Tetra-ethyl lead

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Tetra-ethyl lead
Image:Tera-ethyl-lead-chemical.png
General
Systematic name	Tetraethyllead
Other names	TEL</br>lead tetraethyl tetra-ethyl lead
Molecular formula	C₈H₂₀Pb
SMILES	CC[Pb](CC)(CC)CC
Molar mass	323.44 g/mol
Appearance	colorless, viscous liquid
CAS number	[78-00-2]
Properties
Density and phase	1.653 g/mL at 25 °C
Solubility in water	insoluble
Other solvents	soluble in benzene, hexane
Melting point	−136 °C
Boiling point	84-85 °C@15 mm Hg
Viscosity	? cP at ? °C
Structure
Molecular shape	tetrahedral
Dipole moment	0 D
Hazards
MSDS	External MSDS
Main hazards	toxic, flammable
NFPA 704	Image:NFPA 704.svg 2 3 3
Flash point	346 K - 73 °C - 163 °F
R/S statement	R:61-26/27/28-33-50/53-62 S: 53-45-60-61
RTECS number	TP4550000
Supplementary data page
Structure and properties	n = 1.519
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Tetraphenyllead
Other cations	Tetramethylsilane Tetramethyltin
Related compounds	Lead(II) chloride decaphenylplumbocene
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Tetra-ethyl lead, abbreviated TEL, is an organometallic compound with the formula (CH₃CH₂)₄Pb. Once an common additive in gasoline (petrol), TEL usage was largely discontinued because of the toxicity of lead. It is still used as an additive in aviation fuel.

1 Synthesis and properties
- 1.1 Formulation of ethyl fluid
2 Uses of TEL as an antiknock agent
3 History
4 Alternative antiknock agents
5 Reference
6 See also

[edit] Synthesis and properties

TEL is produced by reacting ethyl chloride with a sodium-lead alloy.<ref>Seyferth, D., "The Rise and Fall of Tetraethyllead. 2", Organometallics, 2003, volume 22, pages 5154-5178.</ref>

4 NaPb + 4 CH₃CH₂Cl → (CH₃CH₂)₄Pb + 4 NaCl + 3 Pb

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.

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:

(CH₃CH₂)₄Pb + 13 O₂ → 8 CO₂ + 10 H₂O + Pb

This lead can oxidize further to give species such as lead oxide:

2Pb + O₂ → 2PbO

The Pb and PbO would quickly accumulate and destroy an engine. For this reason, lead scavengers such as 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).

[edit] 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 ethylene dibromide and ethylene dichloride. 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.

The classical formula for ethyl fluid is:

Tetraethyl lead 61.45%
Ethylene dibromide 17.85%
Ethylene dichloride 18.80%
Inerts & dye 1.90%

[edit] 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. In addition some of the lead was deposited on the valve seats and helped protect them against wear.

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 octance 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 leads results in greater fuel efficiency.

When used as an antiknock agent alcohol will make fuel hygroscopic (causing it absorb water 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 degredation 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. Harder metals were introduced for intake valves and valve-seats. Lower oil prices promoted the development of low compression engines that were not as senstive to gasoline quality. Other anti-knocking additives (MMT) and cheaper methods for making higher octance blending stocks(reformate) reduced the need for TEL.

As of 2006, 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, Central Asia and Southeast Asia. The global market for TEL is estimated to decline by 15% each year. The world's sole remaining manufacturing facility for TEL is owned by Innospec and is located in Ellesmere Port in the UK.

TEL remains an ingredient of aviation gasoline and is also still available from a limited number of outlets as a fuel additive, mostly for owners of classic and vintage cars and motorcycles. 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 historical aviation gasolines did.

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.

[edit] History

TEL was found to be an effective anti-knocking agent by Thomas Midgley in 1921, working under Charles Kettering at General Motors Research.<ref>"Leaded Gasoline, Safe Refrigeration, and Thomas Midgley, Jr." Chapter 6 in S. Bertsch McGrayne "Prometheans in the Lab" McGraw-Hill: New York, 2002. ISBN 0-07-140795-2</ref> Due to its extreme toxicity, many early researchers of TEL became ill (including Midgley himself), and dozens died [1]. In 1924, DuPont 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 [2].

Leaded gasoline phased out in the European Union on the 1st January 2000, & was only recently phased out in China (around 2001).

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).

One of the worlds leaders in removal of Tetra Ethyl Lead is Crest Environmental Services http://crest-enviro.com whose latest decommisioning project removed and decontaminated all leaded Gasoline facilities for Saudi Aramco in the Kingdom of Saudi Arabia. In June 2006 the Kingdom of Saudi Arabia was declared Lead Free.

[edit] 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.