Gas-phase reactions of Cl atoms with hydrochloroethers: relative rate constants. Potential interference by OH radicals.

PABLO R. DALMASSO, RAÚL A. TACCONE, JORGE D. NIETO, PABLO M. COMETTO, SILVIA I. LANE.

Los Cocos. Córdoba. Argentina.

Congreso; 9th Latin American Conference on Physical Organic Chemistry (CLAFQO).; 2007

The determinations of rate constants for the reactions de Cl atoms with organic compounds could show discrepancies when they are carried out in N₂ or air as the diluent gas. Kaiser and Wallington [1] discovered that the apparent rate constant of the  reaction “Cl + C₂H₄ ® Products” increased when the amount of O₂ in the diluent gas was raised. They observed that O₂ affected the measured rate constant  indicating that reactions of C₂H₄ with radicals other than Cl may occur at low pressures in their relative rate determinations, leading to erroneous results in the presence of O₂. In agreement with Michael et al. [2] and Tyndall et al. [3], Kaiser and Wallington indicated that the potential interference is the OH radical formed via secondary reactions involving O₂ and that the magnitude of this interference may depend on the total pressure and on the initial reactant concentrations.    

The aims of this study have been:

* Extending the existing scant data base of rate constants for the reactions of chlorine atoms with hydrochloroethers as part of ongoing work in our laboratory regarding the atmospheric impact of these compounds as acceptable candidates to replace the harmful CFCs and their derivatives in industrial uses.

* Testing the potential systematic errors that could affect the rate coefficients for the reactions studied due to interference by OH radicals.

A relative rate technique was used to study the kinetics of the reactions of Cl atoms with CH₃OCHCl₂ and CH₃OCH₂CH₂Cl at (298 ± 2)K and atmospheric pressure (~ 750 Torr), using synthetic air as diluent gas. All the experiments were carried out in a 80 L collapsible Tedlar chamber and a gas chromatograph equipped with a flame ionization detector (GC-FID) was used for the quantitative analysis of the reactants. Chlorine atoms were generated by the UV photolysis of molecular chlorine. Different reference compounds were employed for the reactions.

To the best of our knowledge, this kinetic study constitutes the first experimental determination of the rate coefficient for the reaction of Cl atoms with CH₃OCHCl₂.

The results obtained will be presented and compared with those obtained previously for the same and related reactions of Cl atoms. The atmospheric implications for the studied hydrochloroethers are considered briefly.

[1] Kaiser, E.W., Wallington, T.J., 1996. Journal of Physical Chemistry 100, 4111.

[2] Michael, J.V., Keil, D.G., Klemm, R.B., 1985. Journal of  Chemical Physics 83, 1630.

[3] Tyndall, G.S., Staffelbach, T.A., Orlando, J.J., Calvert, J.G., 1995. International Journal of           Chemical Kinetics 27, 1009.