Methyl tertiary-butyl ether
(MTBE) is a chemical compound with molecular formula C5H12O. MTBE is a volatile, flammable and colorless liquid that is highly soluble in water. MTBE has a minty odour vaguely reminiscent of diethyl ether, leading to unpleasant taste and odour in water. MTBE is a gasoline additive, used as an oxygenate and to raise the octane number, although its use has declined in the United States in response to environmental and health concerns. It has been found to easily pollute large quantities of groundwater when gasoline with MTBE is spilled or leaked at gas stations. MTBE is also used in organic chemistry as a relatively inexpensive solvent with properties comparable to diethyl ether but with a higher boiling point and lower solubility in water. It is also used medically to dissolve gallstones.
Production
MTBE is manufactured by the chemical reaction of methanol and isobutylene. Methanol is a derived from natural gas, and isobutylene is made from crude oil or natural gas, thus MTBE, as used in motor gasoline, is a fossil fuel. In the United States, it was produced in very large quantities (more than 200,000 barrels per day in the United States in 1999) when it was being used widely as a fuel additive there. Because of widespread releases of MTBE-containing gasoline from Underground Storage Tanks all over the US, various jurisdictions banned the use of MTBE and production was reduced. MTBE contamination in drinking water aquifers is a serious concern in many states (most famous cases are Lake Tahoe and Santa Monica). By late 2006, most American gasoline retailers had ceased using MTBE as an oxygenate, and accordingly, US production had declined. Similarly, lack of growth or even decline of MTBE production has been seen in Western Europe. This not because of environmental reasons; on the contrary, it is because the alternative ethanol-derived ether ETBE has been given more favorable tax treatment. Nevertheless, in other parts of the world, which account for about a half of 2004 production, the use of MTBE will continue and even grow
Physical properties
MTBE forms azeotropes with water (52.6 °C) and methanol (51.3 °C).
Uses
MTBE 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.
As anti-knocking agent
In the US it has been used in gasoline at low levels since 1979 to replace tetra-ethyl lead to increase its octane rating and help prevent engine knocking. Oxygen helps gasoline burn more completely, reducing 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. It is produced from natural gas, which is less expensive than oil.
Since 1992, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by the United States Congress in Clean Air Act amendments; however, since 1999, in California and other locations MTBE has begun to be phased out because of groundwater contamination (California Air Resources Board, 2004). Due to its higher solubility in water MTBE moves more quickly than other fuel components (California Air Resources Board, 2004). The Energy Policy Act of 2005 reduces the federal requirement for oxygen content in reformulated gasoline[2].
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.[3] Subsequent U.S. findings indicate tens of thousands of contaminated sites in water wells distributed across the country. As per toxicity alone, MTBE is not classified as a hazard for the environment. The maximum contaminant level of MTBE in drinking water has not yet been established by the EPA. The leakage problem is partially attributed to the lack of effective regulations for underground storage tanks, but spillage from overfilling remains an important upset scenario. As an ingredient in unleaded gasoline, MTBE is the most soluble part. When dissolved in groundwater, MTBE will lead the contaminant plume with the remaining components such as benzene, toluene, etc. to follow. Thus the discovery of MTBE in public groundwater wells indicates that the contaminant source was a gasoline release. The MTBE concentrations used in the EU (usually 1.0–1.6%) and allowed (maximum 5%) in Europe are lower than in California
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 economic 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. 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. Volatile hydrocarbons from gasoline are known (and severe) carcinogens and 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. This gives rise to political motives for supporting ethanol over MTBE. However, the political stability of the supply is a major advantage for ethanol and other biofuels.
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.
In 2003, California was the first state to start replacing the MTBE with ethanol. Several other states started switching soon afterward.
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.[5],[6] Iso-octane is the ideal gasoline, being the standard reference for the octane rating.
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 esters and oxygen-free alkyl biodiesels are available.
As a solvent
As a solvent, MTBE it possesses one distinct advantage over most ethers- it has a much lower 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), it 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. Research volumes are much smaller, leading to lower risks from other ethers, and in addition the use of MTBE as a solvent is very rare in literature synthetic procedures
As a chemical reagent
Being an ether, MTBE is a Lewis base. However, unlike other ethers such as diethyl ether or THF, it does not coordinate well enough with 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.
Health risks
The IARC, a cancer research agency of the World Health Organization, maintains MTBE is not classifiable as a human carcinogen. However, exposure to large doses of MTBE has significant non-cancer-related health risks. MTBE ruins the taste of water at concentrations of 5 – 15 µg/l[9] so that significant concentrations of MTBE in drinking water are detectable.
MTBE is not classified as a human carcinogen in low exposure levels by the International Agency for Research on Cancer (IARC).[10] However, it has been shown 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, tert-butanol, with a greatly increased residence time. The prolonged presence of this alcohol derivative is not fully understood.
Some industry advocates of MTBE contend that it has little provable effect on humans, although its manufacturers did not test it for its effects on human health before introducing it as an additive. As of 2007, researchers have limited data about the health effects from ingestion of MTBE. The Environmental Protection Agency (EPA) has concluded that available data are not adequate 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
Persistence and pervasiveness in the environment
MTBE is often introduced into water-supply aquifers by leaking underground storage tanks (USTs) at gasoline stations. Although USTs are much better constructed now than in the 1980s, accidental releases still take place because of the very large number of USTs. The high solubility and persistence of MTBE cause it to travel faster and farther than many other gasoline components when released into an aquifer. It is also released whenever gasoline with MTBE is spilled on the ground. Because it is so water soluble, it easily moves through soil, polluting both surface and groundwater.[7]
MTBE has widespread occurrences in the aquifers of North America, where the majority of groundwater chemistry data has been acquired. As one regional example, the San Francisco Bay Area Regional Water Quality Control Board has indicated MTBE is one of the groundwater pollutants of most widespread concern in this major metropolitan region of the USA
ملخص الموضوع
ثالث ميثيل بيوتايل أثير (إم تي بي إي) مركّب كيمياوي بالصيغة الجزيئية C5H12O قابل للإشتعال غير مستقر وسائل عديم اللون قابل للذوبان في الماء. له رائحة خفيفه بشكل مبهم تشبه أيثر داي ايثيل، يؤدّي إلى الطعم والرائحة الكريهه في الماء و يشكّل بالماء آزوتروبس (52.6 °C) و ميثانول(51.3 °C) .
المركب مصنّع بتفاعل كيمياوي ميثانول و ايزو بيوتانول
وقد أنتج بكميات كبيرة جدا في الولايات المتّحدة، إستعملت على نحو واسع كمضاف للوقود.و بسبب الإطلاقات الواسعة الإنتشار من هذا الغازولين في الخزانات تحت الأرضية في جميع أنحاء السلطات القضائية المختلفة الأمريكية منعت إستعماله و خفّض إنتاجه. لأن أيثر ايثانول البديل المشتقّ إي تي بي إي أعطى معالجة ضريبة أكثر مناسبة. إم تي بي إي يستعمل تقريبا بشكل خاص كأحد عناصر الوقود في الغازولين المحرّك. هو أحد مجموعة المواد الكيمياوية معروفة بإسم oxygenates وذلك بسبب رفع محتوى أوكسجين الغازولين.
إم تي بي إي إستعمل كوكيل ضدّ التقطيع عندما يستبدل غازولين رباعي أثيل الرئيسي لزيادة تقدير أوكتانه ويساعد على منع تقطيع المحرّك. المركّبات الأخرى متوفرة كإضافات oxygenate للغازولين، على سبيل المثال ايثانول وأثير ذو علاقة، ومثال على ذلك: رباعي أثير ميثيل أميل (أليف). تتضمّن الأسباب لإستعمال إم تي بي إي إعتبارات إقتصادية، كالبعض من الإنتاج مكتسب من قبل ميثانول مضاف إلى isobutylene أنتج كناتج عرضي من عمليات أخرى. على أية حال، أكثر الوسائل التي يجب توفرها الميثانول و الايزوبيوتالين تطلّبا لإنتاج إم تي بي إي.
إم تي بي إي يمتلك فائدة متميّزة واحدة على الأثيرالذي له ميل أقل بكثير لتشكيل البيروكسيد العضوي المتفجّر. القناني المفتوحة لأثير داي ايثيل أو تي إتش إف يمكن أن يعزّزا مستويات خطرة هذه البيروكسيد في شهور، بينما عينات إم تي بي إي عادة أمينة للسنوات. على خلاف الأثير الآخر مثل أثير داي ايثيل أو تي إتش إف، هو لا ينسّق كافي بشكل جيد جدا بالمغنيسيوم الّذي سيستعمل لجعل كواشف . إنّ مجموعة رباعي البوتيل تخلص بسهولة من تحت الشروط الحامضية بقوة، خصوصا إذا ساخنة، الشّيء الذي يمكن أن يحدّد إستعمال إم تي بي إي.
وكالة بحث سرطان تابعه لمنظمة الصحة العالمية وجدت إم تي بي إي ليس مصنّف كعوامل مسرطنة بالرغم من ان التعرّض إلى الجرع الكبيرة لإم تي بي إي لها أخطار صحية متعلقة بالسرطان غير الهامّة لكن، كان أمرا معروض تسبّب امراض الكلية في الحيوانات.
إم تي بي إي يقدّم في أغلب الأحيان إلى طبقات إمداد المياه الجوفية بتسريب الخزانات تحت أرضية (يو إس تي إس) في محطات الغازولين. قابلية الذوبان العالية وإصرار إم تي بي إي يسبّبانه لارتحال أسرع وعلى نحو إضافي من العديد من مكوّنات الغازولين الأخرى عندما أصدرت إلى طبقة جوفية. إم تي بي إي عنده حوادث واسعة الإنتشار في الطبقات الجوفية لأمريكا الشمالية، حيث أغلبية بيانات كيمياء المياه الجوفيّة إكتسبته.
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Mtbe
رد: Mtbe
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