|Molar mass||323.44 g/mol|
|Appearance||colorless, viscous liquid|
|Density||1.653 g/cm3 (20 °C)|
84–85 °C@15 mm Hg
|Solubility in water||insoluble|
|Solubility||slightly soluble in ethanol;
soluble in benzene, petroleum ether, toluene, gasoline 
|Refractive index (nD)||1.5198|
|Dipole moment||0 D|
|R-phrases||, , , ,|
|S-phrases||, , ,|
|Flash point||346 K - 73 °C - 163 °F|
|Other cations||Tetramethylsilane; tetramethyltin|
|Related compounds||Lead(II) chloride; decaphenylplumbocene|
(what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Tetra-ethyl lead, abbreviated TEL, is an organometallic compound with the formula (CH3CH2)4Pb. Once a common antiknock additive in gasoline (petrol), TEL usage was largely discontinued because of the toxicity of lead and its deleterious effect on catalytic converters. It is still used as an additive in aviation fuel for piston engine-powered aircraft.
Despite decades of research, no reactions were found to improve upon this rather difficult process that involves metallic sodium—a process with lithium was developed, but never put into practice. A related compound, tetramethyl lead, was commercially produced by a different electrolytic reaction. The product, TEL, is a viscous colorless liquid. Because TEL is charge neutral and contains an exterior of alkyl groups, it is highly lipophilic and soluble in petrol (gasoline).
A noteworthy feature of TEL is the weakness of its four C–Pb bonds. At the temperatures found in internal combustion engines (CH3CH2)4Pb decomposes completely into lead and lead oxides and combustible, short-lived ethyl radicals. Lead and lead oxide scavenge radical intermediates in combustion reactions. This prevents ignition of unburnt fuel during the engine's exhaust stroke. Lead itself is the reactive antiknock agent, and TEL serves as a gasoline-soluble lead carrier. When (CH3CH2)4Pb burns, it produces not only carbon dioxide and water, but also lead:
This lead can oxidize further to give species such as lead(II) oxide:
The Pb and PbO would quickly accumulate and destroy an engine. For this reason, the lead scavengers 1,2-dibromoethane and 1,2-dichloroethane are used in conjunction with TEL—these agents form volatile lead(II) bromide and lead(II) chloride, respectively, which are flushed from the engine and into the air.
TEL was supplied for mixing with raw gasoline in the form of ethyl fluid, which was TEL blended together with the lead scavengers 1,2-dibromoethane and 1,2-dichloroethane. 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.
Ethyl fluid was added to gasoline at rate of 1:1260, usually at the refinery. Because of the widespread use and toxic nature of ethyl fluid, the Ethyl Corporation developed an expertise in its safe handling. In the 1920s, before safety procedures were yet developed, some 17 workers for the Ethyl Corporation and Standard Oil died from the effects of exposure to lead.
The formula for ethyl fluid is:
Dibromoethane and dichloroethane act in a synergistic manner, where a particular mixing ratio provides the best lead scavenging ability.
Tetraethyl lead was once used extensively as a gasoline additive for its ability to increase the fuel's octane rating. A high enough octane rating is required to prevent premature detonations known as engine knocking ("knock" or "ping"). Antiknock agents allow the use of higher compression ratios for greater efficiency and peak power. The use of TEL in gasoline was started in the U.S., while in Europe, alcohol was initially used. The advantages of leaded gasoline from its higher energy content and storage quality eventually led to a universal switch to leaded fuel. One of the greatest advantages of TEL over other antiknock agents or the use of high octane blend stocks is the very low concentrations needed. Typical formulations called for 1 part of prepared TEL to 1260 parts untreated gasoline. Competing antiknock agents must be used in higher amounts, often diluting the energy content of the gasoline.
When used as an antiknock agent, alcohol will cause fuel to absorb moisture from the air. Over time fuel humidity can rise leading to rusting and corrosion in the fuel line. Whereas TEL is highly soluble in gasoline, ethanol is poorly soluble and that solubility decreases as fuel humidity increases. Over time, droplets and pools of water can form in the fuel system creating a risk for fuel line icing. High fuel humidity can also raise issues of biological contamination, as certain bacteria can grow on the surface of the water/gasoline interface thus forming bacterial mats in the fuel system. TEL's biocidal properties helped prevent fuel contamination and degradation from bacterial growth.
In most Western countries this additive went out of use in the late 20th century because of the concerns over pollution of air and soil (e.g., the areas around roads) and the accumulative neurotoxicity of lead. The use of TEL as a fuel additive spoils catalytic converters, which became mandatory to meet emissions regulations from the 1970s on in the West. The need for TEL was lessened by several advances in automotive engineering and petroleum chemistry. Lower oil prices promoted the development of low compression engines that were not as sensitive to gasoline quality. Other antiknock additives of various toxicities (MMT, MTBE, ETBE) and safer methods for making higher octane blending stocks (reformate, iso-octane) reduced the need for TEL.
As of 2007, unleaded automotive gasoline is available throughout the world, and the only countries in which leaded gasoline is extensively used are Yemen, Afghanistan and North Korea. Leaded gasoline is still available in parts of Northwest Africa, Europe, Commonwealth of Independent States (CIS), Iraq, Jordan and the Palestinian territories.
TEL remains an ingredient of high-octane racing fuels, and of 100 octane aviation fuel, as an economically favorable replacement for it in the aviation industry has not yet been found. The current formulation of 100LL (low lead) aviation gasoline contains much less lead than in previous fuels.
Many vehicles produced before TEL's phase-out required modification to run successfully on unleaded gasoline. These modifications fell 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 is addressed by the installation of hardened exhaust valves and seats. 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. However, the appearance on the market of high-octane unleaded gasolines has reduced or eliminated the need to alter engines' compression ratios.
Contact with concentrated TEL leads to the familiar symptoms of acute lead poisoning.
Lead pollution from engine exhaust is dispersed into the air and into the vicinity of roads and easily inhaled. Lead is a toxic metal that accumulates and has subtle and insidious neurotoxic effects especially at low exposure levels, such as low IQ and antisocial behavior. It has particularly harmful effects on children. These concerns eventually led to the ban on TEL in automobile gasoline in many countries. For the entire U.S. population, during and after the TEL phaseout, the mean blood lead level dropped from 13 μg/dL in 1976 to only 3 μg/dL in 1991. The U.S. Centers for Disease Control considered blood lead levels "elevated" when they were above 10 μg/dL. Lead exposure affects the intelligence quotient (IQ) such that a blood lead level of 30 μg/dL is associated with a 6.9-point reduction of IQ, with most reduction (3.9 points) occurring below 10 μg/dL.
Also in the U.S., a statistically significant correlation has been found between the use of TEL and violent crime: taking into account a 22-year time lag, the violent crime curve virtually tracks the lead exposure curve. After the ban on TEL, blood lead levels in U.S. children dramatically decreased.
Even though leaded gasoline is largely gone in North America, it has left high concentrations of lead in the soil adjacent to all roads that were constructed prior to its phaseout. Children are particularly at risk if they consume this, as in cases of pica.
Tetraethyl lead was first discovered by a German chemist in 1854, but remained commercially unused for many years. In 1921, TEL was found to be an effective antiknock agent by Thomas Midgley, working under Charles Kettering at General Motors Research. General Motors patented the use of TEL as a knocking agent and called it "Ethyl" in its marketing materials, thereby avoiding the negative connotation of the word "lead". In 1924, Standard Oil of New Jersey (ESSO/EXXON) and General Motors created the Ethyl Gasoline Corporation to produce and market TEL.
The toxicity of concentrated TEL was recognized early; many TEL researchers and workers, including Midgley, became victims of lead poisoning, and dozens died. In 1925, the sales of TEL were suspended for one year to conduct a hazard assessment. The cases of fatal lead poisoning and serious symptoms of lead toxicity were, however, assumed to be restricted to TEL manufacture and handling. The low concentrations present in gasoline and exhaust were not perceived as immediately dangerous. A U.S. Surgeon General committee issued a report in 1926 that concluded there was no real evidence that the sale of TEL was hazardous to human health but urged further study.
In the late 1920s, Dr. Robert Kehoe of the University of Cincinnati was the Ethyl Corporation's chief medical consultant. In 1928, Dr. Kehoe expressed the opinion that there was no basis for concluding that leaded fuels posed any health threat. He convinced the Surgeon General that the dose–response relationship of lead was that of no effect below a certain threshold. As the head of Kettering Laboratories for many years, Kehoe would become a chief promoter of the safety of TEL, an influence that did not begin to wane until about the early 1960s. But by the 1970s, the general opinion of the safety of TEL would change, and by 1976 the U.S. government would begin to require the phaseout of this product.
As early as the late 1940s and early 1950s, Clair Patterson accidentally discovered the pollution caused by TEL in the environment while determining the age of the earth. As he attempted to measure lead content of very old rocks, and the time it took uranium to decay into lead, the readings were made inaccurate by lead in the environment that contaminated his samples. He was then forced to work in a clean room to keep his samples uncontaminated by environmental pollution of lead. After coming up with a fairly accurate estimate of the age of the earth, he turned to investigating the lead contamination problem by examining ice cores from countries such as Greenland. He realized that the lead contamination in the environment dated from about the time that TEL became widely used as a fuel additive in gasoline. Being aware of the health dangers posed by lead and suspicious of the pollution caused by TEL, he became one of the earliest and most effective opponents of its use.
In the U.S. in 1972, the EPA launched an initiative to phase out leaded gasoline, Ethyl Corp's response to which was to sue the EPA. The EPA won the case, so the TEL phaseout began in 1976 and was completed by 1986. A 1994 study indicated that the concentration of lead in the blood of the U.S. population had dropped 78% from 1976 to 1991.
By the year 2000, the TEL industry had moved the major portion of their sales to developing countries and lobbied governments to delay phasing out of the additive. Leaded gasoline was withdrawn entirely from the European Union market on 1 January 2000, although it had been banned much earlier in most member states. It was only recently phased out in China (around 2001). In the United Kingdom a small amount of leaded gasoline ("four star petrol") is still permitted to be manufactured and sold, albeit with a higher rate of fuel duty. In Australia, owners of old cars that run on leaded petrol can buy leaded additives and mix them with octane 98 fuel (premium unleaded).
Antiknock agents are grouped into "high-percentage" additives, such as alcohol, and "low-percentage" additives based on heavy elements. Since the main problem with TEL is its lead content, many alternative additives that contain less poisonous metals have been examined. A manganese-carrying additive, methylcyclopentadienyl manganese tricarbonyl (MMT or methylcymantrene), is used as an antiknock agent in Canada, but its use as a fuel additive had been banned in the U.S. until 1995. Ferrocene, an organometallic compound of iron, has also been reported as an effective antiknock agent.
Lead replacement additives were scientifically tested, and some were approved by the Federation of British Historic Vehicle Clubs, at the UK's Motor Industry Research Association (MIRA) in 1999.
High-percentage additives are organic compounds that do not contain metals, but they require much higher blending ratios, such as 20–30% for benzene and ethanol. It had also been established by 1921 that ethanol was an effective antiknock agent, but TEL was introduced for mainly commercial reasons to replace it. Oxygenates, mainly methanol-derived MTBE and ethanol-derived ETBE, have largely substituted the need for TEL. MTBE has environmental risks of its own and there are also bans on its use. ETBE, on the other hand, requires more expensive ethanol as a starting material.
Improvements of the gasoline itself decrease the need for separate antiknock agents. Synthetic iso-octane and alkylate are examples of such blending stocks. Benzene and other high-octane aromatics can be also blended to raise the octane number, but they are disfavored today because of toxicity and carcinogenity.