Brominated Aromatic Hydrocarbons

The structural range of industrially important representatives of these groups is enormous, and includes chlorobenzenes (solvents), polychlorinated biphenyls (PCBs) (hydraulic and insulating fluids), and polybrominated biphenyls and diphenyl ethers (flame retardants). There is widespread concern over both the persistence and the potential toxicity of all these compounds, and sites that have become contaminated during their production represent a threat both to the environment and to human health. Pathways for the aerobic bacterial degradation of chlorobenzenes and chlorobiphe-nyls, and their brominated analogs have been discussed in Chapter 9, Part 1. 

Although the possibly incompatible pathways for the degradation of chlorobenzene and toluene has been noted, a strain of Ralstonia sp. from a site contaminated with chlorobenzene contained the genes for both chlorocatechol degradation and the dioxygenase system for the degradation of benzene/toluene (van der Meer et al. 1998). The evolution of this strain resulted in a natural lowering 

The anaerobic dechlorination of hexachlorobenzene has been described in anaerobic mixed cultures supplemented with electron donors including lactate, ethanol, or glucose (Holliger et al. 1992) successive and partial dechlorination produced 1,2,4- and 1,3,5-trichlorobenzenes, while the 1,2,3-trichlorobenzene was further dechlorinated. The partial dechlorination of 1,2,3,4-tetra-, 1,2,3,5-tetra-, and pentachlorobenzene has been examined in a methanogenic mixed culture using lactate as electron donor (Middeldorp et al. 1997), and sterile Rhine River sand was needed to maintain dechlorination activity for unresolved reasons. 

There has been renewed interest in DDT in spite of the fact that its use has been banned in many countries for several years. The degradation of DDT has been discussed in Chapter 7, Part 3, and attention has been directed to the apparently recalcitrant DDE (Quensen et al. 1998) and DDA (Heberer and Diinnbier 1999). Uptake and metabolism of p,p -DDT and the isomeric o,p -DDT have been examined in a range of plants, and the results illustrate a number of important issues. 

There is an enormous literature on the degradation of PCBs, and it is not intended to attempt to cover this. The degradation of PCBs is exacerbated not only by the number of congeners, but by the fact that heavily ortho-substituted congeners occur in enantiomeric forms as a result of restricted 

---

An interesting new homogeneous catalytic process was developed by Buijs [184] for the reductive dehalogenation of polychlorinated and -brominated aromatic hydrocarbons and ethers. Cu(I) benzoate catalyzed the reaction under Dow-Phenol conditions in the absence of air at 235 °C (Eq. (8)) ... 

The Fate of Highly Brominated Aromatic Hydrocarbons in Fish... 

Zitko V. 1979. The fate of highly brominated aromatic hydrocarbons in fish. ACS Symp Ser 99 177-182. 

Zitko, V. 1979. The fate of highly brominated aromatic hydrocarbons in fish. In M.A.Q. Khan, J.J. Lech, and J.J. Menn (Eds.), Pesticide and xenobiotic metabolism in aquatic organisms. American Chemical Society Symposium series No. 99, American Chemical Society, Washington, D.C., pp. 177-182. 

Condensed aromatic hydrocarbons may be brominated directly with bromine in the presence of a solvent, such as carbon tetrachloride ... 

The bromination of aromatic hydrocarbons can occur either in a side chain or on the ring, depending on conditions. In the presence of sunlight aLkylben2enes are brominated predominately in the side chain (24). 

Impurities can sometimes be removed by conversion to derivatives under conditions where the major component does not react or reacts much more slowly. For example, normal (straight-chain) paraffins can be freed from unsaturated and branched-chain components by taking advantage of the greater reactivity of the latter with chlorosulfonic acid or bromine. Similarly, the preferential nitration of aromatic hydrocarbons can be used to remove e.g. benzene or toluene from cyclohexane by shaking for several hours with a mixture of concentrated nitric acid (25%), sulfuric acid (58%), and water (17%). 

Although, in most cases, iodine is a fairly inert halogen (in contrast to bromine) and does not normally react with the substances that have been chromatographed there are, nevertheless, examples where chemical changes have been observed. Oxidations can evidently take place (e.g. aromatic hydrocarbons and isoquinoline alkaloids [252,253]) and additions and substitutions have also been observed. Pale zones then appear on a brown background. 

If chlorine and bromine are allowed to act upon an aromatic hydrocarbon like toluene, which has a side-chain, substitution may occur in the nucleus or the side-chain, according to the conditions. Generally speaking, in the cold and in presence of a halogen carrier, nuclear substitution occurs, Irut at a high temperatuie the halogen passes into the side-chain (see Piep. 

Robertson et al.261 measured rates of bromination of some aromatic hydrocarbons in acetic acid containing sodium acetate (to eliminate protonation of the aromatic by liberated hydrogen bromide) and lithium bromide (to reduce the rate to a measurable velocity ) at 25 °C, the second-order rate coefficients for 3-nitro-N,N-dimethylaniline and anisole being 14.2 and 0.016 respectively the former compound was thus stated to be about 1012 times as reactive as benzene (though no measurement of the latter rate coefficient, inferred to be 1.33 xlO-11, could be found in the literature) and this large rate spread gives one further indication of the unreactive nature of the electrophile. Other rates relative to benzene were ... 

Table 2. Bromination of Polycyclic Aromatic Hydrocarbons with CuBr2/Al2033)... 

Action of bromine on acetylene, action of nitric acid on aromatic hydrocarbons. The accidents involving both reactions are very specific.They are the result of application of an unsuitable temperature. If it is too low, the reaction is too slow and causes an accumulation of reagent that is not converted and whose concentration increases, causing an acceleration of the reaction, i.e. a rise in temperature. The temperature then becomes too high and causes a more or less violent speeding-up of the reaction. 

Two examples have been given for the temperature/concentration action in the earlier reference to thermal effects (actions of bromine on acetylene and nitric acid on aromatic hydrocarbons). 

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

Horii Y, Ok G, Ohura T, Kannan K (2008) Occurrence and profiles of chlorinated and brominated polycyclic aromatic hydrocarbons in waste incinerators. Environ Sci Technol 42(6) 1904—1909. doi 10.1021/es703001f... 

Horii Y, Khim JS, Higley EB, Giesy JP, Ohura T, Kannan K (2009) Relative potencies of individual chlorinated and brominated polycyclic aromatic hydrocarbons for induction of aryl hydrocarbon receptor-mediated responses. Environ Sci Technol 43(6) 2159-2165. doi 10.102 l/es8030402... 

Whereas pure NBS effects side chain bromination of aromatic hydrocarbons, aged material effects bromination of aromatic ring. 

Bierbach, A., I. Barnes, and K. H. Becker, Rate Coefficients for the Gas-Phase Reactions of Bromine Radicals with a Series of Alkenes, Dienes, and Aromatic Hydrocarbons at 298 + 2 K, lnt.. J. Chem. Kinet., 28, 565-577 (1996). 

---