Free radicals from tetraethyllead

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

Catalysts that decrease reaction rates are usually referred to as inhibitors. They often act by interfering with the free-radical processes involved in chain reactions, and the mechanism usually differs from that involved in accelerating a reaction. The most familiar example of the use of inhibitors has historically been the addition of additives such as tetraethyllead or methyl terf-butyl ether to gasoline to improve its antiknock properties. 

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 ... 

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