Azomethane pyrolysis

The formal kinetics A B C occurs commonly in practice. Consider the reactivity of the free methyl radical, generated by azomethane pyrolysis, with an alkane, RH ... 

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

Methyl radicals were produced by pyrolysis of azomethane (CH3N2CH3). Azomethane was synthesized as describe earlier [18]. It was purified periodically by fteeze-pump cycles at 77 K, and the gas purity verified by RGA. The methyl radical source was similar to that developed by Stair and coworkers. [10, 11] The source was made of a quartz tube with 3 mm OD and 1 mm ID, resistive heating was supplied by means of a 0.25 mm diameter tantalum wire wrapped outside the quartz tube. The len of the heating zone was 4 cm, recessed from the end of the tube by 1 cm. An alumina tube around the outside of the heating zone served as a radiation shield. Azomethane was admitted to the hot tube at a pressure of 1x10-8 to 1x10-7 Torr via a high-vacuum precision leak valve. The pyrolysis tube was maintained at about 1200 K, adequate to decrease the major peaks in the mass sp trum of the parent azomethane at 58 and 43 amu by at least a factor of 100. 

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. 

The extreme sensitivity of CH3CHO to chain decomposition makes it very susceptible to free radical sensitization. Thus OH3 radicals from the pyrolysis of azomethane can induce the chain decomposition in CIIsCHO at 300°C, the chain length being as great as 500. The photolysis of azomethane at room temperature can also sensitize the decomposition. Letort showed that CH3 radicals from dtBP will decompose as many as 50 molecules of CH3CHO per molecule of dtBP at 160 0. 

The pyrolysis of azomethane which yields mainly nitrogen and ethane, has been extensively studied [138, 139]. At high temperatures the decomposition is explosive [140]. The evidence suggests that the explosion is of thermal origin above 636 °K, while below this temperature there are indications that chain-branching plays a significant part. 

It was not suggested how the aryldiazomethane was initially formed but the minimum reaction temperature was lowered by 150° C on introduction of a phenyldi-azomethane catalyst. In contrast, pyrolysis of benzophenone azine gave no tetra-phenylethylene and only a trace of nitrogen . Benzophenone azine decomposes by a free radical process at 375-500 °C to afford principally benzhydrylideneimine, benzonitrile and 6-phenylphenanthridine accompanied by lesser quantities of benzene, biphenyl, diphenylmethane and benzhydrylideneaniline by the following pathways... 

The present discussion is by no means exhaustive. It is designed to provide a summary of the most significant and reliable kinetic data, at least those that appear so to the author. There is a variety of methods for producing alkyl radicals, and, naturally, there will be certain restrictions on experimental conditions depending on the method chosen. Some of the common methods for generation of methyl radicals, for example, include photolysis of acetone, pyrolysis of di-ferf-butyl peroxide, photolysis of biacetyl, photolysis of azomethane and decarbonylation of acetaldehyde. In the majority of cases discussed here, the reactions were followed by product determinations, employing gas chromatography. 

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