Radical pyrolyzed

An interesting reaction occurs when the dihydropyrazine (102 R = CHaPh) is pyrolyzed under vacuum. Toluene is liberated to give the monobenzylpyrazine (103) in high yield, presumably by a radical mechanism. 

Paralene [para-xylene] Also called Gorham and also spelled parylene. A process for coating articles with poly-p-xylene. The vapor of di-p-xylylene is pyrolyzed at 550°C, yielding p-xylyl free radicals, -CHj-CgH CH, which deposit and polymerize on cooled surfaces. Developed by W. F. Gorham at Union Carbide Corporation. 

It is also necessary to explain why there are parentheses around the collision partner M in reactions (3.94), (3.95), and (3.99). When RH in reactions (3.94) and (3.95) is ethane and R in reaction (3.99) is the ethyl radical, the reaction order depends on the temperature and pressure range. Reactions (3.94), (3.95), and (3.99) for the ethane system are in the fall-off regime for most typical combustion conditions. Reactions (3.94) and (3.95) for propane may lie in the fall-off regime for some combustion conditions however, around 1 atm, butane and larger molecules pyrolyze near their high-pressure limits [34] and essentially follow first-order kinetics. Furthermore, for the formation of the olefin, an ethyl radical in reaction (3.99) must compete with the abstraction reaction. 

All of these problems are overcome if the precursors are pyrolyzed in an inert host gas at high pressure, because under these conditions energy transfer occurs mostly by colhsions with the host gas, and radical recombination is largely suppressed. Of course, a constant stream of hot gas at high pressure is incompatible with the requirements of matrix isolation, so the experiment has to be carried out in a pulsed fashion. Chen and co-workers were the first to propose what they called a hyperthermal nozzle for pulsed pyrolysis at very high temperatures, at that time for gas-phase studies. Several research groups have implemented variants of this design" for work in matrix isolation and have used it successfully for a variety of sffidies. 

Oxidations will be chain reactions when one or more of Reactions 7 to 10 are faster than Reactions 4 to 6 combined. They will be auto-catalytic when products such as ROOH and HOOH pyrolyze to give new free radicals by reactions such as 12 and 12, or when products such as aldehydes and olefins react with oxygen to give new free radicals by reactions such as 13. 

The first quantitative measure of reaction (7) was made by Stavely and Hinshelwood,400 who pyrolyzed CH3OCH3. They found a chain length of about 17 for 157 torr of ether at 540°C. The addition of 0.10 torr of NO reduced the rate to one half of its uninhibited value. Under these conditions, the reaction of CH3 radicals with CH3OCH3 is 11 times as fast with NO. (Some termination still occurs by radical-radical processes.) The rate constant is about 1.0 x 106 A/-1 sec-1 at 540°C430 for... 

To produce CH3 radicals, Birss, Danby, and Hinshclwood44 pyrolyzed 50 torr of (ter/-C4H90)2 at 160°C. With NO present the C2H6 rate fell off, because of the competition between reactions (7) and (10). From the C2H6 rates and the value of 10,377 k7 was found to be 0.9 x 107 Af-1 sec-1, which corresponds to a third-order rate constant of 5.6 x 109 M 2 sec-1. Either constant seems to be too low to be correct. 

Bryce and Chrysochoos64 pyrolyzed isopentane in the presence of NO at 519°C and found that [C2H4] increased with the NO pressure. They explained their results in terms of reaction (23), with R being ethyl radicals. Metcalfe and Trotman-Dickenson308 photolyzed (C2H5)2CO at 145 and 210°C in the presence of 0-100 torr of NO. They found C2H4, which possibly could be attributed to reaction (23). 

Chilton and Gowenlock85,87 pyrolyzed (z -C3H7)2Hg with NO and N2 as a carrier gas in a flow system at 230-280°C. They found (CH3)2 CHN=0 and (CH3)2C=NOH as products,85 the latter arising from the isomerization of the former. Woodall and Gunning454 studied the sensitized [Hg 6(3.Pi) plus NO Ilj)] decomposition of propane and its deuterated analogs at room temperature. Both n-propyl and isopropyl radicals were produced and added to NO. The product isomers, i.e., the respective oximes, were the principal products. An unusual feature of this study was that the oximes were formed readily at room temperature. The authors suggested that the reactant radicals might have been hot, and this coupled with the heat of addition could have facilitated the isomerization. 

Tertiary butyl radicals were examined by Levy and Copeland,285 who pyrolyzed 2,2 -azoisobutane with NO at 180-220°C, and by Kraus and Calvert,267 who performed two runs at 106°C on the photolysis of (tert-C H9)2CO in the presence of NO. In the former study, a blue product, presumably (CH3)3CN=0, was found. In the latter study the results were interpreted with reactions (22) and (23). 

The absolute value for k26 when RO is tert-C4H90 was obtained by Birss, Danby, and Hinshelwood.44 They pyrolyzed 50 torr of (tert-CiH90)2 at 160°C in the presence of NO. The tert-C HgO radical formed initially can add to NO via (26) or decompose by... 

When 3-chloro-l-butene was pyrolyzed in a static system at temperatures 776-835°K, the primary product was methane and some higher molecular weight hydrocarbons37. This result was ascribed to the initial dissociation of the substrate into chloroalkyl and methyl radicals followed by H abstraction (equations 6-9). 

It is a known fact that the gas-phase pyrolysis kinetics of alkyl bromides have not been extensively investigated due to the experimental difficulties as well as to the complexity of concurrent unimolecular and radical chain mechanisms. However, when these organic bromides are pyrolyzed under maximum inhibition, the reaction in the presence of a free radical suppressor is a molecular elimination. Sometimes, these organic bromides are pyrolyzed under maximum catalysis with HBr gas, and the process may proceed by an autocatalytic molecular mechanism. 

When methyl chloroformates, which lack a -hydrogen at the alkyl side of the ester, were pyrolyzed in a seasoned vessel and in the presence of a radical suppressor at 425 480 °C, they gave consistently the elimination products shown in equation 105192. 

In this way, allyl iodide (C3H5I) and nitrosobenzene (CeHsNO) can be pyrolyzed quantitatively to generate allyl and radicals, respectively. For... 

The presence of OII3 radicals has been shown by the isolation of large amounts of formaldoxiine when dtBP is pyrolyzed in the presence of excess NO." This arises from the reaction... 

One of the above-mentioned examples comprises the investigation of hydrocarbon radicals formed by pyrolysis of fluids, such as indene and phenyl-substituted alkanes at about 843 K (36). As a reaction tube, a silica capillary of 1.4 mm inner and 4.7 mm outer diameter was used, which was connected to high-pressure stainless-steel tubing through Teflon seals. It was equipped with pressure transducers on both ends and tested to withstand a maximum pressure of 28 MPa. The hydrocarbon to be pyrolyzed was cycled between a reservoir and the reaction tube by a high-pressure liquid chromatography (HPLC) pump. The tube was heated by a preheated stream of N2 as described above. 

Above 170 °C the amidrazone ylide (36) decomposes with loss of triethylamine and concurrent cyclization to give an 85% yield of 2-phenylbenzimidazole (Scheme 19) (B-75M140800). Poorer yields ( 40%) are obtained when N-benzyl-o-nitroaniline is pyrolyzed in the presence of iron oxalate. No doubt this last reaction is similar in many respects to the reactions shown in Scheme 2. Both 2-phenyl-imidazoles and -benzimidazoles (as well as other 2-substituted analogues) can be obtained as a result of thermal rearrangement of the 1-substituted isomers (Section 4.07.1.2.2), by radical substitution methods (Section 4.07.1.7) or via the 2-lithio derivatives (Sections 4.07.1.6, 4.07.3.7). 

Staveley and Hobbs and Hinshelwood studied the inhibition of the reaction by nitric oxide, and found under certain conditions that some 13 % was uninhibitable. Several investigations have shown that this residual reaction is not a molecular reaction. Stevenson et al studied the pyrolysis of propane containing radioactive carbon, and concluded that isotopic mixing took place at the same rate, relative to the pyrolysis rate, when the reaction was completely inhibited as when it was uninhibited Hinshelwood et al obtained a similar result. Poltorak and Voievodsky showed that when propane is pyrolyzed in the presence of D2, D atoms appear in the hydrocarbon fraction at a rate, relative to the rate of decomposition, that is independent of the amount of nitric oxide present. All of these results show that free radicals are still important in the reaction occurring in the presence of nitric oxide, and provide no support for the view that a molecular mechanism plays a significant role in the propane pyrolysis. Evidence reading to the same conclusion is provided by the experiments of Niclause et which show that in certain reaction vessels the propane pyrolysis is completely... 

A particular a ect, ordinarily one of the first to be explored in a free radical process, has hardly received attention. It is a qualitative proof of the existence and quantitative analysis of the types of radicals present in the system. There are obvious difficulties in a pyrolyzing polymer, but they, we expect, will be overcome. In this manner, it may even become possible to provide estimates of the rate constants in Equation 2. 

Nature of the Free Radicals in Coals, Pyrolyzed Coals, Solvent-Refined Coal, and Coal Liquefaction Products... 

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