Benzene: acylation bromination

Because the position of electrophilic attack on an aromatic nng is controlled by the direct ing effects of substituents already present the preparation of disubstituted aromatic com pounds requires that careful thought be given to the order of introduction of the two groups Compare the independent preparations of m bromoacetophenone and p bromoace tophenone from benzene Both syntheses require a Friedel-Crafts acylation step and a bromination step but the major product is determined by the order m which the two steps are carried out When the meta directing acetyl group is introduced first the final product IS m bromoacetophenone... 

Another substitution reaction that is initiated by photochemical hydrogen abstraction is the replacement of the bromine atom in 2-bromo-8-methoxy-l, 4-naphthoquinone by an acyl group757. Irradiation of a solution in benzene of the quinone, butyraldehyde or capraldehyde and pyridine yields mixtures of acylated quinone and acylated hydro-quinone. In the first step, the excited quinone abstracts the aldehyde hydrogen atom and this is followed by bond formation between the acyl radical and C-2 of the quinone. The radical that is formed after departure of a bromine atom may either lose a hydrogen atom and yield acylated quinone or take up a hydrogen atom and become acylated hydro-quinone. 

The first three reactions are all electrophilic substitution a bromination of a pyrrole, the nitration quinoline, and a Friedel-Crafts acylation of thiophene. Bromination occurs on the pyrrole at only remaining site. Nitration of quinoline occurs on the benzene rather than the quinoline r-fi (actually giving a mixture of 5- and 8-nitroquinolines, but don t worry if you didn t see t- Mi p. 1174), and the acylation would occur next to sulfur. 

Bromination of Aromatic Rings 593 Other Aromatic Substitutions 597 Alkylation of Aromatic Rings The Friedel-Crafts Reaction Acylation of Aromatic Rings 604 Substituent Effects in Substituted Aromatic Rings 605 An Explanation of Substituent Effects 610 Trisubstituted Benzenes Additivity of Effects... 

The treatment of stannylenes (see Section 5.1.5) dissolved in benzene with a bromine solution in the same solvent in the presence of molecular sieves or BujSnOMe as base, gives, at room temperature, the hydroxyl ketone at the rate of titration (David and Thieffrey 1979). The reaction is nearly uniformly regiospecific, that is to say that it gives only one of the two possible hydroxyl ketones (reaction 5.15). The same selective activation by stannylation, already observed in benzylation, allylation, and acylation, is found here. Thus, compound 5.28 is prepared from diol 5.27 in 75% yield. The replacement of bromine by A -bromosuccinimide without any other reagent has recently been recommended (Kong and Grindley 1993). 

The characteristic reaction of benzene and its derivatives is electrophilic aromatic substitution. In these reactions, a hydrogen on the benzene ring is replaced by a chlorine (chlorination), a bromine (bromination), an alkyl or acyl group (Friedel-Crafts alkylation or acylation), a nitro group (nitration), or a sulfonic acid group (sulfonation). 

Compare the independent preparations of m-bromoacetophenone and p-bromoace-tophenone from benzene. Both syntheses require a Friedel-Crafts acylation step and a bromination step, but the major product is determined by the order in which the two... 

Explain the following observations (a) although Friedel-Crafts alkylation reactions often result in polysubstitution products, Friedel-Crafts acylation reactions only give the monosub-stituted product (b) bromination of ethyl 4-methy benzoate gives only one product (c) treatment of 4-chlorotoluene with NaNH, in liquid ammonia gives two products (d) reaction of benzene with I-chloro-2-methylpropane and AlCl gives rm-butylbenzene. 

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