Tetraethyllead
Tetra-ethyl lead, abbreviated TEL, is an organometallic compound with the formula (CH3CH2)4Pb. Once a common anti-knock 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.

# Synthesis and properties
TEL is produced by reacting ethyl chloride with a sodium-lead alloy.
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 before TEL was largely banned. 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 anti-knock 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 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 exhausted from the engine and into the air.

## Formulation of ethyl fluid
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 which would distinguish treated gasoline from untreated gasoline and discourage the diversion of 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, specialised and toxic nature of ethyl fluid, the Ethyl Corporation developed an expertise in the handling and formulation of toxic organometallics. During the first days of TEL manufacture, when safety procedures were yet to be developed, some 17 workers for the Ethyl Corporation and Standard Oil died during the 1920s from the effects of exposure to lead.
The classical formula for ethyl fluid is:
- Tetraethyl lead          61.45%
- 1,2-Dibromoethane        17.85%
- 1,2-Dichloroethane       18.80%
- Inerts & dye             1.90%
Dibromoethane and dichloroethane act in a synergistic manner, where a particular mixing ratio provides the best lead scavenging ability.

# Uses of tetraethyl lead as an antiknock agent
Tetraethyl lead was once used extensively as an additive in gasoline (petrol) for its ability to increase the fuel's octane rating. A high enough octane rating is required to prevent its detonation during the compression stroke ("knocking") in the engine. Anti-knock agents allow the use of higher compression ratios for greater efficiency and peak power.
The use of TEL in gasoline was started in the US 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 anti-knock agents or the use of high octane blend stocks is the very low concentrations needed. Typical formulations called for 1 part of ethyl fluid (prepared TEL) to 1260 parts untreated gasoline. Competing anti-knock agents must be used in higher amounts and/or have a much lower energy level than natural gasoline. The higher energy content of leaded gasoline results in greater fuel efficiency.
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 help 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, the areas around roads and the accumulative neurotoxicity of lead. Use of TEL as a fuel additive would result in the fouling of catalytic converters. 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 anti-knocking additives (MMT, MTBE, ETBE) and cheaper 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 a suitable 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 did historical aviation gasolines.
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 nonleaded 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.

# Toxicity
The toxicity of concentrated TEL was recognized early, and the sales of TEL were initially 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 ethyl fluid handling. The low concentrations present in gasoline and exhaust were not perceived as immediately dangerous. In the engine exhaust, compounds of lead are dispersed as fine particulates. Lead pollution is dispersed into the air and into the vicinity of roads. Lead is a toxic metal that accumulates and has subtle and insidious neurotoxic effects at even very low levels, such as low IQ and antisocial behavior. It has particularly deleterious effects on children. These concerns led to the ban on TEL in automobile gasoline. In a 2000 study, the highest blood lead levels were about 27 g/dL (county averages). A blood lead level of 30 g/dL is associated with a 6.9-point reduction of IQ, with most reduction (3.9 points) occurring below 10 g/dL, the level at which CDC considers blood lead level "elevated". 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 children have been dramatically reduced.
Even though leaded gasoline is largely gone in North America, it has left high concentrations of lead in the dirt adjacent to all roads that were constructed prior to its phaseout. Child development specialists often advise parents to not let their children play in such dirt, especially because some children like to eat dirt (see pica).

# History
TEL was found to be an effective anti-knocking agent by Thomas Midgley in 1921, working under Charles Kettering at General Motors Research. Due to its extreme toxicity, many early TEL researchers, including Midgley, became lead poisoned, and dozens died.. In 1924, Standard Oil of New Jersey (ESSO/EXXON) and General Motors created the Ethyl Gasoline Corporation to produce and market TEL. In the US in 1972, the EPA launched an initiative to phase out leaded gasoline, in response to which Ethyl Corp. to sue the EPA. The EPA won, so the phaseout began in 1976 and was completed by 1986. A 1994 study indicated that the concentration of lead in blood dropped 78% from 1978 to 1991.
As early as the late 1940s and early 1950s, Clair Cameron 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 tetra-ethyl lead 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 tetra-ethyl lead, he was one of the earliest opponents of its use. Since the lead industry was powerful and used its influence to protect its practices, Patterson often found research funding withdrawn, and the trustees of his university were pressured to fire him or keep him quiet. He was even excluded from the 1971 US National Research Council panel that was investigating lead poisoning, despite the fact that he was the leading expert on atmospheric lead.
Leaded gasoline was withdrawn entirely from the European Union market on the 1 January 2000, although it had been banned much earlier in some 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.

# Alternative antiknock agents
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 US until 1995. Ferrocene, an organometallic compound of iron, has also been reported as an effective antiknock agent.
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.