Kinetics and thermochemistry of the equilibria CH 2Cl+O 2⇔CH 2ClO 2 and CHCl 2+O 2⇔CHCl 2O 2

Abstract

The equilibria, CH 2 Cl+O 2 ⇔CH 2 ClO 2 and CHCl 2 +O 2 ⇔CHCl 2 O 2 , were studied between 562 and 664 K and 498 and 562 K respectively. Equilibrium constants were measured as a function of temperature. The chlorinated methyl radicals were produced by pulsed UV excimer laser photolysis. The decay of the radicals to their equilibrium concentrations in the presence of O 2 was monitored in time-resolved experiments using photoionization mass spectrometry. The measured equilibrium constants were combined with calculated entropy and heat capacity changes in Third Law calculations to obtain the enthalpy changes (ΔH 298 o ) for the two reactions: −121(±11) kJ mol −1 for the CH 2 Cl+O 2 →CH 2 ClO 2 reaction and −106(±6) kJ mol −1 for the CHCl 2 +O 2 →CHCl 2 O 2 reaction. The results indicate a considerable weakening of the C−O bond in the methylperoxy radical (which is 136 kJ mol −1 ) due to chlorine substitution in the CH 3 group. This effect of chlorine substitution on bond strengths is compared with that found for other compounds of the type CH 3 -X. The implications of weaker R-O 2 bond strengths caused by chlorine substitution in the oxidation chemistry of chlorinated hydrocarbons is briefly discussed. Heats of formation of CH 2 ClO 2 (ΔH f .298 o =−8(±12) kJ mol −1 ) and CHCl 2 O 2 (ΔH f .298 o =−21(±10) kJ mol −1 ) were also obtained using the experimental and theoretical results.