Tetraethyl lead (TEL)
Tetraethyl lead (TEL) is a liquid with the chemical formula (CH3CH2)4 Pb. Once widely used (circa 1925 to 1990) to increase the octane rating of gasoline (petrol), TEL usage in gasoline has been largely phased out by most nations primarily because of the toxicity of the lead emissions from spark-ignited internal combustion engines fueled by gasoline containing TEL.
Another reason for discontinuing TEL usage was that it degraded the efficiency of the catalytic converters installed in automotive vehicles to reduce their emissions of air pollutants.
TEL is still available for use as an additive to increase the octane rating of aviation fuel for aircraft powered by internal combustion engines.
Manufacture and properties
Pure tetraethyl lead, as distinguished from tetraethyl lead fluid (TEL fluid), is a colorless liquid that is highly lipophilic and soluble in fats, oils and lipids as well as gasoline and other non-polar hydrocarbons.
The various other physical and chemical properties of tetraethyl lead are listed in the adjacent table.
TEL is produced by the alkylation of a sodium-lead alloy using chloroethane as expressed by this chemical reaction:
4 CH3CH2Cl + 4 NaPb ⇒ (CH3CH2)4Pb + 4 NaCl + 3 Pb
which can also be written as:
4 mols chloroethane + 4 mols sodium-lead alloy ⇒ 1 mol tetraethyl lead + 4 mols sodium chloride + 3 mols lead
At the temperatures found in spark-ignited internal combustion engines, TEL decomposes completely into lead, lead oxide (PbO) and combustible, short-lived ethyl radicals. Lead itself is the reactive agent that enhances the octane rating of gasolines and tetraethyl lead serves as a gasoline-soluble lead carrier.
TEL fluid formulation
(CH3CH2)4Pb + 13 O2 ⇒ 8 CO2 + 10 H2O + Pb
The lead can the oxidize further to give lead oxide (PbO):
2 Pb + O2 ⇒ 2 PbO
The Pb and PbO would soon accumulate and destroy an engine. For this reason, the TEL used in gasoline was actually part of a blended liquid formulation known as TEL fluid or ethyl fluid that contained the halocarbons 1,2-dibromoethane and 1,2-dichloroethane liquids known as lead scavengers. Those scavengers formed lead bromide (PbBr) and lead chloride (PbCl) which are volatile and were emitted from the engine exhaust to the atmosphere. The complete composition of TEL fluid was:
- 61.45% Tetraethyl lead
- 17.85% 1,2-Dibromoethane
- 18.80% 1,2-Dichloroethane
- 1.90% Inerts and color dye
The addition of as little TEL fluid as 0.8 ml per liter (three ml per gallon) of gasoline was equivalent to adding 0.5 grams of lead per liter of gasoline, and resulted in significant increases in the octane rating of the gasoline.
History of tetraethyl lead as a gasoline octane enhancer
In the 1920s, petroleum refining technology was rather primitive and produced gasolines with an octane rating of about 40 – 60. But automotive engines were rapidly being improved and required better gasolines, which led to a search for octane rating enhancers. That search culminated in 1921 with the development and use of tetraethyl lead as an octane enhancer.
Its utility was discovered by Charles Kettering and Thomas Midgely. By 1923, when Thomas Midgley accepted the Nichols Medal in March, 1923, he had almost returned to normal after fighting a winter-long battle with lead poisoning. Despite ethanol being widely recognized as an alternative octane rating enhancer, the less expensive TEL quickly became a commercial success. In 1924, Standard Oil of New Jersey and General Motors created the Ethyl Corporation to produce and market TEL.
In 1924, New York City Medical Examiner Charles Norris, and his forensic chemist, Alexander Gettler, were asked to investigate possible occupational exposure and toxicity in a New Jersey refinery. Workers there called the TEL facility the "looney gas building" because hallucinations were common. Within a year, 33 TEL workers were hospitalized and seven were dead. At a press conference, Thomas Midgely, the General Motors engineer that helped to develop TEL, put his hand in a bowl of TEL, saying "I'm taking no chances whatsoever. Nor would I take any chances doing that every day." Midgely, a few months after the press conference, traveled to Europe for treatment of lead poisoning.
Production and sale of "leaded gasoline" was briefly banned in 1925 by the Surgeon General, and a panel of experts was appointed to investigate a number of fatalities that had "occurred in the manufacture and usage of TEL. In 1927, the Surgeon General set a voluntary standard for the petroleum refining industry to follow in mixing tetraethyl lead with gasoline. The standard was three cubic centimeters per gallon, corresponding to the maximum then in use by refineries, and thus imposed no real restraint.
For about the next 50 years, TEL was used as the most cost effective way to raise the octane rating of gasolines. During that period, petroleum refining technology grew until high-octane gasolines could, in fact, be produced without using TEL. Also, in about the 1940's, it was discovered that the lead being emitted in the exhaust gases from vehicular internal combustion engines was a toxic air pollutant that seriously affected human health.
Because of its toxicity and the fact that the catalytic converters being installed in vehicles (to reduce smog-forming air pollutant emissions) could not tolerate the presence of lead, the U.S. EPA launched an initiative in 1972 to phase out the use of TEL in the United States and it was completely banned for use in on-road vehicles as of January 1996. Using TEL in race cars, airplanes, marine engines and farm equipment is still permitted.
TEL usage has also been phased out by most other nations worldwide. As of 2008, the only nations still allowing extensive use of TEL are the Democratic People's Republic of Korea, Burma, and Yemen.
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