Tetraethyllead
Tetraethyllead, abbreviated TEL, is an organolead compound with the formula Pb4. 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 mid-20th century, scientists discovered that TEL caused lead poisoning and was highly neurotoxic to the human brain, especially in children. The United States and many other countries began phasing out the use of TEL in automotive fuel in the 1970s. With EPA guidance and oversight, the United States achieved the total elimination of sales of leaded gasoline for on-road vehicles 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 to declare an "official end" of its use in cars on August 30, 2021. In 2011, researchers retroactively estimated the annual impact of tetraethyl lead worldwide to be 1.1 million excess deaths, 322 million lost IQ points, 60+ million crimes, and 4% of worldwide GDP.
Synthesis and properties
TEL is produced on an industrial scale by reacting chloroethane 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. 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. The lead ions 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.
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 oxide:
Pb and PbO would quickly over-accumulate and foul an engine. For this reason, 1,2-dichloroethane and 1,2-dibromoethane were also added to gasoline as lead scavengers—these agents form volatile lead chloride and lead bromide, respectively, which flush the lead from the engine and into the air:
In motor fuel
TEL was extensively used as a gasoline additive beginning in the 1920s, wherein it served as an effective antiknock agent and reduced exhaust valve and valve seat wear. Concerns were raised in reputable journals of likely health outcomes of fine particles of lead in the atmosphere as early as 1924.Valve wear preventive
Tetraethyllead helps cool intake valves and is an excellent buffer against microwelds forming between exhaust valves and their seats. Once these valves reopen, the microwelds pull apart and abrade the valves and seats, leading to valve recession. When TEL began to be phased out, the automotive industry began specifying hardened valve seats and upgraded materials which allow for high wear resistance without requiring lead.Antiknock agent
A gasoline-fueled reciprocating engine requires fuel of sufficient octane rating to prevent uncontrolled combustion. Antiknock agents allow the use of higher compression ratios for greater efficiency and peak power. Adding varying amounts of additives to gasoline allowed easy, inexpensive control of octane ratings. TEL offered the business advantage of being commercially profitable because its use for this purpose could be patented. Aviation fuels with TEL used in WWII reached octane ratings of 150 to enable turbocharged and supercharged engines such as the Rolls-Royce Merlin and Griffon to reach high horsepower ratings at altitude. In military aviation, TEL manipulation allowed a range of different fuels to be tailored for particular flight conditions.In 1935 a license to produce TEL was given to IG Farben, enabling the newly formed German Luftwaffe to use high-octane gasoline for high altitude flight. A company, Ethyl GmbH, was formed that produced TEL at two sites in Germany with a government contract from 10 June 1936.
In 1938 the United Kingdom Air Ministry contracted with ICI for the construction and operation of a TEL plant. A site was chosen at Holford Moss, near Plumley in Cheshire. Construction started in April 1939 and TEL was being produced by September 1940.
"Ethyl Fluid"
For mixing with raw gasoline, TEL was most commonly supplied in the form of "Ethyl Fluid", consisting of TEL blended with 1,2-dichloroethane and 1,2-dibromoethane, which prevent lead from building up in the engine. Ethyl Fluid also contained a reddish dye to distinguish treated from untreated gasoline and discourage the use of leaded gasoline for other purposes such as cleaning.In the 1920s, before safety procedures were strengthened, 17 workers for the Ethyl Corporation, DuPont, and Standard Oil died from the effects of exposure to lead. The grim news was not well received by US legislators and a brief ban was put into place. However, it was lifted on recommendation of the United States Surgeon General and a panel of scientists in 1929, after extensive lobbying efforts by the aforementioned companies. It would be another half century until a similar effort was made to rein in the additive, spearheaded by the EPA in the 1980s. This time, concerns for health and the environment were aided by the increasing use of catalytic converters, which cannot tolerate TEL.
Ethyl Fluid's formulation consisted of:
- 61.45% tetraethyllead
- 18.80% 1,2-dichloroethane
- 17.85% 1,2-dibromoethane
- 1.90% inerts, preservatives, and dyes
Phaseout and ban
In most industrialized countries, a phaseout of TEL from road vehicle fuels was completed by the early 2000s because of concerns over air and soil lead levels and the accumulative neurotoxicity of lead. In the European Union, tetraethyllead has been classified as a Substance of Very High Concern and placed on the Candidate List for Authorization under Registration, Evaluation, Authorization and Restriction of Chemicals. Potential use of TEL would need to be authorized through the REACH authorization procedure. While not a complete ban, it introduces significant obligations such as a mandatory analysis of alternatives and socioeconomic analysis.The use of catalytic converters, mandated in the United States for 1975 and later model-year cars to meet tighter emissions regulations, started a gradual phase-out of leaded gasoline in the U.S. The need for TEL was lessened by several advances in automotive engineering and petroleum chemistry. Safer methods for making higher-octane blending stocks such as reformate and iso-octane reduced the need to rely on TEL, as did other antiknock additives of varying toxicity including metallic compounds such as methylcyclopentadienyl manganese tricarbonyl as well as oxygenates including methyl tert-butyl ether, tert-amyl methyl ether, and ethyl tert-butyl ether.
The first country to completely ban leaded gasoline was Japan in 1986.
Since January 1993, all gasoline powered cars sold in the European Union and the United Kingdom have been required to use unleaded fuel. This was to comply with the Euro 1 emission standards which mandated that all new cars to be fitted with a catalytic converter. Unleaded fuel was first introduced in the United Kingdom in June 1986.
Leaded gasoline was removed from the forecourts in the United Kingdom on January 1, 2000, and a Lead Replacement Petrol was introduced although this was largely withdrawn by 2003 due to dwindling sales. An exemption to the ban exists for owners of classic cars.
Vehicles designed and built to run on leaded fuel often require modification to run on unleaded gasoline. These modifications fall into two categories: those required for physical compatibility with unleaded fuel, and those performed to compensate for the relatively low octane of early unleaded fuels. Physical compatibility requires the installation of hardened exhaust valves and seats, or by use of additives. Compatibility with reduced octane was addressed by reducing compression, generally by installing thicker cylinder head gaskets and/or rebuilding the engine with compression-reducing pistons, and/or by retarding ignition timing.
Leaded gasoline remained legal as of late 2014 in parts of Algeria, Iraq, Yemen, Myanmar, North Korea, and Afghanistan. North Korea and Myanmar purchased their TEL from China, while Algeria, Iraq, and Yemen purchased it from the specialty chemical company Innospec, the world's sole remaining legal manufacturer of TEL. In 2011 several Innospec executives were charged and imprisoned for bribing various government state-owned oil companies to approve the sale of their TEL products.
the UNEP-sponsored phase-out was nearly complete: only Algeria, Iraq, and Yemen continued widespread use of leaded gasoline, although not exclusively. In July 2021, Algeria had halted its sale.