Electrophilic aromatic substitution reactions brominations

Like Friedel-Crafts reactions, halogenation such as bromination and iodination of aromatic compounds are classified as electrophilic aromatic substitution reactions. Bromine and iodine substituents are weakly electron-withdrawing groups, and introduction of such substituents causes a decrease in reactivity, and therefore, the first halogenation is faster than the second halogenation. However, the reactions in batch macroreactors suffer from the problem of disguised chemical selectivity, i.e, the formation of dihalogenated products. 

There are many other kinds of electrophilic aromatic substitutions besides bromination, and all are thought to occur by the same general mechanism. Let s look at some of these other reactions briefly. 

SAMPLE SOLUTION (a) The ring that bears the hydroxyl group is much more reactive than the other ring. In electrophilic aromatic substitution reactions of rings that bear several substituents, it is the most activating substituent that controls the orientation. Bromination occurs para to the hydroxyl group. 

Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions. Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete n electron delocalization (resonance). Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved. For example, benzene reacts with bromine to form bromobenzene. 

An electrophilic aromatic substitution reaction begins in a similar wav. but there are a number of differences. One difference is that aromatic ring.< are less reactive toward electrophiles than alkenes are. For example, Br in CH2CI2 solution reacts instantly with most alkenes but does not react at room temperature with benzene. For bromination of benzene to take plai a catalyst such as PeBrj is needed. The catalyst makes the Br2 molecu.. more electrophilic by polarizing it to give an FeBr4" Br species that reaci as if it were Br. ... 

A good example of a back titration involving iodine and thiosulfate is the assay of resorcinol in Resorcinol Solution BP. Resorcinol is an antiseptic that was widely used in the past, although less so now. The assay of resorcinol involves a quantitative electrophilic aromatic substitution reaction using bromine as the reagent, as shown in Figure 6.4. 

The same groups have also examined electrophilic aromatic substitution reactions, using both 223 and 255 as substrates. Bromination of 223 gave the 6-bromo derivative (256 X = Br) or the 6,8-dibromo derivative (257 X = Y = Br) depending on the conditions.147 Under all bromination conditions examined, compound 255 (R = alkyl) afforded only a 6-mono-brominated product 258 (X = Br, Y = H). 

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