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 for the preparation of polystyrene. 

Laboratory methods for preparing alkenylbenzenes include dehydration and dehydrohalogenation. 

The second of these two examples illustrates the regioselective preference for formation of the isomer in which the double bond is conjugated with the benzene ring. A hydrocarbon in which a C=C unit is conjugated with an aromatic ring is stabilized to about the same extent as a double bond in a conjugated diene. 

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. 

Part of the evidence for this comes from acid-catalyzed alcohol dehydrations, which are known to yield the most stable alkene (Section 7.8A). For example, dehydration of an alcohol such as the one that follows yields exclusively the conjugated system  

Because conjugation always lowers the energy of an unsaturated systan by allowing the 7T electrons to be delocalized, this behavior is just what we would expect. 

In the presence of peroxides, hydrogen bromide adds to the double bond of 1-phenylpropene to give 2-bromo-l-phenylpropane as the major product  

The addition of hydrogen bromide to 1-phenylpropene proceeds through a benzylic radical in the presence of peroxides and through a benzylic cation in their absence (see Review Problem 15.15 and Section 10.9). 

Write mechanisms for the reactions whereby HBr adds to 1-phenylpropene (a) in the presence of peroxides and (b) in the absence of peroxides. In each case account for the regiochemistry of the addition (i.e., explain why the major product is 2-bromo-l-phenylpropane when peroxides are present and why it is 1-bromo-1-phenylpropane when peroxides are absent). 

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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 leav mg the ring unaffected... 

Section 1115 The simplest alkenylbenzene is styrene (C6H5CH=CH2) An aryl group stabilizes a double bond to which it is attached Alkenylbenzenes are usu ally prepared by dehydration of benzylic alcohols or dehydrohalogena tion of benzylic halides... 

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... 

Dehydrogenation of alkylbenzenes although useful m the industrial preparation of styrene is not a general procedure and is not well suited to the laboratory prepara tion of alkenylbenzenes In such cases an alkylbenzene is subjected to benzylic bromi nation (Section 11 12) and the resulting benzylic bromide is treated with base to effect dehydrohalogenation... 

Reactions involving benzylic cations, benzylic radicals, and alkenylbenzenes will be discussed in Sections 11.12 through 11.17. 

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

Benzene derivatives. Tbe nomenclature is a combination of the lUPAC system and traditional names. Many of the derivatives are named by the substituent group appearing as the prefbt. These may be considered a subclass of the aliphatic-aromatic hydrocarbon family, which contains both aliphatic and aromatic units in its structures. Thus, alkylbenzenes are made up of a benzene ring and alkane units alkenylbenzenes are Composed of a benzene ring and alkene units and alkynylbenzenes comprise a benzene ring and alkyne units. Examples of alkylbenzenes include... 

Problem 16.21 Styrene, the simplest alkenylbenzene, is prepared commercially for use in plastics manufacture by catalytic dehydrogenation of ethylbenzene. How might you prepare styrene from benzene using reactions you ve studied ... 

Alkenylbenzene by-products, 23 335 Alkenylborates, 13 653 Alkenyl coupling, 13 653 Alkenyl halides, 12 167 Alkenylpyridine derivatives, physical properties of, 21 94t Alkenylsuccinic anhydride (ASA),... 

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