Bromine chemistry

This symposium addressed several important issues in bromine chemistry. A major part has been devoted to stereochemistry and mechanism of electrophilic bromination of olefins. Other topics included new selective methods of bromination and oxybromination, brominations in presence of solid supports and catalysts, organobromine compounds as synthons, recent developments in brominated fire retardants and toxicological and environmental aspects of brominated compounds. 

A third factor comes into play in bromine chemistry, which is that atmospheric solutions containing bromide and chloride are most typically formed from seawater. Wave action generates small airborne droplets of seawater, which thus initially contain the elements in the ratios found in seawater. The molar ratio of Br- to Cl- is 1 650. However, despite the relatively small amounts of bromide relative to chloride, it plays a disproportionate role because of its reactivity and because its chemistry is closely intertwined with chloride ion chemistry. Table 8.16, for example, shows some of the interhalogen reactions of bromide and chloride. It can be seen that the chemistry preferentially generates Br2 rather than Cl2. 

TABLE 8.15 Some Aqueous-Phase Bromine Chemistry... 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

In Section C.3, we saw that gas-phase chlorine chemistry in the stratosphere is inextricably intertwined with bromine chemistry. Because of this close interrelationship, altering the concentrations of only one of the halogens (e.g., through controls) may not have the proportional quantitative result that might be initially expected. We explore in this section in more detail the role of brominated organics in stratospheric ozone destruction and the interrelationship with chlorine chemistry. 

FIGURE 12.45 Schematic of gas-phase and heterogeneous bromine chemistry in the stratosphere. The heavier dark lines show the heterogeneous (het) chemistry. 

However, although as much as 50% of the loss of O-, could be attributed to bromine chemistry at a Bry concentration of 25 ppt, a reduction in bromine did not give a proportional change in the total destruction of 03. Figure 12.46 shows the predicted cumulative O, loss as a function of the Br concentration as bromine decreases, the contribution of the BrO-CIO cycle decreases as well. However, the net effect on total ozone loss is quite small because near-total ozone destruction occurs even without a significant contribution from BrO. 

Thus, the effect of heterogeneous bromine chemistry is primarily to amplify the chlorine-catalyzed destruction of ozone through the more rapid conversion of the reservoir species HC1 back into active forms of chlorine (Lary et al., 1996 Tie and Brasseur, 1996). This becomes particularly important under conditions of enhanced aerosol particles, e.g., after major volcanic eruptions. 

Erie, F., A. Grendel, D. Perner, U. Platt, and K. Pfeilsticker, Evidence of Heterogeneous Bromine Chemistry on Cold Stratospheric Sulphate Aerosols, Geophys. Res. Lett., 25, 4329-4332 (1998). 

Lee, T. J., S. Parthiban, and M. Head-Gordon, Accurate Calculations on Excited States New Theories Applied to the -X, -XO, and -X02 (X = Cl and Br) Chromophores and Implications for Stratospheric Bromine Chemistry, Spectrochim. Acta A, 55, 561-574 (1999b). 

These data also demonstrate the impact of bromine chemistry on the stratosphere (see Chapter 12.D). The initial ODP for methyl bromide is 15, due primarily to the large a factor associated with bromine chemistry. However, since it is removed by reaction with OH in the troposphere as well as by other processes such as hydrolysis in the oceans and uptake by soils and foliage (see Chapter 12.D), it has a short atmospheric lifetime of 1.3 years and hence the ODP decreases rapidly with time, toward a long-term steady-state value. 

Abbatt, J.P.D., and Nowak, J.B. (1997) Heteogeneous interactions of HBr and HOC1 with cold sulfuric add solutions implications for Arctic boundary layer bromine chemistry, J. Phys. Chem. A 101, 2131-2137. 

This process is a lower-cost method of providing bromine chemistry to large cooling systems, where conventional BCDMH or similar chemistry is cost-prohibitive (see Section 6.2.7 on bromine chemistry). In addition, bleach is often used as a raw material, with sodium chlorite, for the on-site production of chlorine dioxide (see Section 6.2.6). 

There are derivations of this chemistry now available that provide an isocyanurate/sodium bromide blend in tablet form. This niche product is often suitable for cooling systems that suffer from small, but regular, process leaks and other contaminating, oxidizable material in the cooling water (see Section 6.2.7 on bromine chemistry). 

During the last 20 years, the use of programs based on bromine chemistry has become standard practice. Bromine/chlorine-based biocides have become internationally accepted as mainstream products for all types of cooling water systems, brewery pasteurizers, and many other... 

Bromine chemistry has gained significant ground because it offers several advantages over those of chlorine. 

Bromine retains a high biocidal activity (in the form of undissociated hypobromous acid) in alkaline cooling water environments. Bromine chemistry will permit a circulating water pH of at least 1.0 unit higher, compared with chlorine, at any acceptable percentage of undissociated hypohalous acid. 

First, bromine chemistry works The combination of high pH activity, high bromamine activity, and a fast kill rate has given it a significant edge over chlorine chemistry. 

The most recent enhancement of bromine biocide chemistry is the development of a stabilized single liquid bromine, under the brand Stabrex , from Nalco (now owned by Degremont/Suez Lyonnaise Des Eaux). The product is most likely a sodium hypochlorite/sodium bromide mix, stabilized with sulfamic acid. It contains 14% available halogen as Br (6% as Cl), and is stable for several months. Stabrex probably has some benefits over other bromine chemistries, but is also a premium priced product. 

As yet, little is available in terms of properly documented iodine treatment case histories, but it is difficult to see the advantages of iodine chemistry over bromine chemistry for most general-purpose cooling systems. 

Conley, Jeffery C. Puzig, Edward H. Bromine Chemistry An Alternative to Dechlorination in Cooling Water and Wastewater Disinfection. Waterline, UK (reprinted from the proceedings of the 48th International Water Conference, USA), November 2-4, 1987. 

Yang X, Cox RA, Warwick NJ, Pyle JA, Carver GD, O Connor FM, Savage NH (2005) Tropospheric Bromine Chemistry and Its Impact on Ozone A Model Study. J Geophys Res 110 D23311... 

An important aspect of stratospheric bromine chemistry is the possibility of synergistic interactions between bromine and chlorine cycles via the following reaction,... 

Strong bromine activation and ozone depletion have also been recorded at salt lakes like the Dead Sea and the Great Salt Lake. Besides the regions where the impact of bromine chemistry is clearly noted, there are indications that halogens can also play a significant role in the chemistry of the marine boundary layer and perhaps even the free... 

Stratospheric Bromine Chemistry Insights from Computational Studies (S. Guha J. S. Francisco)... 

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