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Tetra-ethyl lead

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Tetra-ethyl lead
Tetra-ethyl lead
General
Systematic name ?
Other names TEL
lead tetraethyl
Molecular formula C8H20Pb
SMILES ?
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 solubie in benzene, hexane
Melting point −136 °C
Boiling point 84-85 °C15 mm Hg
refractive index 1.519
Basicity (pKb) ?
Viscosity ? cP at ? °C
Structure
Molecular shape tetrahedral
Crystal structure ?
Dipole moment 0 D
Hazards
MSDS External MSDS
Main hazards ?
NFPA 704
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, εr, etc.
Thermodynamic
data 		Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Other anions ?
Other cations ?
Related ? ?
Related compounds ?
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
[Chemical infoboxInfobox disclaimer and references]

Tetra-ethyl lead (also known as TEL, lead tetraethyl and tetraethyllead) is a toxic organometallic chemical compound, with formula (CH3CH2)4Pb, which was once used as a gasoline (petrol) additive. It is still in use today as an additive in aviation fuel.

Contents

Synthesis and properties

TEL is produced by reacting ethyl chloride with a sodium-lead alloy.
:Pb + 4 Na + 4 CH3CH2Cl → (CH3CH2)4Pb + 4 NaCl
The product, TEL, is a viscous colorless liquid. Because TEL is charge neutral (vs. a salt) and contains an exterior of alkyl groups, it is highly lipophilic and soluble in petrol.

The most 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 an ethyl radicals. These radicals help to propagating the combustion, which itself is a radical reaction. 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)
:Pb + 0.5 O2 → PbO
The Pb and PbO would quickly accumulate and destroy an engine. For this reason, lead scavengers such as ethylene dibromide 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).

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 deposited on the valve seats and helped protect them against wear.

The use of TEL in gasoline was mostly dominant in the US while in Europe alcohol was used instead. However the dominance of the US oil companies eventually led to a switch to leaded fuel. When used as an antiknock agent alcohol would make the fuel hydroscopic (it would absorb water from the air) over time fuel humidity could lead to rusting and corrosion, as well as issues of biological contamination.

In most Western countries this additive went out of use in the late 20th century, chiefly because of the realization that most of its lead—which is toxic to humans and other organisms—ended up in the exhaust fumes and became a major health and environmental problem. The need for that additive was also lessened by the introduction of harder metals for valves and valve-seats, a general reduction in engine compression ratios and the introduction of other anti-knocking additives. The deployment of the catalytic converter (which lead oxides from TEL would foul) further reduced TEL use. 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. In addition, TEL is still in use as an ingredient in the fuels used by NASCAR in its top three racing series. A more environmental friendly additive is composed of contains 10 ppm potassium (sometimes sodium), solvents and 75% diesel, and is added to unleaded fuel to boost the octane rating by 2-3 points.

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 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. 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.

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 [link]. 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 [link].

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).

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.

Reference

"Leaded Gasoline, Safe Refrigeration, and Thomas Midgley, Jr." Chapter 6 in S. Bertsch McGrayne "Prometheans in the Lab" McGraw-Hill: New York, 2002. ISBN 0071407952

See also

External links

 

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