Bromination, of bromobenzene

Our group suppressed (ref. 19) the deactivation by HBr by the addition of NaHC03 as a scavenger for HBr and zeolite KA for removal of water formed in the latter reaction. With these additives a p/o-selectivity of 97/3 was achieved in the bromination of bromobenzene over CeY. 

There are instances in which the mode of fission and therefore reaction mechanism changes with the environment or experimental conditions. This can be illustrated with the dissociation of hexaphenyl-ethane, the bromination of bromobenzene, and the reactions of diazonium ions ... 

His proposal also accounted for the fact that the bromination of bromobenzene gives three (and only three) isomeric dibromobenzenes ... 

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 ... 

Kekule s proposal accounted nicely for the fact that bromination of benzene gives only one bromobenzene and that bromination of bromobenzene gives three isomeric dibromobenzenes. 

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

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. 

What is an immediate precursor of toluene " Benzene, which could be methylated in a Friedel-Crafts reaction. Alternatively, "What is an immediate precursor of bromobenzene " Benzene, which could be brominated. 

Bromodiphenyl has been obtained along with some of the ortho derivative from the bromination of diphenyl.1 However, according to Schlenk,2 the product so obtained is contaminated with some p, -dibromodiphenyl which is very difficult to remove. Bamberger obtained j>-bromodiphenyl from the action of benzene on solid p-bromobenzene diazoanhydride.3 The method described in the procedure has recently been described in the literature.4... 

Chlorobromobenzene has been prepared by the diazotiza-tion of o-bromoaniline followed by replacement of the diazonium group by chlorine 1 by the elimination of the amino group from 3-chloro-4-bromoaniline 2 by the chlorination of bromobenzene in the presence of thallous chloride,3 aluminum chloride,4 or ferric chloride 4 by the bromination of chlorobenzene without a catalyst6 or in the presence of aluminum,4 iron,4 or ferric bromide 6 by the diazotization of o-chloroaniline followed by replacement of the diazonium group with bromine 4,6 and from o-chlorophenylmercurie chloride by the action of bromine.7... 

For comparison, fluorous-phase-soluble Pd complexes are only 74-98% selective towards the trans product [168-170]. The isolated yields of the product approached 70% when a threefold excess of olefin to iodobenzene was used (Table 3) however, the percent yield decreased with the use of bromobenzene as expected since activation of bromine-carbon bonds is less favorable than iodo-carbon bonds. It was also possible to catalyze the reaction in the absence of additional triethylamine base (Table 3). In this case, the tertiary amines of the den-drimer most likely act as the base. The catalysts, in general, were fully recover-... 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

C. Brornotris[2-(perfluorohexyl)ethyl]tin. The fluorous phenyltin product (17.2 g, 13.9 mmol) and dry ether (80 mL) are transferred to a 250-mL, three-necked flask that had been dried in an oven and cooled to 0°C under argon. Bromine (0.71 mL, 14 mmol) is added dropwise over 30 min to the mixture. The addition rate is adjusted to keep the temperature between 0° and 1°C. The mixture is warmed to 25°C and stirred for 7 hr. The reaction mixture is transferred to a 250-mL, round-bottomed flask. The ether and excess bromine are removed under reduced pressure to leave a yellow oil. The oil is dissolved in FC-72 (75 mL) and transferred to a 250-mL separatory funnel. The bromine and bromobenzene by-products are removed by washing three times with methylene chloride (3 x 75 mL) leaving the fluorous layer colorless. The FC-72 is removed under reduced pressure to provide 15.8 g (12.7 mmol, 92%) of a colorless oil (Note 6). 

Bromobenzaldehyde from aluminum chloride catalyzed bromination of benzaldehyde, 40, 9 Bromobenzene, conversion to Grignard reagent, 41, 91... 

The synthetic utility of many of the substitution reactions described so far is limited because there are well-established thermal routes to the same products. However, a third group of photochemical nucleophilic substitutions involves aryl halides and nucleophiles based on sulfur, phosphorus or, of particular importance, carbon. Two examples are the reaction of bromobenzene with the anion of t-butyl methyl ketone 13.12), and the replacement of bromine by cyanomethyl in 2-bromopyridine (3.13). This type of reaction offers a clear advantage over lengthy thermal alternatives, and intramolecular versions have been used in the synthesis of indoles (e.g. 3.14) or benzofurans from o-iodoaniline or o-iodoanisole respectively. 

Bromomandelic acid has been prepared by the bromination of -bromoacetophenone followed by alkaline hydrolysis 3 by the alkaline hydrolysis of the product formed by the addition of chloral to -bromophenylmagnesium bromide 4 and by the condensation of bromobenzene and ethyl oxomalonate in the presence of stannic chloride followed by hydrolysis and decarboxylation.5 />-Bromomandelic acid is a valuable reagent in the analyses of zirconium and hafnium.6... 

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. 

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. 

This method is a modification of that of Buckles, Hausman, and Wheeler.2 4,4 -Dibromobiphenyl has also been prepared by the bromination of biphenyl in water,3 carbon disulfide,4 and glacial acetic acid 6 by the bromination of a mixture of biphenyl-sulfonic acids in dilute sulfuric acid 6 by the action of sodium carbonate on the perbromide obtained from the reaction of di-azotized benzidine with bromine water 7 and by passing />-di-bromobenzene vapor through a red-hot tube.8... 

Dibromobibenzyl has been prepared by the bromination of bibenzyl in water 9 by the reaction of />-bromobenzyl bromide with zinc dust in water 10 and by the reaction of bromobenzene with ethylene oxide in the presence of anhydrous aluminum chloride.11... 

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