Pressure Effect on CH3 and C2H3 Cross-Radical Reactions

Published

November 1, 1999

Author(s)

Askar Fahr, Allan H. Laufer, D C. Tardy

Abstract

The effect of pressure on the cross- radical reactions of vinyl and methyl radicals have been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH₃COC₂H₃) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.287 k Pa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C₃H₆), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C₂H₆), the methyl radical combination product. At 27 kPa pressure a value of [C₃H₆]/[C₂H₆] = 1.28 was derived. However, the value of this ratio was reduced by nearly a factor of two to about 0.75 when the pressure was reduced to about 0.28 kPa. Kinetic modeling results indicated the contribution of the C₂H₃ + CH₃ --M-->C₃H₆ + M to the total cross-radical reactions, is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C₂H₃+CH₃ >C₃H₅ + H, becomes available, producing a host of allyl radical reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5 hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C₃H₆. Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. In has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The appearent pathological behavior in this unsaturated system is attributed to the formation of a stronger C-C bond as contrasted to the weaker C-C bond formed from combination of saturated hydrocarbon radicals. This C-C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C-H bond or isomerize to cyclopropane.

Citation

Journal of Physical Chemistry

Volume

103

Pub Type

Journals

Keywords

combination, cross-radical reactions, disproportionation, methyl, pressure effect, RRKM calculation, vinyl

Citation

Fahr, A. , Laufer, A. and Tardy, D. (1999), Pressure Effect on CH₃ and C₂H₃ Cross-Radical Reactions, Journal of Physical Chemistry (Accessed November 8, 2024)