Hydroxyl radical alkane reactions

No systematic studies of a number of compoimds have yet appeared to discover correlations suggestive of mechanism. This paper presents the fractional conversions and reaction rates measured under reference conditions (50 mg contaminants/m ) in air at 7% relative humidity (1000 mg/m H2O), for 18 compounds including representatives of the important contaminant classes of alcohols, ethers, alkanes, chloroethenes, chloroalkanes, and aromatics. Plots of these conversions and rates vs. hydroxyl radical and chlorine radical rate constants, vs. the reactant coverage (dark conditions), and vs. the product of rate constant times coverage are constructed to discern which of the proposed mechanistic suggestions appear dominant. 

DeMore, W.B., Bayes, K.D.(1999) Rate constants for the reactions of hydroxyl radical with several alkanes, cycloalkanes, and dimethyl ether. J. Phys. Chem. A, 103, 2649-2654. 

Greiner, N.R. (1970) Hydroxyl radical kinetics by kinetic spectroscopy. VI. Reactions with alkanes in the range 300-500 K. J. Chem. Phys. 53, 1070-1076. 

Harris, S.J., Kerr, J.A. (1988) Relative rate measurements of some reactions of hydroxyl radicals with alkanes studied under atmospheric conditions. Int. J. Chem. Kinet. 20, 939-955. 

Lloyd, A.C., Darnall, K.R., Winer, A.M., Pitts, Jr., J.N. (1976) Relative rate constants for reaction of the hydroxyl radical with a series of alkanes, alkenes, and aromatic hydrocarbons. J. Phys. Chem. 80, 189-794. 

The hydroxyl radical will be the predominant entity which attacks the alkane to regenerate an alkyl radical (Reaction 10) under conditions where isomerization and decomposition are the usual fate of alkylperoxy radicals. The activation energy for attack on an alkane molecule by OH, although difficult to determine accurately (30), is low (I, 3) (1-2 kcal. per mole). This has an important consequence. The reaction will be unselective, being insensitive to C—H bond strength. Each and every alkyl radical derived from the alkane skeleton will therefore be formed. To describe the chain-propagation steps under conditions where isomerization is a frequent fate of alkylperoxy radicals it is necessary, then, to consider each and every alkylperoxy radical derived from the alkane and not just the tertiary radicals. 

Figure 9.25 shows the correlation coefficient of 0.9636 between the hydroxyl radical rate constants and UV/Ti02 rate constants for chlorinated alkanes. Therefore, the reaction mechanism for the degradation of chlorinated alkanes by UV/Ti02 is similar to the reaction with hydroxyl radical. This correlation suggests that the reaction proceeds via hydroxyl radical attack on the chlorinated alkane. 

The intermediates formed in AOPs sometimes are more toxic than the parent compounds and are required to be decomposed completely using either combination of AOPs or combination of AOP and some other treatment methods such as adsorption and biodegradation. Carbonyl compounds, particularly aldehydes, are quite toxic, and some of the secondary compounds formed from aldehydes, especially peroxyacylnitrates are more dangerous than the parent compounds. Organic peroxy radical (ROj) reactions are of significance because they represent an important class of intermediates formed in the oxidation process of hydrocarbons (15). Intermediates such as ethers and alcohols have enhanced reactivity toward hydroxyl radical. The rate constant of oxidation of these compounds is of similar order of magnitude as of the alkanes. 

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