Tetraethyl lead

Tetraethyl lead (TEL) is a liquid with the chemical formula (CH₃CH₂)₄Pb. Once widely used (circa 1925 to 1990) to increase the octane rating of gasoline (petrol), TEL usage in gasoline has been largely phased out by most nations primarily because of the toxicity of the lead emissions from spark-ignited internal combustion engines burning gasoline containing TEL. Another reason for discontinuing TEL usage was that it degraded the efficiency of the catalytic converters installed in automotive vehicles to reduce their emissions of air pollutants.

Manufacture and properties

TEL is produced by the alkylation of a sodium-lead alloy using chloroethane as expressed by this chemical equation:

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

which can also be written as:

4 moles of chloroethane + 4 moles of sodium-lead alloy → 1 mole of tetraethyl lead + 4 moles of sodium chloride + 3 moles of lead

Properties Property Value Common name tetraethyl lead Other names tetra-ethyl lead, lead tetraethyl, tetraethyl plumbane, TEL IUPAC name tetraethylplumbane CAS number 78-00-2 Molecular formula (CH3CH2)4Pb Molecular mass 323.4 g/mol Density 1.653 g/ml for liquid at 20 °C, 1 atm Boiling point 455.7 K (182.6 °C) at 1 atm Melting point 142.94 K (-130.2 °C) Flash point 366.48 K (93.3 °C) Specific heat, cp 0.956 J/(g·K) for liquid at 20 °C Heat of vaporization 175.0 J/g for liquid at 182.6 °C Heat of fusion 27.2 J/g for solid at -130.2 °C Viscosity 0.862 mPa·s (0.862 cP) at 20 °C Refractive index 1.5198 nD20

The product tetraethyl lead is a viscous colorless liquid that is highly lipophilic and soluble in fats, oils and lipids as well as gasoline and other non-polar hydrocarbons. Other properties of tetraethyl lead are listed in the adjacent table.

At the temperatures found in spark-ignited internal combustion engines, TEL decomposes completely into lead and lead oxide (PbO) and combustible, short-lived ethyl radicals. Lead itself is the reactive agent that enhances the octane rating of gasolines and tetraethyl lead serves as a gasoline-soluble lead carrier.

TEL fluid formulation

When TEL burns, it produces not only carbon dioxide (CO₂) and water (H₂O), but also lead (Pb):

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

The lead can the oxidize further to give lead oxide (PbO):

2 Pb + O₂ → 2 PbO

The Pb and PbO would soon accumulate and destroy an engine. For this reason, the TEL used in gasoline was actually part of a blended liquid formulation known as TEL fluid or ethyl fluid that contained 1,2-dibromoethane and 1,2-dichloroethane liquids known as lead scavengers. Those scavengers formed lead bromide (PbBr) and lead chloride (PbCl) which are volatile and were emitted from the engine exhaust to the atmosphere. The complete composition of TEL fluid was:

• Tetraethyl lead       61.45%
• 1,2-Dibromoethane 17.85%
• 1,2-Dichloroethane 18.80%
• Inerts & color dye    1.90%

The addition of as little as 0.8 ml of TEL fluid per liter of gasoline (3 ml/gallon) of gasoline was equivalent to adding 0.5 g of lead per liter of gasoline and resulted in significant increases in the octane rating of the gasoline.

History of tetraethyl lead as a gasoline octane enhancer

In the 1920's, petroleum refining technology was rather primitive and produced gasolines with an octane rating of about 40 – 60. But automotive engines were rapidly being improved and required better gasolines, which led to a search for octane rating enhancers. That search culminated in 1921 in the development and use of tetraethyl lead as an octane enhancer. Despite ethanol being widely recognized as an alternative octane rating enhancer, the less expensive TEL quickly became commercially available as what was called TEL fluid, which contained 61.5 weight % TEL. The addition of as little as 0.8 ml of that TEL fluid per litre (equivalent to 0.5 gram of lead per litre) of gasoline resulted in significant octane rating increases. Production and sale of "leaded gas" was briefly banned in 1925 by the Surgeon General, and a panel of experts was appointed to investigate a number of fatalities that had "occurred in the manufacture and mixing of the concentrated tetraethyl lead". Then, in 1927, the Surgeon General set a voluntary standard for the petroleum refining industry to follow in mixing tetraethyl lead with gasoline. The standard was 3 cubic centimeters per gallon (cm³/gal), corresponding to the maximum then in use among refiners, and thus imposed no real restraint. For about the next 50 years, TEL was used as the most cost effective way to raise the octane rating of gasolines. During that period, petroleum refining technology grew until high-octane gasolines could, in fact, be produced without using TEL. Also, in about the 1940's, it was discovered that the lead being emitted in the exhaust gases from vehicular internal combustion engines was a toxic air pollutant that seriously affected human health. Because of its toxicity and the fact that catalytic converters being installed in vehicles could not tolerate the presence of lead, the U.S. EPA launched an initiative in 1972 to phase out the use of TEL in the United States and it was completely banned for use in on-road vehicles as of January 1996. Using TEL in race cars, airplanes, marine engines and farm equipment is still permitted. TEL usage has also been phased out by most nations worldwide. As of 2008, the only nations still allowing extensive use of TEL are the Democratic People's Republic of Korea, Burma, and Yemen.