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Tetra-ethyl lead
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Tetra-ethyl lead

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

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 bromide and lead chloride respectively.

Uses

TEL's was once extensively used as a additive in gasoline (petrol), for its ability to increase the fuel's octane rating (i.e. to reduce its premature ignition ("knocking") in the engine), thus allowing the use of higher compression ratios for greater efficiency and power. Also, 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 environmentalal 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) was the coup de grace.

Many vehicles produced before TEL's phase-out require 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 old 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.

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]. 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 [1]. However, in many European countries leaded gasoline is still available.

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