Tetraethyl lead

TEL is a viscous colorless liquid with a sweet odor.

TEL is produced on an industrial scale by reacting chloroethane (ethyl chloride) with a sodium–lead alloy.

The product is recovered by steam distillation, leaving a sludge of lead and sodium chloride.

Despite decades of research, no reactions were found to improve upon this process; it is rather difficult, involves reactive metallic sodium, and converts only 25% of the lead to TEL. A related compound, tetramethyllead, was commercially produced by a different electrolytic reaction. However, tetramethyllead was even more difficult to make, and it did not find use beyond niche applications. A highly efficient pathway utilizing ethyl chloride with a slight excess of lithium was developed, with a TEL yield over lead of over 90%. However, by then the fuel additive had started to fall out of favor and into disrepute, and the process was never put into practice.

A noteworthy feature of TEL is the weakness of its four C–Pb bonds. At the temperatures found in internal combustion engines, TEL decomposes completely into lead as well as combustible, short-lived ethyl radicals. Lead and lead oxide scavenge radical intermediates in combustion reactions. Engine knock is caused by a cool flame, an oscillating low-temperature combustion reaction that occurs before the proper, hot ignition. Lead quenches the pyrolyzed radicals and thus kills the radical chain reaction that would sustain a cool flame, preventing it from disturbing the smooth ignition of the hot flame front. Lead itself is the reactive antiknock agent, and the ethyl groups serve as a gasoline-soluble carrier.

When TEL burns, it produces not only carbon dioxide and water, but also lead and lead(II) oxide: Pb ( C 2 H 5 ) 4 + 13 O 2 ⟶ 8 CO 2 + 10 H 2 O + Pb {\displaystyle {\ce {Pb(C2H5)4 + 13 O2 -> 8 CO2 + 10 H2O + Pb}}} 2 Pb ( C 2 H 5 ) 4 + 27 O 2 ⟶ 16 CO 2 + 20 H 2 O + 2 PbO {\displaystyle {\ce {2 Pb(C2H5)4 + 27 O2 -> 16 CO2 + 20 H2O + 2 PbO}}}

Pb and PbO would quickly over-accumulate and foul an engine. For this reason, 1,2-dichloroethane and 1,2-dibromoethane were also added to gasoline as lead scavengers—these agents form volatile lead(II) chloride and lead(II) bromide, respectively, which flush the lead from the engine and into the air: Pb ( C 2 H 5 ) 4 + C 2 H 4 X 2 + 16 O 2 ⟶ 10 CO 2 + 12 H 2 O + PbX 2 {\displaystyle {\ce {Pb(C2H5)4 + C2H4X2 + 16 O2 -> 10 CO2 + 12 H2O + PbX2}}}

TEL was extensively used as a gasoline additive beginning in the 1920s, wherein it served as an effective antiknock agent and reduced exhaust valve and valve seat wear. Concerns were raised in reputable journals of likely health outcomes of fine particles of lead in the atmosphere as early as 1924.

Tetraethyllead helps cool intake valves and is an excellent buffer against microwelds forming between exhaust valves and their seats. Once these valves reopen, the microwelds pull apart and abrade the valves and seats, leading to valve recession. When TEL began to be phased out, the automotive industry began specifying hardened valve seats and upgraded materials which allow for high wear resistance without requiring lead.

A gasoline-fueled reciprocating engine requires fuel of sufficient octane rating to prevent uncontrolled combustion (pre-ignition and detonation). Antiknock agents allow the use of higher compression ratios for greater efficiency and peak power. Adding varying amounts of additives to gasoline allowed easy, inexpensive control of octane ratings. TEL offered the business advantage of being commercially profitable because its use for this purpose could be patented. Aviation fuels with TEL used in WWII reached octane ratings of 150 to enable turbocharged and supercharged engines such as the Rolls-Royce Merlin and Griffon to reach high horsepower ratings at altitude. In military aviation, TEL manipulation allowed a range of different fuels to be tailored for particular flight conditions.

In 1935 a license to produce TEL was given to IG Farben, enabling the newly formed German Luftwaffe to use high-octane gasoline for high altitude flight. A company, Ethyl GmbH, was formed that produced TEL at two sites in Germany with a government contract from 10 June 1936.

In 1938 the United Kingdom Air Ministry contracted with ICI for the construction and operation of a TEL plant. A site was chosen at Holford Moss, near Plumley in Cheshire. Construction started in April 1939 and TEL was being produced by September 1940.

For mixing with raw gasoline, TEL was most commonly supplied in the form of "Ethyl Fluid", consisting of TEL blended with 1,2-dichloroethane and 1,2-dibromoethane, which prevent lead from building up in the engine. …

6 mL of tetraethyllead is enough to induce severe lead poisoning. The hazards of TEL content are heightened due to the compound's volatility and high lipophilicity, enabling it to easily cross the blood–brain barrier.

Because TEL is charge neutral and contains an exterior of alkyl groups, it is highly lipophilic and soluble in petrol (gasoline). This property, which allows it to dissolve so evenly and effectively in motor fuel, also allowed easy absorption by body fats and lipids and diffusion through the blood–brain barrier (BBB). The lead (II) ions (Pb) would accumulate within the limbic forebrain, frontal cortex, and hippocampus. Practically speaking, TEL is a "central nervous system toxin which produces an acute toxic psychosis."

There is no cure for direct poisoning by TEL. Inorganic lead compounds, such as those present in engine exhausts, could be removed from the system through the administration of chelating agents, which bind to the inorganic lead and flush them out of the body. However, highly lipid-soluble TEL cannot be removed this way, and treatments are of a supportive nature.

Early symptoms of acute exposure to tetraethyllead can manifest as irritation of the eyes and skin, sneezing, fever, vomiting, and a metallic taste in the mouth. Later symptoms of acute TEL poisoning include pulmonary edema, anemia, ataxia, convulsions, severe weight loss, delirium, irritability, hallucinations, nightmares, fever, muscle and joint pain, swelling of the brain, coma, and damage to cardiovascular and renal organs. Chronic exposure to TEL can cause long-term negative effects such as memory loss, delayed reflexes, neurological problems, insomnia, tremors, psychosis, loss of attention, and an overall decrease in IQ and cognitive function.

The carcinogenity of tetraethyllead is debatable. It is believed to harm the male reproductive system and cause birth defects.

Concerns over the toxicity of lead eventually led to the ban on TEL in automobile gasoline in many countries. Some neurologists have speculated that the lead phaseout may have caused average IQ levels to rise by several points in the US (by reducing cumulative brain damage throughout the population, especially in the young). For the entire US population, during and after the TEL phaseout, the mean blood lead level dropped from 16 μg/dL in 1976 to only 3 μg/dL in 1991. The U.S. …

In 1853, German chemist Karl Jacob Löwig (1803–1890) first prepared what he claimed was Pb2(C2H5)3 from ethyl iodide and an alloy of lead and sodium. In 1859, English chemist George Bowdler Buckton (1818–1905) reported what he claimed was Pb(C2H5)2 from zinc ethyl (Zn(C2H5)2) and lead(II) chloride. Later authors credit both methods of preparation with producing tetraethyl lead.

TEL remained unimportant commercially until the 1920s. In 1921, at the direction of DuPont Corporation, which manufactured TEL, it was found to be an effective antiknock agent by Thomas Midgley Jr., working under Charles Kettering at General Motors Corporation Research. General Motors patented the use of TEL as an antiknock agent and used the name "Ethyl" that had been proposed by Kettering in its marketing materials, thereby avoiding the negative connotation of the word "lead". Early research into "engine knocking" (also called "pinging" or "pinking") was also led by A.H. Gibson and Harry Ricardo in England and Thomas Boyd in the United States. The discovery that lead additives modified this behavior led to the widespread adoption of their use in the 1920s, and therefore more powerful, higher-compression engines. In 1924, Standard Oil of New Jersey (ESSO/EXXON) and General Motors created the Ethyl Gasoline Corporation to produce and market TEL. Deepwater, New Jersey, across the river from Wilmington, was the site for production of some of DuPont's most important chemicals, particularly TEL. After TEL production at the Bayway Refinery was shut down, Deepwater was the only plant in the Western hemisphere producing TEL up to 1948, when it accounted for the bulk of the Dupont/Deepwater's production.

The toxicity of concentrated TEL was recognized early on, as lead had been recognized since the 19th century as a dangerous substance that could cause lead poisoning. In 1924, a public controversy arose over the "loony gas", after five workers died, and many others were severely injured, in Standard Oil refineries in New Jersey. There had also been a private controversy for two years prior to this controversy; several public health experts, including Alice Hamilton and Yandell Henderson, engaged Midgley and Kettering with letters warning of the dangers to public health. After the death of the workers, dozens of newspapers reported on the issue. …