Benzene, acylation chlorobenzene from

Decomposition of Aromatic Acyl Peroxides. The decomposition of aromatic acyl peroxides in liquid aromatic compounds is similar to the decomposition of nitrosoacetylarylamines and appears to involve the intermediate formation of free radicals. When dibenzoyl peroxide is heated in benzene, biphenyl, benzoic acid, and small amounts of phenyl benzoate, p-terphenyl, and quaterphenyl are produced. 4 That the second component enters into the reaction is shown by the formation of 4-chlorobiphenyl from the decomposition of di-p-chlorobenzoyl peroxide in benzene 84 and of dibenzoyl peroxide in-chlorobenzene 86... 

How is this useful Consider the preparation of l-chloro-3-ethylbenzene from benzene. Retrosynthetic analysis (Section 8-9) indicates that neither chlorobenzene nor ethylbenzene is suitable as the immediate precursor to product, because each substituent is ortho, para directing. However, we recognize that ethyl can be produced from acetyl, a meta director. Hence, acetylbenzene, readily prepared by Friedel-Crafts acylation, is a perfect relay point, because it can be chlorinated meta. After the carbonyl group has completed its directing job, it is reduced to the desired alkyl group. 

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