Tetraethyllead, decomposition

Tetraethyllead is a transparent colourless liquid in small concentrations its vapours have a sweetish fruity smell in large concentrations it has an unpleasant odour. Tetraethyllead boils at 200 °C (with decomposition) d420 = 1.6524 nD20= 1.5198. It does not dissolve in water but can be easily dissolved in alcohols, diethyl ether, petrol, acetone and other organic solvents. 

The thermal decompositions of tetramethyllead 297>, tetraethyllead 255> and tetraphenyllead 112,192) have been shown to proceed via a free radical mechanism, the initial step of which is represented by the equation ... 

The liquid phase pyrolysis of tetraethyllead has been investigated by Razuvaev and coworkers 25B> with the aid of spectrophotometric techniques. Both hexaethyldilead and diethyllead were detected as intermediate decomposition products. The following reaction sequence was proposed to account for the formation of these lead compounds. 

The radical mechanism was justified by the polymerization of various olefins by the [Pb(CH3)4 + Cu(NOg)2] system, but not by methylcopper. Recently, Clinton and Kochi 49, 50) have reexamined each stage of the reaction for tetraethyllead in acetic acid solution. The decomposition was strongly catalyzed by copper(I), and acetolysis was found to occur by a metastable intermediate, ethylcopper ... 

Halogenation of Alkanes - Reaction Mechanism Section 4.4D The ethyl radicals formed from the decomposition of tetraethyllead can react with methane to form methyl radicals or with chlorine to form chlorine radicals. Both of these are part of the propagation steps. 

Tetraethyllead is stable for long periods of time if stored in brown, glass-stoppered bottles. Direct exposure to sunlight results in gradual decomposition. 

One other item of historical interest may suffice. Paneth was an inorganic chemist who reported on simple free alkyl radicals, which he generated by the pyrolysis of alkylmetallic compounds of metals such as lead and whose half-life period he measured. About 3 years before, H. Taylor (a ph)rsical chemist) published in a less generally used periodical, his ideas and experimental observations on studies concerned with catalytic reduction processes of hydrogen-ethylene mixtures. He wrote that the simplest method of liberating free alkyl radicals into a mixture of these gases was by decomposition of metal alkyls such as diethylmercury and tetraethyllead. 

If organolead compounds are heated, the radicals are easily produced by a decomposition. Organolead compounds have been investigated as the polymerization initiators of various kinds of unsaturated compounds [9,10,39,40]. The unsaturated eompounds are ethylene, propylene, butadiene, styrene, acrylonitrile and vinyl chloride, etc. Organolead compounds are tetraethyllead, tetracyclo-hexyllead, tetravinyllead, tetraethyllead and TiCl3, the mixtures with organoalu-minum compounds or transition metal halides [9,10,39,40]. 

Leermakers, j. a. The effect of ethyl radicals on the thermal decomposition of azomethane. The kinetics of the thermal decomposition of tetraethyllead. Ethyl radicals and hydrogen. J. Amer. Chem. Soc. 55, 4508 (1933). 

Among the early works on free radicals, the importance of experiments of F. Paneth and W. Hafeditz is often quoted. They supplied evidence that aliphatic free radicals occur in the decomposition of metallic alkyls, such as dimethyl-mercury (Hg(CH3)2), and tetramethyl- or tetraethyllead (Pb(CH3)4 or Pb(C2H5)4). Paneth saturated a stream of nitrogen with Pb(CH3)4 vapor. The vapors were then heated to 450 °C. Decomposition of Pb(CH3)4 deposited a lead mirror on the heated part of the tube. When the vapors from the decomposition passed over the deposited lead mirror at 100 °C, the mirror slowly disappeared. It was concluded that the following reaction had taken place ... 

Since a potassium electrolyte has better conductivity but separation of metallic potassium from the amalgam is difficult, a method for regeneration of potassium-containing electrolytes has been developed on the basis of the reaction of the potassium amalgam with NaAlR4 [57, 102]. Use of this double-decomposition reaction has made it possible to develop a scheme for the preparation of tetraethyllead with complete regeneration of the electrolyte ... 

Huntzicker et al. [34] have shown the photochemical reactivity of tetraethyllead and tetramethyllead, finding that there is a light induced decomposition in the atmosphere. These organic compounds are less volatile than gasoline, but small amounts may be emitted to the atmosphere by evaporation from fuel storage and transportation systems. 

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