Alkyl radical decomposition models

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

Section 2.5 examines addition reactions which are the reverse of the radical decomposition reactions considered in Section 2.4. These reactions in themselves are comparatively unimportant in hydrocarbon oxidation, but they have provided a good source of thermodynamic data on radicals. Thermodynamic parameters are central to the modelling of autoignition because of the importance of heat release, but also because of their use in determining the rate parameters for the reverse of well characterized reactions. Section 2.5 includes a brief review of the currently accepted alkyl radical heats of formation. This field has been in turmoil in recent years because of disagreements on the values, which largely derive from kinetic measurements. Consensus is emerging but controversy still remains. 

The reaction between HO and even simple carbohydrates is very complex [240]. Therefore, some basic assumptions will be made on the example of glucose as a simple model compovmd The first step of the reaction is the abstraction of one H. Nearly all positions are affected to the same extent, while the positions C-1, C-2, and C-6 are slightly preferred [240]. Please note that under these conditions, twelve different radicals are generated because in aqueous solution, glucose exists as a- and P-anomers. Molecular oxygen is subsequently added to the alkyl radical, whereby peroxyl radicals are generated. The addition of O2 is also diffusion-controlled. The decomposition of these initially generated radicals yields a considerable variety of reaction products, which are listed in [240]. 

R. J. Wolf, Theoretical studies of the formation and decomposition of vibrationally excited model alkyl radicals, Ph.D. thesis, Wayne State University, Detroit, 1980. 

Scheme 1 Decomposition outline ofthe aromatic part of Apollofix-Red dye, modeled with a substituted benzene (lower part) and achieving its complete mineralization (upper part). A disproportionation reaction between two cyclohexadienylic radicals regenerates the aromatic ring structure In a phenol derivative. The R-group can be an alkyl group ora halogen atom.

Later studies showed that the mechanism of reactions, in particular ionic versus free-radical, could vary. Townsend [15] has studied the reaction of a series of coal model compounds (alkyl-aryl hydrocarbons and ethers) in supercritical water. For the hydrocarbons a free-radical pyrolysis route does not take advantage of the medium. However, for the ethers enhanced rates of reaction through a hydrolysis route occurs. As a result of different possible pathways, decomposition products of some organics in supercritical water have been shown by several workers to vary with solvent strength. In the absence of water, Pr(H20) = 0, pyrolysis is dominant and yields a variety of products including polycondensates. The main products of decomposition of neat methoxy... 

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