| Methyl tert-butyl ether | |
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2-Methoxy-2-methylpropane |
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Other names
Methyl tertiary-butyl ether |
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| Identifiers | |
| CAS number | 1634-04-4 |
| PubChem | 15413 |
| ChemSpider | 14672 |
| KEGG | C11344 |
| ChEBI | CHEBI:27642 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C5H12O |
| Molar mass | 88.15 g/mol |
| Density | 0.7404 g/cm³ |
| Melting point |
-109 °C, 164 K, -164 °F |
| Boiling point |
55.2 °C, 328 K, 131 °F |
| Hazards | |
| NFPA 704 |
3
1
0
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| Flash point | -10 °C |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Methyl tert-butyl ether, also known as methyl tertiary butyl ether and MTBE, is an organic compound with molecular formula (CH3)3COCH3. MTBE is a volatile, flammable, and colorless liquid that is immiscible with water. It has a minty odor vaguely reminiscent of diethyl ether, leading to unpleasant taste and odor in water. MTBE is a gasoline additive, used as an oxygenate to raise the octane number. Its use is controversial in the US and declining in that country in part because of its occurrence in groundwater and legislation favoring ethanol. Worldwide production of MTBE has been constant at about 18 million tons/y (2005) owing to growth in Asian markets which are less subject to ethanol subsidies.[1]
Contents |
MTBE is manufactured via the chemical reaction of methanol and isobutylene. Methanol is derived from natural gas, and isobutylene is derived from butane obtained from crude oil or natural gas, thus MTBE is derived from fossil fuels. In the United States, it was produced in very large quantities (more than 200,000 barrels (32,000 m3) per day in 1999) during its use as a fuel additive.
MTBE is almost exclusively used as a fuel component in fuel for gasoline engines. It is one of a group of chemicals commonly known as oxygenates because they raise the oxygen content of gasoline.
In the US it has been used in gasoline at low levels since 1979 to replace tetraethyl lead and to increase its octane rating helping prevent engine knocking. Oxygenates help gasoline burn more completely, reducing tailpipe emissions from pre-1984 motor vehicles; dilutes or displaces gasoline components such as aromatics (e.g., benzene) and sulfur; and optimizes the oxidation during combustion. Most refiners chose MTBE over other oxygenates primarily for its blending characteristics and low cost.
Other compounds are available as additives for gasoline including ethanol and some ethers such as tert-amyl methyl ether (TAME).
Ethanol has been advertised as a safe alternative by the agricultural interest groups in the USA and Europe. Its lack of toxicity is not different from MTBE, but as a polar solvent, it drives off nonpolar hydrocarbons from the gasoline, a problem that MTBE does not cause. Some volatile hydrocarbons in gasoline vapors are carcinogens, and gasoline vapors produce photochemical smog. Ethanol's higher cost requires government intervention in the form of subsidies or mandated usage to be competitive. In 2003, California was the first U.S. state to start replacing MTBE with ethanol. Several other states started switching soon thereafter.
Advocates of both sides of the debate in the United States sometimes claim that gasoline manufacturers have been forced to add MTBE to gasoline by law. It might be more correct to say they have been induced to do so, although any oxygenate would fulfill the law.
An alternative to straight ethanol is the related ether ETBE, which is manufactured from ethanol and isobutene. Its performance as an additive is similar to MTBE, but due to the higher price of ethanol compared to methanol, it is more expensive.
Higher quality gasoline is also an alternative, i.e., so that additives such as MTBE are unnecessary. Iso-octane itself is used. MTBE plants can be retrofitted to produce iso-octane from isobutylene.[2],[3]
As a solvent, MTBE possesses one distinct advantage over most ethers - it has a reduced tendency to form explosive organic peroxides. 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 (but they should still be tested periodically). For this reason (as well as its higher boiling point), MTBE is used as a solvent extensively in industry, where safety concerns and regulations make working with diethyl ether, THF, or other ethers much more difficult and expensive. However, despite the popularity of MTBE in industrial settings, it is rarely used as a solvent in academia with some exceptions.[4]
MTBE forms azeotropes with water (52.6 °C; 96.5% MTBE).[5] and methanol (51.3 °C; 68.6% MTBE).[6]
Although an ether, MTBE is a poor Lewis base and does not support formation of Grignard reagents. It is also unstable toward strong acids. It reacts dangerously with bromine.[7]
MTBE gives water an unpleasant taste at very low concentrations, and thus can render large quantities of groundwater non-potable. MTBE is often introduced into water-supply aquifers by leaking underground storage tanks (USTs) at gasoline stations or by gasoline containing MTBE spilled onto the ground. The higher water solubility and persistence of MTBE cause it to travel faster and farther than many other components of gasoline when released into an aquifer.[8][9]
MTBE is biodegraded by the action of bacteria. In the proper type of bioreactor, such as a fluidized bed bioreactor, MTBE can be rapidly and economically removed from water to undetectable levels. Activated carbon produced from coconut shells and optimized for MTBE adsorption can also reduce MTBE to undetectable levels.[10]
According to the IARC, a cancer research agency of the World Health Organization, MTBE is not classified as a human carcinogen. MTBE can be tasted in water at concentrations of 5 – 15 µg/l.[11]
As of 2007, researchers have limited data about the health effects of ingestion of MTBE. The United States Environmental Protection Agency (EPA) has concluded that available data are inadequate to quantify 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.[12]
MTBE removal from groundwater and soil contamination in the U.S. is estimated to cost from $1 billion[13] to $30 billion,[14] including removing the compound from aquifers and municipal water supplies and replacing leaky underground oil tanks. In one case, the cost to oil companies to clean up the MTBE in wells belonging to Santa Monica is estimated to exceed $200 million.[15] In another case, the City of New York estimated a $250 million cost for cleanup of a single wellfield in Queens, NY.[16]
MTBE is banned in the US states of California and New York. 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 at the United States Department of Energy website.[17])
In 2000, the U.S. EPA drafted plans to phase out the use of MTBE nationwide over four years. As of fall 2006, hundreds of lawsuits are still pending regarding MTBE contamination of public and private drinking water supplies.
The Energy Policy Act of 2005, passed in the U.S. House of Representatives, did not include a provision for shielding MTBE manufacturers from water contamination lawsuits. This provision was first proposed in 2003 and had been thought by some to be a priority of Tom DeLay and Rep. Joe Barton, then chairman of the Energy and Commerce Committee.[18] This bill did include 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.[19] Due to opposition in the Senate,[20] the conference report dropped all MTBE provisions. The final bill was passed by both houses and signed into law by President Bush.[21] The lack of MTBE liability protection is resulting in a switchover to the use of ethanol as a gasoline additive. Some traders and consumer advocates are blaming this for an increase in gasoline prices.[22]
The U.S. EPA currently lists MTBE as a candidate for a maximum contaminant level (MCL) in drinking water.[23] MCLs are determined by the EPA using toxicity data.
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