Free radicals Paneth mirror technique

Thermal cracking of organic substances is an important reaction in the petroleum industry and has been extensively studied for over seventy years. At least for simple alkanes, the decay is first order in good approximation and therefore was long believed to occur in a single, unimolecular step [21]. However, in the 1930s, Rice and coworkers [22-24] established the presence of free radicals under the conditions of the reaction by means of the Paneth mirror technique [25,26], This observation led Rice and Herzfeld to propose a chain mechanism [22,27,28], Extensive later studies proved the essential features of their mechanism to be correct not only for hydrocarbons, but also for many other types of organic substances. 

In terms of physical methods, by 1937 there had been only a few advances beyond the mirror technique of Paneth or the invoking of forbidden reactivity in solution to establish that a mechanism involved free radicals (or not). As noted above, mass... 

In a similar manner, Pearson and Purcell 205) could obtain dimethyl-tellurium from photolysis of acetone if the Te mirror were not heated, but dimethylditellurium formed from a hot mirror. The Paneth technique can be used as a test for the presence of organic free radicals in the vapor phase. Other early work on organometals and free radicals is given in a book by Waters (275). 

An important early method (1929) for the detection of gaseous free radicals was the metallic mirror technique of Paneth. Since then, many compounds have been used to determine the presence and extent of free radical reactions in the gas phase decomposition of organic compounds. The subject of inhibition in gas-phase reactions has been reviewed by Ashmore and by Gowenlock . 

Experimental confirmation of free radical participation in chemical reactions was first obtained by Paneth and Hofeditz by the chemical method of mirror removal. This most familiar technique has been used subsequently in many studies of free radicals. The apparatus consisted of a Sow tube operated at pressures of about 1-2 torr. In the original experiments of Paneth and Hofeditz the reactant was lead tetramethyl, and the carrier gas was specially purified hydrogen. A lead mirror was deposited in the downstream part of the tube by local application of heat. If now the tube was heated upstream from the mirror, the first mirror gradually disappeared, suggesting that methyl radicals, produced pyrolytically from lead tetramethyl, removed the first mirror by the reaction... 

Paneth technique. Method demonstrating the existence of free radicals (e.g., methyl) or atoms, which is based on the removal of a metallic mirror by a stream of gas containing the radicals. The reaction products can be collected and assayed. 

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