Pressure Dependence of the Reaction Cl + C3H6

The rate constant for the reaction Cl + C 3 H 6 ( k 1 ) has been measured relative to that of Cl + C 2 H 6 over the range 0.3−700 Torr in N 2 at 298 K. UV irradiation was used to generate Cl atoms in mixtures of C 3 H 6, C 2 H 6, Cl 2, and N 2 in two different reactors using FTIR or GC analysis. The yields of the two major products, allyl chloride (3-C 3 H 5 Cl) and 1,2-dichloropropane were measured. k 1 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 k 1 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 C 3 H 6, at each pressure allows a determination of the rate constant ( k 1a ) for the addition of Cl to C 3 H 6 . Assuming a typical center broadening factor ( F c = 0.6), the high- and low-pressure limiting constants are calculated to be k 1a (∞) = (2.7 ± 0.4) × 10 -10 cm 3 molecule -1 s -1 and k 1a (0) = (4.0 ± 0.4) × 10 -28 cm 6 molecule -2 s -1 . The pressure dependence of the yield of 3-C 3 H 5 Cl 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 C 3 H 6 is (2.3 ± 0.3) × 10 -11 cm 3 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 ( k 6 ) and 1,2-dichloropropane ( k 7 ) relative to C 3 H 6, C 2 H 5 Cl, or CH 3 Cl to correct the product yield experiments for secondary consumption. The observed values of k 6 / k 1 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 k 7 = (3.9 ± 0.6) × 10 -12 cm 3 molecule -1 s -1 over the range 1−700 Torr.