Aromatic substitution reactions bromination

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. 

A wide variety of reactions other than substitutions and hydrolyses have been performed in microemulsions. Examples include alkylations [29], Knoevenagel condensations [13], oxidations [30,31], reductions [32], formation and decomposition of Meisenheimer complexes [33], aromatic substitution reactions such as nitration and bromination [34-36], nitrosation [37] and lactone formation, i.e. esterification [38-40]. Microemulsions have also been used for photochemical and electrochemical reactions [41-45]. 

Intramolecular nucleophilic substitution reactions of chlorine " " or bromine with alco-holates lead to tetrahydropyrans. The alcoholate can be formed in situ by deprotccting an alcohol.Activated aromatic chlorides give in nucleophilic aromatic substitution reactions a six-membered heterocyclc, e.g, reaction of 1 to give 2. ... 

Just by knowing the effects summarized in these short lists, we can now predict fairly accurately the course of hundreds of aromatic substitution reactions. We now know, for example, that bromination of nitro enzene will yield chiefly the /M-isomer and that the reaction will go more slowly than the bromination of benzene itself indeed, it will probably require severe conditions to go at ail. We now know that nitration of CeHsNHCOCH, acetanilide) will yield chiefly the o-and / -isomers and will take place more rapidly than nitration of benzene. 

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. ... 

---