Methyl tert-butyl ether

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Image:MTBE.png Methyl tert-butyl ether (MTBE) is a chemical compound that is manufactured by the chemical reaction of methanol and isobutylene. MTBE is produced in very large quantities (more than 200,000 barrels per day in the United States in 1999) and is almost exclusively used as a fuel component in motor gasoline. It is one of a group of chemicals commonly known as oxygenates because they raise the oxygen content of gasoline. MTBE is a volatile, flammable and colorless liquid that is relatively soluble in water. When in groundwater MTBE moves more quickly than other fuel components. (California Air Resources Board, 2004). MTBE has a typical odor reminiscent of diethyl ether, leading to unpleasant taste and odor in water. MTBE is used in organic chemistry as a cheap solvent with properties comparable to diethyl ether but with a higher boiling point and lower solubility in water.

MTBE has been used in U.S. gasoline at low levels since 1979 to replace tetra-ethyl lead to increase its octane rating and help prevent engine knocking. Since 1992, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by Congress in Clean Air Act amendments; however, since 1999, in California and other locations MBTE has begun to be phased out because of its health risk related to widespread groundwater contamination (California Air Resources Board, 2004). The Energy Policy Act of 2005 drops the federal requirement for oxygen content in reformulated gasoline [1].

Oxygen helps gasoline burn more completely, reducing harmful tailpipe emissions from pre-1984 motor vehicles. In more modern vehicles, the emissions reduction is negligible. In one respect, the oxygen dilutes or displaces gasoline components such as aromatics (e.g. benzene) and sulfur. In another, oxygen optimizes the oxidation during combustion. Most refiners have chosen to use MTBE over other oxygenates primarily for its blending characteristics and for economic reasons.

In 1995 high levels of MTBE were unexpectedly discovered in the water wells of Santa Monica, California, and the U.S. Geological Survey reported detections Subsequent U.S. findings indicate tens of thousands of contaminated sites in water wells distributed across the country. The World Health Organization has made the finding that MTBE is a carcinogen and induces liver lesions in animals[2]. Although the extent of MTBE as a health risk is debatable, a transparent disadvantage is that MTBE ruins the taste of water even at low concentrations of 5–15 mg/liter. As a result, significant concentrations of MTBE in drinking water are immediately detectable. The leakage problem is partially attributed to the lack of effective regulations for underground storage tanks, but spillage from overfilling remain important upset scenarios. The EU does not predict the same problem in Europe. The MTBE concentrations used in the EU (usually 1.0–1.6%) and allowed (maximum 5%) in Europe are lower than in California³.

1 Health risks
2 Alternatives
3 Legislation
- 3.1 United States
4 Chemistry
5 References
6 External links

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Health risks

MTBE is known to be a carcinogen [3] and to cause kidney lesions in animals. As an ether, MTBE acts as an emulsifier, increasing the solubility of other, harmful components of gasoline (for example, the known carcinogen benzene). It thus may increase the risk of contamination by other compounds. MTBE is biodegraded very slowly, remaining in water for decades or more. In addition some MTBE degrades in blood to the associated alcohol, with a greatly increased residence time. The prolonged presence of this alcohol derivative is not fully understood.

Industry advocates of MTBE contend that it has little effect on humans, although its manufacturers did not test it for its effects on human health before introducing it as an additive. As of today, researchers have limited data about what the health effects from ingestion of MTBE. The Environmental Protection Agency (EPA) has concluded that available data are not adequate to estimate potential health risks of MTBE at low exposure levels in drinking water but that the data support the conclusion that MTBE is a potential human carcinogen at high doses. [4]

MTBE often ends up in drinking water, e.g. when fuel storage tanks leak near populated areas. Aside from health risks, MTBE negatively affects the taste and odor of drinking water, even at very low concentrations.

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Alternatives

Other compounds are available as oxygenate additives for gasoline, for example ethanol and related ethers, e.g. tert-amyl methyl ether (TAME). Reasons for using MTBE include economical considerations, as some of the production is obtained by adding methanol to isobutylene produced as a by-product of other processes. However, most MTBE facilities have to manufacture the methanol and isobutylene required to produce MTBE.

Ethanol has been advertised as a safe alternative by the agricultural interest groups in the USA and Europe. It is indeed nontoxic, but as a polar solvent, it drives off nonpolar hydrocarbons from the gasoline; volatile hydrocarbons are the main contributor to photochemical smog. EU's agricultural subsidies have produced an oversupply of wine, and the excess low-quality wine is being refined to ethanol fuel in Europe. However, the political stability of the supply is a major advantage for ethanol and other biofuels.

Advocates in the United States sometimes claim that gasoline manufacturers have been forced to add MTBE to gasoline by law. However, this is incorrect, since any oxygenate would fulfill the law.

In 2003, California was the first state to start replacing the MTBE with ethanol. Several other states started switching soon afterward. Another option is to add nothing to straight-run gasoline except antiknock compounds and detergents.

Higher quality gasoline is also an alternative. Iso-octane itself is used. MTBE plants can be retrofitted to produce iso-octane from isobutylene, which is a lighter-than-gasoline hydrocarbon and thus more difficult to sell. [5] [6]

In the long run, diesel fuel is also an alternative, although it requires a major switchover to diesel-run cars. There are several varieties of biodiesel; both oxygen-containing methyl ethers and no-oxygen alkyl biodiesels are available.

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Legislation

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United States

The clean-up of all MTBE in the U.S. in the form of groundwater and soil contamination is estimated to cost as much as $140 billion, including removing the compound from aquifers and municipal water supplies and replacing leaky underground oil tanks. Some controversy centers on who will pay the costs of this remediation.

Recent state laws have been passed to ban MTBE in certain areas. California and New York, which together accounted for 40% of U.S. MTBE consumption, banned the chemical starting January 1, 2004, and as of September, 2005, twenty five states had signed legislation banning MTBE. (A table of state by state information, as of 2002, is available here at the U.S. Department of Energy website.

In the year 2000, the U.S. EPA drafted plans to phase out the use of MTBE nationwide over four years. Upon taking office, the Bush administration cancelled those plans. In April of 2002, a California jury found several oil companies guilty of irresponsibly manufacturing and distributing MTBE, stating that the companies acted with malice in failing to warn customers about the dangers of MTBE contamination. There are hundreds of other lawsuits currently active regarding the compound. An amendment to provide blanket immunity from MTBE-related lawsuits was inserted into the House version of the 2003 Energy Bill, which did not pass.

The Energy Policy Act of 2005, passed in the House on April 21, 2005, did not include a provision for shielding MTBE manufacturers from water contamination lawsuits. This provision was thought by some to be a priority of Tom DeLay and Rep. Joe Barton, chairman of the Energy and Commerce Committee. [7] This bill also includes a provision that gives MTBE makers, including some major oil companies, $2 billion in transition assistance as MTBE is phased out over the next nine years. [8] Due to opposition in the Senate, the conference report dropped all MTBE provisions. The final bill was passed by both houses and signed into law by President Bush. [9] The lack of MTBE liability protection is resulting in a switchover to the use of ethanol as a gasoline additive, which is in limited supply in April 2006. Some traders and consumer advocates are blaming this for an increase in gasoline prices. [10]

Certain patents important in the manufacture of MTBE are not held by American companies; for example, United States patent 5536886, Process for preparing alkyl ethers [11], is owned by the Finnish company Neste. (The same corporation also went on to patent the replacement of the MTBE process, an octane production process trademarked NExOCTANE.)

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Chemistry

MTBE has a boiling point of 55 - 56 °C and forms azeotropes with water (52.6 °C) and methanol (51.3 °C).

Being an ether, MTBE is a Lewis base. However, unlike other ethers such as diethyl ether or THF, it is does not coordinate well enough to magnesium to be used for making Grignard reagents. The tert-butyl group is easily cleaved off under strongly acidic conditions (forming a moderately stable carbocation), particularly if heated (isobutylene is lost), something which can limit the use of MTBE as a solvent. However it possesses one distinct advantage over most ethers- it has a much lower tendency to form explosive organic peroxides than most ethers. Opened bottles of diethyl ether or THF can build up dangerous levels of these peroxides in months, whereas samples of MTBE are usually safe for years - however they should still be tested periodically.

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References

California Air Resources Board, Cleaner Burning Gasoline without MTBE, updated August 9 2004
Harper's Magazine, August 2004, pp 73-64
European Commission. MTBE and the Requirements for Underground Storage Tank Construction and Operation in Member States.
UKPIA on MTBE. [12]

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