Kinetics of phenyl radical reactions with propane, n ‐butane, n ‐hexane, and n ‐octane: Reactivity of C 6 H 5 toward the secondary CH bond of alkanes

The kinetics of C 6 H 5 reactions with n ‐C n H 2 n +2 ( n = 3, 4, 6, 8) have been studied by the pulsed laser photolysis/mass spectrometric method using C 6 H 5 COCH 3 as the phenyl precursor at temperatures between 494 and 1051 K. The rate constants were determined by kinetic modeling of the absolute yields of C 6 H 6 at each temperature. Another major product C 6 H 5 CH 3 formed by the recombination of C 6 H 5 and CH 3 could also be quantitatively modeled using the known rate constant for the reaction. A weighted least‐squares analysis of the four sets of data gave k (C 3 H 8 ) = (1.96 ± 0.15) × 10 11 exp[−(1938 ± 56)/ T ], and k ( n ‐C 4 H 10 ) = (2.65 ± 0.23) × 10 11 exp[−(1950 ± 55)/ T ] k ( n ‐C 6 H 14 ) = (4.56 ± 0.21) × 10 11 exp[−(1735 ± 55)/ T ], and k ( n −C 8 H 18 ) = (4.31 ± 0.39) × 10 11 exp[−(1415 ± 65) T ] cm 3 mol −1 s −1 for the temperature range studied. For the butane and hexane reactions, we have also applied the CRDS technique to extend our temperature range down to 297 K; the results obtained by the decay of C 6 H 5 with CRDS agree fully with those determined by absolute product yield measurements with PLP/MS. Weighted least‐squares analyses of these two sets of data gave rise to k (n−C 4 H 10 ) = (2.70 ± 0.15) × 10 11 exp[−(1880 ± 127)/ T ] and k (n−C 6 H 14 ) = (4.81 ± 0.30) × 10 11 exp[−(1780 ± 133)/ T ] cm 3 mol −1 s −1 for the temperature range 297‐‐1046 K. From the absolute rate constants for the two larger molecular reactions (C 6 H 5 + n ‐C 6 H 14 and n ‐C 8 H 18 ), we derived the rate constant for H‐abstraction from a secondary CH bond, k s −CH = (4.19 ± 0.24) × 10 10 exp[−(1770 ± 48)/ T ] cm 3 mol −1 s −1 . © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 49–56, 2004