Tetra-ethyl lead

From Academic Kids

Tetra-ethyl lead (also known as TEL, lead tetraethyl and tetraethyllead) is a toxic organometallic chemical compound, with formula (CH2CH3)4Pb, which was once used as a gasoline (petrol) additive.

Contents

Chemistry

TEL is a viscous colorless liquid, produced by reacting ethyl chloride with a sodium-lead alloy. TEL has a very weak carbon-lead bond, and at the temperatures found in internal combustion engines it decomposes into lead and ethyl radicals, propagating the combustion by radical reactions.

When TEL burns, it produces lead and many lead compounds (including lead oxide), which would quickly build up and destroy an engine. That is why scavengers such as ethylene dibromide and ethylene dichloride are used, which form volatile lead(II) bromide and lead(II) chloride respectively.

Uses

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 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.

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 TEL caused to clog up) further reduced TEL use. TEL is still available from a limited number of outlets as an additive, mostly for owners of classic and vintage cars and motorcycles.

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.

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 [1] (http://yarchive.net/chem/tetraethyl_lead.html). 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] (http://old.fairfieldweekly.com/articles/leadout_timeline.html). However, in many European countries leaded gasoline is still available.

Leaded gasoline 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).

See also: Lead poisoning.

External links

ja:四エチル鉛 pl:Tetraetyloołów zh:四乙基鉛

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