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Tetraethyllead
Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb(C2H5)4. First synthesized in 1853 by German chemist Carl Jacob Löwig, TEL was widely used as a fuel additive for much of the 20th century before being globally banned due to its dangerous neurotoxicity.
In the early 1920s, American chemical engineer Thomas Midgley Jr. of General Motors discovered that TEL was a cheap and effective antiknock agent for gasoline. 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. Between 1926 and 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.
In the mid-20th century, scientists discovered that TEL causes lead poisoning and is highly toxic to the human brain, especially in children. Many countries began phasing out TEL in automotive fuel in the 1970s. The United States achieved the total elimination of sales of leaded gasoline for road vehicles on January 1, 1996. By the early 2000s, most countries had banned the use of TEL in gasoline. The global phase-out was completed in 2021 when Algeria ended its leaded-gasoline production, prompting the United Nations Environment Program to declare an "official end" of the use of TEL in gasoline. Researchers have retroactively estimated the global harm caused annually by the use of TEL in leaded gasoline during the 20th century to have been 1.1 million excess deaths, 322 million lost IQ points, 60 million crimes, and 4% of worldwide GDP (about US$2.4 trillion per year).
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. TEL is a viscous colorless liquid with a sweet odor. Because TEL is charge neutral and contains an exterior of alkyl groups, it is highly lipophilic and soluble in petrol (gasoline). This property, which allows it to dissolve so evenly and effectively in motor fuel, also allowed easy absorption by body fats and lipids and diffusion through the blood–brain barrier (BBB). The lead (II) ions (Pb2+) would accumulate within the limbic forebrain, frontal cortex, and hippocampus. Practically speaking, TEL is a "central nervous system toxin which produces an acute toxic psychosis."
There is no cure for direct poisoning by TEL. Inorganic lead compounds, such as those present in engine exhausts, could be removed from the system through the administration of chelating agents, which bind to the inorganic lead and flush them out of the body. However, highly lipid-soluble TEL cannot be removed this way, and treatments are of a supportive nature.
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.
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.
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Tetraethyllead
Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb(C2H5)4. First synthesized in 1853 by German chemist Carl Jacob Löwig, TEL was widely used as a fuel additive for much of the 20th century before being globally banned due to its dangerous neurotoxicity.
In the early 1920s, American chemical engineer Thomas Midgley Jr. of General Motors discovered that TEL was a cheap and effective antiknock agent for gasoline. 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. Between 1926 and 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.
In the mid-20th century, scientists discovered that TEL causes lead poisoning and is highly toxic to the human brain, especially in children. Many countries began phasing out TEL in automotive fuel in the 1970s. The United States achieved the total elimination of sales of leaded gasoline for road vehicles on January 1, 1996. By the early 2000s, most countries had banned the use of TEL in gasoline. The global phase-out was completed in 2021 when Algeria ended its leaded-gasoline production, prompting the United Nations Environment Program to declare an "official end" of the use of TEL in gasoline. Researchers have retroactively estimated the global harm caused annually by the use of TEL in leaded gasoline during the 20th century to have been 1.1 million excess deaths, 322 million lost IQ points, 60 million crimes, and 4% of worldwide GDP (about US$2.4 trillion per year).
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. TEL is a viscous colorless liquid with a sweet odor. Because TEL is charge neutral and contains an exterior of alkyl groups, it is highly lipophilic and soluble in petrol (gasoline). This property, which allows it to dissolve so evenly and effectively in motor fuel, also allowed easy absorption by body fats and lipids and diffusion through the blood–brain barrier (BBB). The lead (II) ions (Pb2+) would accumulate within the limbic forebrain, frontal cortex, and hippocampus. Practically speaking, TEL is a "central nervous system toxin which produces an acute toxic psychosis."
There is no cure for direct poisoning by TEL. Inorganic lead compounds, such as those present in engine exhausts, could be removed from the system through the administration of chelating agents, which bind to the inorganic lead and flush them out of the body. However, highly lipid-soluble TEL cannot be removed this way, and treatments are of a supportive nature.
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.
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.