Bromine Cycles

Fig. 11 AFM topography of PE films a virgin, b after one cycle, c after three cycles, d after two bromination cycles...

Halons such as Halon-1211 (CF2BrCl) and Halon-1301 (CF3Br) are bromi-nated CFCs which are used as fire extinguishers. Like CFCs, Halons are chemically inert in the troposphere but photolyze in the stratosphere. Photolysis releases bromine atoms, which can remove stratospheric ozone in a cycle that is analogous to the chlorine-based cycle above. The bromine cycle can couple... 

Similarly, by adding metal oxide catalysts to the Ispra Mark 13 sulfnr-bromine cycle. General Atomics snlfnr-iodine cycle and sulfur-iron cycle (Reactions (56) to (64)), a number of new, modified metal sulfate based... 

Closing the bromine cycle, with initiatives of the European Brominated Flame Retardant Industry Panel (EBFRIP) in cooperation with the Bromine Science and Environmental Forum (BSEF), including a study prepared by ECN (Petten) on a two-stage pyrolysis/high-temperature gasification process. 

This reaction has been discussed as potentially important for bromine cycling on sulphate aerosol [22] and for autocatalytic bromine release from sea-salt aerosol [19,23]. Because of the large [CF]/[Br ] ratio in sea water, the forward reaction (1) is much more important than forward reaction (2). Although the hydrolysis reaction (-1) is faster than reaction (-2), this is of lesser significance because a substantial fraction of the BrCl will react with Br leading to autocatalytic Br activation ... 

Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

The cycle accounts for - 30 and 20—30% of the halogen-controUed loss for chlorine and bromine, respectively. 

Flame-Retardant Resins. Flame-retardant resins are formulated to conform to fire safety specifications developed for constmction as well as marine and electrical appHcations. Resins produced from halogenated intermediates (Table 5) are usually processed at lower temperatures (180°C) to prevent excessive discoloration. Dibromoneopentyl glycol [3296-90-0] (DBNPG) also requires glass-lined equipment due to its corrosive nature. Tetrabromophthahc anhydride (TBPA) and chlorendic anhydride (8) are formulated with ethylene glycols to maximize fiame-retardant properties reaction cycle times are about 12 h. Resins are also produced commercially by the in situ bromination of polyester resins derived from tetrahydrophthahc anhydride... 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

The mechanism of benzylic bromination is similar to that discussed in Section 10.4 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a concurrent reaction of HBr with NBS. 

McElroy et al (18) suggested a cycle in which chlorine and bromine are coupled in the destruction of ozone ... 

There are natural sources of brominated hydrocarbons as well as man-made sources, such as the "halons , which are used in fire extinguishers. Reaction 21 is very fast and generates Cl and Br atoms directly the cycle does not require a photolytic step. Although this cycle occurs with high efficiency, it is less important than the chlorine peroxide cycle because of the much smaller concentrations of bromine compounds in the stratosphere-parts per trillion vs. parts per billion for the chlorine compounds. 

This radical (2) captures molecular bromine to give the product and a new bromine radical so that the cycle continues. 

C. The Rheodyne Model 7010 injection valve, equipped with a 20-pl loop, was switched to injection at the apex of the sample band, as observed on the refractive index detector. The complex kinetics of the production of mono-, di-, and tri-brominated glycols is shown in Figure 14. Optimization of parameters such as the flow rate of acid resulted in a 15% reduction in batch cycle time and eliminated the need for manual analysis and intervention to obtain a desired endpoint composition. 

Sakai S-i, Hirai Y, Aizawa H, Ota S, Muroishi Y (2006) Emission inventory of deca-brominated diphenyl ether (DBDE) in Japan. J Mater Cycles Waste Manag 8(1 ) 56—62... 

Chlorination with CI2, CCI4, SO2CI2 or ClCH20Me also took place with predominant retention of configuration33. Reduction of the chlorogermanes with lithium aluminum hydride proceeded mainly with inversion to complete the Walden cycle (equation 15). Bromination, on the other hand, led to a nearly 1 1 mixture of cis and trans bromoger-manes. 

Figure 13.10 Catalytic cycle of bromination with HT-WD42 catalyst. (From Sels, B. F. et at, Nature, 400, 855,1999.)...

---