Fumigant depletion

Methyl bromide has been identified as an ozone-depleting substance and is being gradually removed from world markets. Current legislation and plans call for the elimination of methyl bromide in most industrial countries by 2005, with possible exemptions for quarantine (UNEP, 1996). Currently there is an extensive search worldwide for products that are alternatives to methyl bromide (Kawakami, 1999). These alternatives are broadly defined and include components of management plans such as sanitation, monitoring, contact insecticides, heat treatments, and modified atmospheres, in addition to new fumigants (Batchelor, 1998). 

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). 

One of the most effective fumigants is methyl bromide. It essentially sterilizes soil when applied under a ground covering, because it kills insects, nematodes, and weed seed but also is used to fumigate warehouses. Overexposure to this compound causes respiratory distress, cardiac arrest, and central nervous effects. The inhalation LC50 is 0.06 mg/L (15 min) of air (rat) and 7900 ppm (1.5 h) (human). Methyl bromide has been classified as an ozone depleter under the Clean Air Act and is due to be phased out of use by 2005. 

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. 

As a soil fumigant methyl bromide leaves no toxic residue in soils. The volatile gas rises into the atmosphere. Methyl bromide is an ozone-depleting substance. Although methyl bromide is very soluble in water, its high vapor pressure in various soil types indicates a low tendency to adsorb to soils and rapid evaporation. Methyl bromide has a half-life in air estimated from 0.3 to 1.6 years. Degradation is primarily due to photolysis. In soils, the half-life is 0.2-0.5 days. In water, a half-life of 3 h was calculated. 

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

Products and Uses Banned from all uses in 1996 due to ozone depletion concerns. Previously found in spot removers, fire extinguishers and dry-cleaning products as a fumigant, solvent, and pesticide. 

Although useful in agriculture as a soil fumigant, methyl bromide is an ozone-depleting chemical, and its production is being phased out. The industrial preparation of methyl bromide is from methanol, by reaction with hydrogen bromide. Write a mechanism for this reaction and classify it as S l or 8 2. 

Irradiation of herbs and spices is widely practiced because many are grown and harvested by small landowners in tropical countries where insects and disease organisms are prevalent Previously used chemical fumigants, such as ethylene oxide, are banned because they deplete atmospheric ozone. 

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