Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb(C2H5)4. It was widely used as a fuel additive for much of the 20th century, first being mixed with gasoline beginning in the 1920s. This "leaded gasoline" had an increased octane rating that allowed engine compression to be raised substantially and in turn increased vehicle performance and fuel economy. TEL was first synthesized by German chemist Carl Jacob Löwig in 1853. American chemical engineer Thomas Midgley Jr., who was working for the U.S. corporation General Motors, was the first to discover its effectiveness as a knock inhibitor on December 9, 1921, after spending six years attempting to find an additive that was both highly effective and inexpensive. Of the some 33,000 substances in total screened, lead was found to be the most effective antiknock agent, in that it necessitated the smallest concentrations necessary; a treatment of 1 part TEL to 1300 parts gasoline by weight is sufficient to suppress detonation. The four ethyl groups in the compound served to dissolve the active lead atom within the fuel. When injected into the combustion chamber, tetraethyllead decomposed upon heating into ethyl radicals, lead, and lead oxide. The lead oxide scavenges radicals and therefore inhibits a flame from developing until full compression has been achieved, allowing the optimal timing of ignition, as well as the lowering of fuel consumption. Throughout the sixty year period from 1926 to 1985, an estimated 20 trillion liters of leaded gasoline at an average lead concentration of 0.4 g/L were produced and sold in the United States alone, or an equivalent of 8 million tons of inorganic lead, three quarters of which would have been emitted in the form of lead chloride and lead bromide. Estimating a similar amount of lead to have come from other countries' emissions, a total of more than 15 million tonnes of lead may have been released into the atmosphere. In the 1950s, Clair Patterson discovered wide spread lead contamination including surfaces, blood, hair and the atmosphere during his experiments using lead dating to establish the age of the solar system, and determining it was a cause for alarm he began the campaign against lead poisoning in 1965. Lead poisoning was discovered during the second century BCE in Greece, but it wasn't until the mid-20th century, that scientists agreed that TEL contributed to lead poisoning and was highly neurotoxic to the human brain, especially in children. After a prolonged lobbying campaign headed by the Ethyl Corporation to continue using TEL unabated, the United States and many other countries finally began phasing out the use of TEL in automotive fuel in the 1970s with the total elimination of sales of leaded gasoline for on-road vehicles not occurring until on January 1, 1996. By the early 2000s, most countries had banned the use of TEL in gasoline. In July 2021, the sale of leaded gasoline for cars was completely phased out worldwide following the termination of production by Algeria, prompting the United Nations Environment Program (UNEP) to declare an "official end" of its use in cars on August 30, 2021. While TEL is no longer used in automobile gasoline, it remains in use for some specialized fuels such as aviation gasoline (Avgas). In 2011, researchers retroactively estimated the annual impact of tetraethyl lead worldwide to be 322 million lost IQ points, 60+ million crimes, and 4% of worldwide GDP (around 2.4 trillion United States dollars per year).
Synthesis and properties TEL is a viscous colorless liquid with a sweet odor. TEL is produced on an industrial scale by reacting chloroethane (ethyl chloride) with a sodium–lead alloy.
The product is recovered by steam distillation, leaving a sludge of lead and sodium chloride. Despite decades of research, no reactions were found to improve upon this process; it is rather difficult, involves reactive metallic sodium, and converts only 25% of the lead to TEL. A related compound, tetramethyllead, was commercially produced by a different electrolytic reaction. However, tetramethyllead was even more difficult to make, and it did not find use beyond niche applications. A highly efficient pathway utilizing ethyl chloride with a slight excess of lithium was developed, with a TEL yield over lead of over 90%. However, by then the fuel additive had started to fall out of favor and into disrepute, and the process was never put into practice.
Reactions A noteworthy feature of TEL is the weakness of its four C–Pb bonds. At the temperatures found in internal combustion engines, TEL decomposes completely into lead as well as combustible, short-lived ethyl radicals. Lead and lead oxide scavenge radical intermediates in combustion reactions. Engine knock is caused by a cool flame, an oscillating low-temperature combustion reaction that occurs before the proper, hot ignition. Lead quenches the pyrolyzed radicals and thus kills the radical chain reaction that would sustain a cool flame, preventing it from disturbing the smooth ignition of the hot flame front. Lead itself is the reactive antiknock agent, and the ethyl groups serve as a gasoline-soluble carrier. When TEL burns, it produces not only carbon dioxide and water, but also lead and lead(II) oxide:
Pb ( C 2 H 5 ) 4 + 13 O 2 ⟶ 8 CO 2 + 10 H 2 O + Pb {\displaystyle {\ce {Pb(C2H5)4 + 13 O2 -> 8 CO2 + 10 H2O + Pb}}}
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