Tetra-ethyl lead
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 disadvantageous effects on catalytic converters.  It is still used as an additive in the aviation fuel known as avgas.

# Synthesis and properties
TEL is produced by reacting ethyl chloride with a sodium-lead alloy.
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, first into (CH3CH2)3Pb  and ethyl radicals.  These radicals scavenge other radicals, thereby preventing the initiation of combustion, which itself is a radical reaction, in order to delay ignition.  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, lead scavengers such as 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
Tetra-ethyl lead was supplied for mixing with raw gasoline in the form of "ethyl fluid", which was tetra-ethyl lead 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. Despite this, 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%

# Uses of TEL as an antiknock agent
TEL was once used extensively as an additive in gasoline (petrol) for its ability to increase the fuel's octane rating (that is, to prevent its premature detonation ("knocking") in the engine) thus allowing the use of higher compression ratios for greater efficiency and power.
The use of TEL in gasoline was started in the US while in Europe alcohol was initially used. The advantages of ethyl 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 ethyl gasoline results in greater fuel efficiency.
When used as an antiknock agent, alcohol will cause fuel to absorb moisture from the air. Over time high 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 in on 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 air pollution. 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) and cheaper methods for making higher octane blending stocks (reformate) 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 aviation gasoline and high-octane racing fuels.  TEL is still in use today as a component 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.
In earlier times many vehicles produced before TEL's phase-out required modification to a greater or lesser extent to run successfully on unleaded gasoline. The installation of new hardened valve seats can be done by a competent automotive machine shop. A major engine rebuild, generally by the use of dished pistons, is required to reduce the compression ratio of some older high-performance engines (which required 100-octane leaded fuel) to a ratio that is compatible with currently available gasoline ratings and this reform necessarily entails a decrease in engine power. However by the 21st century additives were available to allow continued use of even these sensitive engines, more or less to their normal function.

# 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 researchers of TEL became ill (including Midgley himself), 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, which caused Ethyl Corp. to sue the EPA. The EPA won, so in 1976 the phase out began and was completed by 1986. A 1994 study indicated that the concentration of lead in blood dropped 78% from 1978 to 1991 .
Leaded gasoline phased out European Union-wide on the 1st 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.
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).

# Alternative antiknock agents
Since the main problem with TEL is its lead content, many alternative additives that contain less poisonous metals have been examined.   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 has also been reported as an effective antiknock agent. 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.
Improvements of the gasoline itself decrease the need for separate antiknock agents. Synthetic iso-octane and alkylate are examples of such blending stocks.