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) 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

Good sources for TEL history.