Methyl Bromide CH3Br

The ocean acts as both a source and sink for CH3Br the net oceanic flux to the atmosphere is negative, estimated as —21 Ggyr-1 (Table 2.17). Oceanic uptake of CH3Br is currently estimated at 77Ggyr . Combined with atmospheric OH reaction and photolysis (86Ggyr ) and soil uptake (46.8Ggyr-1), this leads to a total estimated 

TABLE 2.17 Global Budget of Methyl Bromide (CH3Br) 

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Some HAPs impact not only the troposphere but also the stratosphere. The most obvious example is highly toxic methyl bromide, CH3Br, used as a soil fumigant as well as for treatment of buildings for termites. As discussed in Chapter 12, this is a significant source of stratospheric bromine and hence contributes to stratospheric ozone depletion. Its continued use has been controversial and is being phased out (e.g., see Thomas, 1996 Ristaino and Thomas, 1997 and Duafala, 1996). 

Consider the reversible transformation of the soil fumigant methyl bromide (CH3Br) to methyl chloride (CH3C1) in aqueous solution (a nucleophilic substitution reaction, see Chapter 13) ... 

Experiments show that doubling the concentration of methyl bromide, [CH3Br], doubles the rate of reaction. Doubling the concentration of hydroxide ion, [ OH], also doubles the rate. Thus, the rate is proportional to both [CH3Br] and [ OH], so the rate equation has the following form ... 

Although they are the most abundant ozone-depleting substance, CFCs are not the only culprits. Other chemicals that damage the ozone layer include methyl bromide, CH3Br, carbon tetrachloride, CCI4, and halons such as carbon trifLuorobromide, CF3Br. 

Methyl Bromide CH3Br Fumigant insecticide and nematicide Animals/Plants. Metabolism not totally elucidated inorganic bromide ion is formed... 

Methyl bromide, CH3Br (boiling point 4.5°C)—Methyl bromide is a gaseous fumigant used both in the soil and on commodities. In the soil, it is used for preplant soil fumigation with chloropicrin to control nematodes, insects, and fungi. Its use was banned in 2005, except for "critical use" exemptions, because it is an ozone depleter. Methyl bromide has an oral LD50 in rats of 100 mg/kg. 

Methyl chloride CH3CI5138 (calc.) 5 5166 Methyl bromide CH3Br 5715 7 Methylamine CH3NH2 6169 8 Dimethylamine (CH3)2NH 6330 9 Mercaptan CH3SH 587270 Dimethyl sulphide (CH3)2S 668871 Formic acid H.COOH 531372 Ether (C2Hs)20 514173... 

Bromine is also important in the manufacture of pesticides, chemicals used to kill pests. For many years, one of the most popular bromine-based pesticides was methyl bromide (CH3Br). Methyl bromide is sprayed on the surface or injected directly into the ground to control pests. 

Bromine is used in the production of silver bromide for light-sensitive eyeglasses and photographic film in the production of sodium bromide, a mild sedative and in methyl bromide, CH3Br, a soil fumigant that contributes to the destruction of the ozone layer. 

Draw a reasonable Lewis structure for methyl bromide, CH3Br, which is an ozone depleting gas used as a fumigant. 

As with methyl chloride, large quantities of methyl bromide, CH3Br, are also produced naturally and some of it eventually reaches the stratosphere, where it decomposes photochemically to yield atomic bromine. Like chlorine, bromine atoms can destroy ozone by the mechanism shown in reactions (5) and (6). These reactions are as follows ... 

While there is a total chlorine compound mixing ratio in the stratosphere of approximately 3400 ppt, that for bromine gases is only 20 ppt (Figure 5.1). Remarkably, with 150 times less abundance than chlorine, bromine is approximately as important as chlorine in overall ozone destruction. Methyl bromide (CH3Br) constitutes about half the source of bromine in the stratosphere (see Section 2.5). The H-atom-containing bromine compounds, CH3Br, CH2Br2, and CHBr, release their Br almost immediately on entry into the stratosphere the... 

Figure 10.7 A mass spectrum of bromomethane (or methyl bromide), CH3Br, showing the characteristic 1 1 ratio of the M/(M + 2) peaks for a compound containing a single Br atom.

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