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In the case of hydrogen-containing compounds such as CH3Br, reaction (51) with OH also generates a bromine-containing organic radical. As is the case for the chlorine compounds, subsequent reactions of these radicals are expected ultimately to release the remaining bromine atoms as well. For example, the CH2Br radical formed from the CH3Br + OH reaction has been shown (Chen et al., 1995; Orlando et al., 1996) in laboratory studies in 1 atm air or 02/N2 mixtures, and at temperatures from 228 to 298 K, to undergo the following reactions, as might be anticipated from the mechanisms discussed in Chapter 6:

FIGURE 12.44 Organic bromine compounds observed at the earth's surface from 1988 to 1996 (adapted from Wamsley et al., 1998).

Chapter 4, the absorption cross sections of the major sources of bromine to the stratosphere, CH3Br,

CBrClF2 (Halon-1211), and

CBr3F

(Halon-1301), are substantial well out into the 250-nm region. The C-Br bond is even weaker than the C-Cl bond, as is generally the case for bromine compared to chlorine bonds (e.g., Lee et al., 1999a). For example, the C-Br bond strength is — 70 kcal mol-1 versus 85 kcal mol-1 for C-Cl and -110 kcal mol"1 for C-F; the C-Br bond breaks first:

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

CH2Br + 02

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