Bromomethane

The chemical compound bromomethane, commonly known as methyl bromide, is an organic halogen compound with formula CH₃Br. It is a colorless, nonflammable gas with no distinctive smell. Its chemical properties are quite similar to those of chloromethane. Trade names for bromomethane include Embafume and Terabol.

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Origin

Bromomethane originates from both natural and human sources. It occurs naturally in the ocean, where it is probably formed by algae and kelp. It is also produced by certain terrestrial plants, such as members of the Brassicaceae family. It is manufactured for agricultural and industrial use by reacting methanol with hydrobromic acid.

Uses

Until its production and use was curtailed by the Montreal Protocol, it was widely used as a soil sterilant, mainly for production of seed but also for some crops such as strawberries. In commercial large-scale monoculture seed production, unlike crop production, it is of vital importance to avoid contaminating the crop with off-type seed of the same species. Therefore, selective herbicides cannot be used. While bromomethane is dangerous to use, it is considerably safer and more effective than the few other soil sterilants available. Its loss to the seed industry has resulted in changes to cultural practices, with increased reliance on mechanical rogueing and fallow seasons.

Bromomethane was also used as a general-purpose fumigant to kill a variety of pests including rats and insects. Bromomethane has poor fungicidal properties. (Bromomethane is the preferred fumigant for ISPM number 15, regulations when exporting wooden packaging to certain countries). It is also a precursor in the manufacture of other chemicals as a methylation agent, and has been used as a solvent to extract oil from seeds and wool.

While the Montreal Protocol has severely restricted the use of bromomethane internationally, the United States has successfully pushed for critical-use exemptions of the chemical. In 2004, the most recent year with available data, over 7 million pounds of bromomethane were applied to California fields, according to pesticide use statistics compiled by the California Department of Pesticide Regulation.

Ozone depletion

Bromomethane is on the list of banned ozone-depleting substances of the Montreal Protocol. Because bromine is 60 times[1] more destructive to ozone than chlorine, even small amounts of bromomethane cause considerable damage to the ozone layer. In 2005 and 2006, however, it was granted a critical use exemption under the Montreal Protocol.

Controversy

Bromomethane is used to prepare golf courses and sod for golf courses and elsewhere, particularly to control Bermuda grass. The Montreal Protocol stipulates that bromomethane use be phased out. The Bush Administration has adopted exceptions "to prevent market disruptions".

Health effects

If inhaled in high concentration for a short period, it produces headaches, dizziness, nausea, vomiting and weakness; this may be followed by mental excitement, convulsions and even acute mania. More prolonged inhalation of lower concentrations may cause bronchitis and pneumonia.^[2]

The liquid burns the skin, producing itching and reddening, then blisters several hours after contact. Both liquid and vapour severely damage the eyes.^[2]

Exposure levels leading to death vary from 1,600 to 60,000 ppm, depending on the duration of exposure.

The respiratory, kidney, and neurologic effects are of the greatest concern to people. No cases of severe effects on the nervous system from long-term exposure to low levels have been noted in people, but studies in rabbits and monkeys have shown moderate to severe injury.

Sources and sinks

Sources of CH₃Br include oceanic production, biomass burning, leaded fuel combustion, plant and marsh emissions, and fumigation of soils, durable goods, perishables, and structures. Sinks include photochemical decomposition in the atmosphere (reaction with hydroxyl radicals (OH) and photolysis at higher altitudes), loss to soils, chemical and biological degradation in the ocean, and uptake by green plants.

See also

• Haloalkane
• Halomethane
• Fluoromethane
• Chloromethane
• Iodomethane
• Dibromomethane
• Bromoform
• Tetrabromomethane

References

1. ^ Merck Index, 11th Edition, 5951.
2. ^ ^{a b} Muir, GD (ed.) 1971, Hazards in the Chemical Laboratory, The Royal Institute of Chemistry, London.