Pressure Dependence of the Reaction Cl + C3H6

Abstract

The rate constant for the reaction Cl + C3H6 (k1) has been measured relative to that of Cl + C2H6 over the range 0.3−700 Torr in N2 at 298 K. UV irradiation was used to generate Cl atoms in mixtures of C3H6, C2H6, Cl2, and N2 in two different reactors using FTIR or GC analysis. The yields of the two major products, allyl chloride (3-C3H5Cl) and 1,2-dichloropropane were measured. k1 decreases by a factor of 5 between 700 and 1 Torr. Below 1 Torr, the rate constant becomes independent of pressure. The results indicate that k1 is a composite of three reaction channels, each having a different pressure dependence. Measurement of the yield of 1,2-dichloropropane, the final product formed from the addition of Cl to C3H6, at each pressure allows a determination of the rate constant (k1a) for the addition of Cl to C3H6. Assuming a typical center broadening factor (Fc = 0.6), the high- and low-pressure limiting constants are calculated to be k1a(∞) = (2.7 ± 0.4) × 10-10 cm3 molecule-1 s-1 and k1a(0) = (4.0 ± 0.4) × 10-28 cm6 molecule-2 s-1. The pressure dependence of the yield of 3-C3H5Cl indicates that the allyl radical is likely formed by both abstraction and addition−elimination channels. The rate constant of the abstraction reaction from the methyl radical in C3H6 is (2.3 ± 0.3) × 10-11 cm3 molecule-1 s-1. At pressures below 10 Torr, the rate constant for formation of the allyl radical increases by 50%, and this is ascribed to an addition−elimination process. Relative rate constant ratios were also measured for Cl atom reactions with allyl chloride (k6) and 1,2-dichloropropane (k7) relative to C3H6, C2H5Cl, or CH3Cl to correct the product yield experiments for secondary consumption. The observed values of k6/k1 are 0.75 for total pressures of 10−700 Torr, 0.44 at 1 Torr, and 0.33 at 0.4 Torr. On the basis of the relative rate measurements k7 = (3.9 ± 0.6) × 10-12 cm3 molecule-1 s-1 over the range 1−700 Torr.