Bromination aromatic

Iron, reaction with nitroarenes, 928 lron(lll) bromide, aromatic bromination and, 548 Iron sulfate, LD5q of, 26 Isoamyl group, 89 lsobutane, molecular model of, 80 lsobutyl group, 84... 

The usual aromatic bromination are performed by free bromine in the presence of a catalyst, most often iron. However, liquid bromine is not easy to handle because of its volatile and toxic character. On the other hand, alumina-supported copper(II) bromide can be treated easily and safely as a solid brominating reagent for aromatic compounds. The advantages of this procedure using the solid reagent are simple workups, mild conditions, and higher selectivities. Products can be isolated in good yield by simple filtration and solvent evaporation, and no extraction steps are required. 

Aromatic brominations have been mainly carried out using X, Y and L zeolites. Improved para/ortho ratios have been observed upon brominating halobenzenes, benzyl halides, and biphenyl. The side product HBr leads to decreased activity and selectivity. This problem has been addressed by adding scavengers, by working in the gas phase, and by applying oxidative bromination. 

Quite some papers and patents deal with zeolite-catalyzed aromatic bromination. Most work pertains to bromination of halobenzenes and of toluene, using X-, Y-and L-type zeolites. 

The important question how inner and outer surface of the zeolite crystals contribute in the aromatic bromination has been discussed by Sasson et al. (20). Zeolite CaY was recently applied in the bromination (CH2CI2, 20 °C) of benzyl bromide (ref. 25). Selectivity to the 4-bromo derivative was 79% at a conversion of 76%. 

The Ag-silica-alumina material is furthermore suited to assist in sterically hindered aromatic brominations. As an example we converted 1,3,5-tri-t-butylbenzene into 2,4,6-tri-t-butylbromobenzene. When using an acidic zeolite and Br2 de-alkylation prevails and 3,5-di-t-butylbromobenzene is formed (ref. 29). 

Aromatic bromination is also achieved in high yields for a variety of substrates including phenols (ref. 4) and anilines (ref. 5). 

BEHAVIOUR AND FATE OF AROMATIC BROMINE COMPOUNDS IN THE ENVIRONMENT... 

The fate of aromatic bromine compounds such as brominated dibenzodioxins occurring on fly ash of municipal waste incinerators has been deduced from appropriate laboratory experiments. Stereoselective, first order ipso-substitution of bromine by chlorine is observed. 

Fate of Aromatic Bromine Compounds During Municipal Waste Incineration... 

Aromatic bromine compounds can be formed and transformed during various thermal processes, like aromatic chlorine compounds (ref. 22). Brominated dibenzodioxins and -furans and mixed brominated/chlorinated compounds have been detected in trace levels in the fly ash of a municipal waste incinerator (ref. 23).Chlorine is generally abundant compared to the bromine of typical municipal waste the chlorine vs. bromine ratio is in the range of 250 1. 

Fig. 9. Schematic Drawing of the Apparatus for the Study of Aromatic Bromine Compounds on...

The chlorine content of the fly ash is 4.9 %, the bromine content 0.065 %. A series of brominated aromatic compounds has been adsorbed on this fly ash and treated for 1 h. with air at 300°C. The extraction and analysis yield mixed brominated/chlorinated as well as completely chlorinated products. This is shown for 5 aromatic bromine compounds in Table 2. 

All products were identified and followed by GC/MS. The lateral positions (C-2, C-3) of 1,2,3,4-TBDD are slightly more reactive compared to the peri positions (C-1, C-4) the two Br3Cl-isomers appear in a ratio of approximately 1 1,5. The relative rate constants of 5 aromatic bromine compounds were determined (Table 3). 

Table 3. Relative rate constants (Kfgi) and half lifes (tj/2> for a mixture of aromatic bromine compounds calculated from the concentrations of educts after reaction with 3.5 g fly ash containing chlorine. 

These results show the fate of aromatic bromine compounds during municipal waste incineration bromine is exchanged by chlorine on the surface of fly ash at the electrostatic precipitator at 250-3(X)°C. But the toxic potential at brominated dibenzodioxins and furans is not reduced by these transformations. The increase of PCDD/F concentration in MWI by adding bromine compounds has been pointed out by Lahl and coworkers (ref. 26). 

The use of aromatic brominated compounds as flame retardants has been a potential source of environmental contamination. Incomplete incineration of these compounds and wastes (plastics, textiles, oils etc...) containing brominated flame retardants caused formation of brominated/chlorinated dibenzodioxines (PBDDs/ PCDDs) and dibenzofurans (PCDFs/PBDFs) (refs. 1 - 4). 

Except for polybrominated biphenyls (PBB), a limited number of studies regarding the toxicity of aromatic brominated compounds has been performed. Some experiments suggest a moderate acute toxicity of these compounds (ref. 1). 

Among aromatic bromine derivates, only polybrominated biphenyl is known to have a porphyrogenic effect (ref. 24). From scarce data about the toxicity of 4 and 5 porphyrogenic activity in the case of repeated administration could be expected (refs. 25,26). No data are available concerning the remaining compounds. 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

Brominated PPO. The electrophilic aromatic bromination of the PPO was carried out in chloroform solution to yield polymers with different degrees of substitution. For the highly brominated structures the TG expanded over a considerable temperature range. A maximum value of 273 C was obtained for 100 mol % brominated PPO. 

Organolithium reagent 35 was added to aldehyde 31 (Scheme 7.6) to obtain alcohol 36 as an inconsequential 1 1 mixture of diastereomers. The benzylic alcohol was removed using a Barton two-step radical deoxygenation protocol, followed by electrophilic aromatic bromination to provide the desired coupling partner 37. 

The first question to be raised about the viability of Scheme 27 is that the driving force (—AGEt) for the charge-transfer step (equation 100) is insufficient to account for the fast rate of aromatic bromination. 

The remarkable generality of FERET to predict the behavior of Br2 in aromatic bromination as well as in olefin addition is established by the single linear correlation in Fig. 21b when the solvation difference (see equation (103)) between the cation radicals of arene and olefin donors is taken into account.263... 

Polypropylene. 3 series of experiments, in which the first stage was performed with Rr2 solutions of 3 different concentrations, at 28 C and various time intervals are summarized in Figure 1. The amounts of NH Br found in the polymer Increase with B concentration to a maximum concentration of 5.7% of NH Br. The 0.1. values of the samples (Fig. 1) appear to correlate closely with the NH Br. Table I shows the average effectivity, e.g. the slope of the plot of O.I. vs % Br for the PP fabric to be 1.24. This value is seen in Table II to be considerably higher than the value of 0.6 reported by van Krevelen [12] for aliphatic and aromatic bromine compounds and of 0.5 found bv Green [13] for aliphatic Br compounds. According to Green [13], 10% of aliphatic bromine are needed to produce a PP with an O.I. of 23.5, as compared to 4,6% Br from NH Br, which produced an 0.1. of 24.2 in the present experiments. 

In Figure 2, taken from reference [12], we inserted curve 5, which contains our present results. It is evident that the NH Br is more effective for PP than aliphatic or aromatic bromine throughout the 1-6% bromine range when no synergists are added. 

Commercially available flame retardants include chlorine- and bromine-containing compounds, phosphate esters, and chloroalkyl phosphates. Recent entry into the market place is a blend of an aromatic bromine compound and a phosphate ester (DE-60F Special) for use in flexible polyurethane foam (8). This paper describes the use of a brominated aromatic phosphate ester, where the bromine and phosphorus are in the same molecule, in high temperature thermoplastic applications. 

Replacement, of acetylenic hydrogen atom by bromine, 45, 86 of acyl chlorine atom by fluorine atom using hydrogen fluoride, 46, 3 of amino hydrogen atoms by thio-carbonyl group, 46, 19 of aromatic bromine atom by potassium /-butoxide, 46, 89 of a-chloro atom in ester using azide ion, 46, 47... 

Chlorinated alkanes and aromatics Brominated neopentyl alcohol and derivatives Dibromophenol Flame retardants... 

The triethyl phosphite deoxygenation process tolerates aromatic bromine, methoxyl, ° ° ° phenyl, and in-... 

Zier B, Lenoir D, Lahaniatis ES, et al. 1991. Surface catalyzed halogenation-dehalogenation reactions of aromatic bromine compounds adsorbed on fly ash. Chemosphere. 22(12) 1121-1129. 

Notes Used for allylic and benzylic brominations (Wohl-Zieeler Reaction). With moist DMSO the reagent is useful for bromohydrin formation, providing trans addition of bromine and water. Can brominate alpha to carbonyl in carbonyl (carboxyl)-containing compounds. With DMF useful for aromatic bromination of activated aromatic rings, such as phenols, aromatic ethers, aniline derivatives and activated heterocyclic compounds. For similar chemistry, see also NBA, N-Bromoacetamide. 

Using 4-methylstyrene as comonomer results in a saturated backbone that cannot be vulcanized as such. However, the pendant benzene rings can be brominated. Thus pending aromatic bromine moieties can be used as the starting point for vulcanization. 

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