Pressure dependence of the reaction Cl+C3H6

The rate constant for the reaction Cl + C3H6 (k(1)) has been measured relative to that of Cl + C2H6 over the range 0.3-700 Ton in N-2 at 298 K. UV irradiation was used to generate Cl atoms in mixtures of C3H6, C2H6, Cl-2, and N-2 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. k(1) decreases by a factor of 5 between 700 and 1 Ton. Below 1 Ton, 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 C3H6, at each pressure allows a determination of the rate constant (k(1a)) for the addition of Cl to C3H6. Assuming a typical center broadening factor (F-c = 0.6), the high- and low-pressure limiting constants are calculated to be k(1a)(infinity) = (2.7 +/- 0.4) x 10(-10) cm(3) molecule(-1) s(-1) and k(1a)(0) = (4.0 +/- 0.4) x 10(-28) cm(6) 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) x 10(-11) cm(3) molecule(-1) s(-1). At pressures below 10 Ton, 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 C3H6, C2H5Cl, or CH3Cl 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 Ton, 0.44 at 1 Ton, and 0.33 at 0.4 Ton. On the basis of the relative rate measurements k(7) = (3.9 +/- 0.6) x 10(-12) cm(3) molecule(-1) s(-1) over the range 1-700 Ton.