Initiation mechanisms, radical reactions thermolysis

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. 

Thermolysis of 44 produced products derived from the Myers-Saito cyclization reaction. However, when 43 having a trimethylsilyl substituent at the acetylenic terminus was subjected to heating in the presence of 1,4-CHD at 70 °C for 3 h, the 1H-cyclobut[a]indene 46 was produced. A reaction mechanism involving an initial Schmittel cyclization to generate the benzofulvene biradical 45 followed by an intramolecular radical-radical coupling was proposed to account for the formation of the formal [2 + 2]-cycloaddition product 46. 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The most characteristic reactions for these compounds are thermolysis and decomposition under the action of both acids and nucleophilic reagents. 1,2,4-Dioxazolidines 15 are thermally stable only below 100 °C. The thermolysis of compound 15a in solution at 130 °C afforded benzophenone and benzamide in quantitative yields. The formation of these products is consistent with a radical mechanism involving initial 0-0 homolysis (Scheme 21). The base-catalyzed decomposition of compound 15a afforded the same products. Similar results were observed for the decomposition of compounds 15b and 15c. The interaction of dioxazolidine 15a with triphenylphosphine resulted in deoxygenation of the system with the concomitant formation of benzophenone and its imine with aniline <1995J(P1)41>. 

It has been observed that the activation energy for thermal decomposition of ethyl nitrate is substantially reduced in the presence of lead oxide or copper surfaces. Above 200° C approximately, the thermolysis of ethyl nitrate becomes much more complex and detonation in the gas phase is common. In the range 242-260° C the reaction was found to be half-order, with an overall rate coefficient" k = 10 - exp(—46,800)/J 7 mmole. l . sec apparently the initiation step is unaltered but the subsequent radical chain mechanism affects the overall rate of decomposition. 

Sang et al. studied the antioxidant mechanism of epicatechin upon reaction with a peroxyl radical generated by thermolysis of the initiator 2,2 -azo-bisisobutyronitrile (AIBN). The progress of the reaction was thought to proceed via the following stages ... 

To evaluate the antioxidant mechanism of garcinol Sang et al. developed studies with the objective of characterize the reaction products of garcinol with the stable radical DPPH and with peroxyl radicals generated by thermolysis of the azo initiator azo-bis-isobutyrylnitrile (AIBN) [77,78],... 

Oxirane formation can also occur via free radical mechanisms, as in the reaction of certain fluoroalkenes with oxygen. Under pressure at elevated temperatures, oxygen alone can suffice, but activation is frequently provided in the form of radical initiators (e.g., tribromofluoromethane) and ultraviolet light. Thermolysis of dioxole 5, comonomer from which DuPont s Teflon-AF is made, offers an unusual route to an oxirane. Rearrangement of the heterocycle presumably takes place via a biradical intermediate. ... 

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