Tetra-ethyl lead Totally Explained

Everything about Tetra-ethyl Lead totally explained

| Section2 = | Section3 = | Section7 =,,,, | SPhrases =,,, }} | Section8 = }} 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. ť :4 NaPb + 4 CH₃CH₂Cl → (CH₃CH₂)₄Pb + 4 NaCl + 3 Pb

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's 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. 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 hasn't 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. 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, 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. 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 don't 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're disfavored today because of toxicity and carcinogenity.

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