Volatile organic compounds (VOCs) are organic chemicals that have a high vapor pressure at ordinary room temperature. VOCs represent a class of atmospheric pollutants that can have a significant impact on human health, both through their own properties and those of their degradation products. In fact, their reactions in the atmosphere can also influence the concentrations of reactive inorganic species such as O3 and NOx either directly or through their reactions with other reactive species such as Cl and OH. Studies by adept researchers have established that the dominant atmospheric oxidant of VOCs is generally the OH free radical. However, degradation of VOCs by their reactions with Cl can also be significant. From a technical perspective, both OH and Cl have been found to react with VOCs by hydrogen abstraction. Consequently, the organic free radicals formed in this way can react rapidly with atmospheric O2 forming a wide range of secondary VOCs such as ketones, aldehydes, esters, and acids each with its own characteristic atmospheric chemistry. As of now, most of the rate coefficients reported for reactions of Cl with VOCs have been reported for experiments at atmospheric pressure and a temperature close to 298 K.
In fact, previous research by Canadian scientists at Acadia University led by Professor John Roscoe measured the rate coefficients for the reactions of Cl with several VOCs over a modest range of temperature and reported that; while for the most part the rate coefficients were independent of temperature, the reactions of Cl with isobutane and 1,4-dioxane had a weak temperature dependence. Therefore, it is evident that further investigation of Cl with other reactants at various temperature ranges ought to be carried out. On this account, researchers at Acadia University: Mr. W. James Frazee and Professor John Roscoe examined the effect of temperature on the kinetics of the reactions of Cl with toluene and xylenes. Their work is currently published in International Journal of Chemical Kinetics.
In their experimental approach, chemical analysis was conducted by gas chromatography. The authors used the relative rate method for kinetic analysis, and the relative rate coefficients were converted to absolute values using absolute rate coefficients for the reference reactions obtained from recent critical evaluations. Generally, most of the measurements were made at a total pressure of approximately 100 kPa in argon.
The authors reported that there was very little temperature dependence, and variation of the total pressure and the concentration of O2 had no effect. In addition, it was seen that the values of the absolute rate coefficients were independent of the reference reaction used. Further, the reaction of Cl with toluene was studied relative to the reactions of Cl with isobutane, ethane, propane, and n-butane.
In summary, the temperature dependence of the rate coefficients for the reactions of Cl with toluene and the xylenes was carefully examined from 290 to 362 K. The results obtained were in satisfactory agreement with those reported before, all of which were measured at fixed temperatures very close to 298 K. More so, the kinetic results were discussed in relation to published descriptions of the dynamics of reactions of Cl and OH with organic compounds. In a statement to Advances in Engineering, Professor John Roscoe highlighted that now there is a consensus regarding Cl reactions with alkanes in that it proceeds via a direct abstraction process without formation of an addition complex with the organic reactant.
W. James Frazee, John M. Roscoe. Temperature dependence of the reactions of Cl with toluene and the xylenes. International Journal of Chemical Kinetics 2019; volume 51: page 579–589.