electrophilic reagents. Many of the reactions of alkenes that were discussed in Chapter 6 find a parallel in the reactions of alkenylbenzenes. Thus, hydrogenation and halogen addition to a side-chain double bond can be achieved while leaving the ring unchanged. 

In a similar way, alkenyl benzenes undergo the other addition reactions characteristic of the carbon-carbon double bond. Let us look further at the reactions of conjugated alkenylbenzenes, and the way in which the ring affects orientation and reactivity,... 

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