Kinetics of the reactions of unsaturated free radicals (methylvinyl and i-C4H3) with molecular oxygen

Abstract

The kinetics and mechanisms of the reactions of two unsaturated hydrocarbon free radicals, methylvinyl (CH3CH=CH) and i-C4H3 (CH2=C=C=CH) with molecular oxygen were investigated using a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature and density in time-resolved experiments. Searches were conducted for possible products to identify reactive routes. The methylvinyl+O2 reaction was studied betweeen 296 and 600 K. The rate constant is essentially constant throughout this range, 7.2(±1.4)×10−12 cm3 molecule−1 s−1. The only reactive route observed is that which yields CH3CHO and HCO, a route analogous to that of the C2H3+O2 reaction. The i-C4H3+O2 reaction rate constant was measured from 296 to 900 K and was found to decrease sharply with increasing temperature, from 4.5(±0.9)×10−12 cm3 molecule−1 s−1 at 296 to 6.4(±1.3)×10−13)×10−13 cm3 molecule−1 s−1 at 900 K corresponding to a T−1.8 power dependence in this temperature range. The reaction rate constants of both reactions studied did not depend on density. No products of the i-C4H3+O2 reaction could bedetected. Possible mechanisms of this reaction are discussed. The role of elementary reactions between unsaturated free radicals and molecular oxygen in combustion processes is briefly reviewed.