Radical halogenation, alkylbenzenes

The comparative ease with which a benzylic hydrogen is abstracted leads to high selectivity m free radical halogenations of alkylbenzenes Thus chlorination of toluene... 

TABLE 2.20 Selected examples of the radical halogenation of alkylbenzenes ... 

Many side-chain halogen compounds can be synthesized by reactions that also are applicable to alkyl halides (see Table 14-5), but there are other methods especially useful for the preparation of arylmethyl halides. The most important of these are the chloromethylation of aromatic compounds (to be discussed later in this section) and radical halogenation of alkylbenzenes. 

Alkylbenzenes undergo free-radical halogenation much more easily than alkanes because abstraction of a hydrogen atom at a benzylic position gives a resonance-stabilized benzylic radical. For example, ethylbenzene reacts with chlorine in the presence of light to give cr-chlon >e thy I benzene. Further chlorination can occur to give a dichlorinated product. 

Halogenation (Section 11.12) Free-radical halogenation of alkylbenzenes is highly selective for substitution at the benzylic position. In the example shown, elemental bromine was used. Alternatively, A/-bromosuccinimide is a convenient reagent for benzylic bromination. 

As in the free-radical halogenation of alkanes, chlorination of alkylbenzenes is less selective than bromination. Given the relative rates per hydrogen for hydrogen atom abstraction from 1-phenylbutane by chlorine for the elementary step shown, calculate the percentage of 1-chloro-1-phenylbutane in the C10H13CI product. 

Alkylbenzenes are susceptible to free-radical halogena-tion. The halogen is added to the alkyl side chain such that the most stable radical intermediate is formed. 

Alkylbenzenes can be halogenated using bromine and light—the light generates bromine radicals from bromine molecules by homolytic fission (Equation 13.10) ... 

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