Bromobenzene electrophilic bromination

The first step in electrophilic bromination of benzene involves addition of Br, leading to an intermediate bromobenzenium ion. This is then rapidly followed by loss of a proton to give bromobenzene. 

Solution It cannot be prepared by direct electrophilic bromination of bromobenzene, because the Br group is o,p-directing (Sec. 4.11). But we can take advantage of the m-directing effect of a nitro group and then convert the nitro group to a bromine atom, as follows ... 

Bromine is an ortho, para director, acetyl a meta director. Reasoning backward from the target, disconnection a makes electrophilic bromination of acetophenone the last synthetic step disconnection b makes Friedel-Crafts acylation of bromobenzene the last step. Of the two approaches, only bromination of acetophenone delivers the desired meta relationship of the two substituents and suggests the following synthesis. 

Electrophilic Bromination of Bromobenzene Results in ortho- and para-Dibromobenzene... 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

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. 

For the synthesis of orfbo-bromotoluene we use a sulphonic acid. o-Bromotoluene could be synthesised by bromination of toluene or by Friedel-Crafts alkylation of bromobenzene (Fig. T). However, the reaction would also give the para substitution product and this is more likely if the electrophile is hindered from approaching the ortho position by unfavourable steric interactions. Alternatively we can substitute a group at the para position before carrying out the bromination. 

It is evident from the above arguments that the addition of halogens to alkynes may occur by a multiplicity of mechanisms. One more argument is offered by a kinetic study of the addition of bromine to diphenylacetylene in bromobenzene reported by Sinn et al. (1965). In this case kinetic data are better interpreted on the basis of a nucleophilic mechanism involving the intermediacy of a vinyl anion, rather than an electrophilic reaction. 

This mixing of such levels of property occurs consciously or sub-consciously even amongst teachers and professors. For example, an organic chemist might do the following as he or she explains the mechanism of an electrophilic substitution reaction, the bromination of benzene the bromine approaches the benzene (...) bromine attacks the benzene core (...) the electrons relocate and the bromine splits (...) bromobenzene results (...) a bromine has substituted a hydrogen . 

The reaction of benzene with bromine or chlorine in the presence of a Lewis acid catalyst (such as FeBr3, FeCl3 or A1C13) leads to the formation of bromobenzene or chlorobenzene, respectively. The Lewis acid, which does not have a full outer electron shell, can form a complex with bromine or chlorine. This polarises the halogen-halogen bond (making the halogen more electrophilic), and attack occurs at the positive end of the complex. 

Because it is aromatic, benzene does not react directly with reagents such as HBr or HCl, or even with diatomic bromine or chlorine. Benzene reacts with cationic species to ve a resonance-stabilized carbocation intermediate, which loses a hydrogen to give a substitution product. This reaction is called electrophilic aromatic substitution. The most common method for generating reactive cations in the presence of benzene is to treat certain reagents with strong Lewis acids. Lewis acids or mixtures of strong acids can be used to convert benzene to chlorobenzene, bromobenzene, nitrobenzene, or benzenesulfonic acid. 

That benzene has different chemical properties from ordinary polyenes was well known to the early chemists. Polyenes, or even alkenes, react with a variety of nucleophiles, electrophiles, and free radicals, nnder various circumstances, to yield addition products. For example, bromine reacts more or less instantaneously with alkenes and polyenes, which is readily apparent when one drips bromine into an alkene solution, and the color disappears. The product in the simplest case will be simply a 1,2-dibromide. Benzene, on the other hand, requires prolonged reflnxing with bromine, in the presence of a catalyst, to convert it to bromobenzene. And the latter reaction is not a simple bromine addition, it is a substitution of one hydrogen by one bromine. And a variety of compounds related to benzene (snch as those discnssed previously in this chapter) similarly are aromatic compounds, and chemically quite different from alkenes and polyenes. Why is this ... 

The bromination of phenyl n-pentyl ether is more para-selective in anionic micelles than it is in water. This contrasts with the lower para-selectivity of nitration of bromobenzene in the cationic micelles formed by dissolving lauric acid in 95% H2S04. It is not clear whether these effects are due to substrate orientation or to micelle-induced changes in the selectivity parameter for electrophilic aromatic substitution. The rates of solvolysis of alkyl p-trimethyl-ammoniumbenzenesulphonate trifluoromethanesulphonates (42) are strongly inhibited by anionic micelles of sodium lauryl sulphate or sodium dodecanoate. In water, homomicelles of (42) or sodium dodecanoate micelles, undergo inversion of stereochemistry, but in sodium lauryl sulphate 22% retention of... 

So why even bring the subject up Notice that this process does look a bit like another reaction that fails to give product—the attempt to form chlorobenzene or bromobenzene from chlorine or bromine and benzene. Those halogenations are transformed into a success by the addition of a catalyst, either FeCl3 or FeBr3, which converts the halogen, a weak electrophile, into a much stronger Lewis acid... 

As mentioned above, selection of proper synthons with active functions and high-yield coupling reactions for the chain extension is quite important in dendrimer synthesis. Siloxane bonds are usually prepared by the reaction between nucleophilic silanols and electrophilic silanes (Si. Lithe divergent synthesis of the polysiloxane dendrimers, the protection of Si was employed instead of the protection of silanol [9]. Conversion from the synthon to active Si species (deprotection) had to be carried out under neutral conditions, because siloxanes are easily hydrolyzed under both acidic and basic conditions. It was found that one of the most suitable synthons for is phenylsilane, which reacts with bromine to form bromobenzene andbromosilane. Surprisingly, siloxanes are stable with bromine for short periods of time at room temperature. 

---