Rate constants for the gas-phase reactions of OH radicals with a series of C6-C14 alkenes at 299 +/- 2 K.

Rate constants for the gas-phase reactions of OH radicals with the C(6)-C(14) 2-methyl-1-alkenes and the C(6)-C(10) trans-2-alkenes have been measured at 299 +/- 2 K and atmospheric pressure of air using a relative rate technique. The rate constants obtained (in units of 10(-11) cm(3) molecule(-1) s(-1)) were as follows: 2-methyl-1-pentene, 5.67 +/- 0.21; 2-methyl-1-hexene, 6.50 +/- 0.11; 2-methyl-1-heptene, 6.71 +/- 0.21; 2-methyl-1-octene, 7.02 +/- 0.16; 2-methyl-1-nonene, 7.28 +/- 0.21; 2-methyl-1-decene, 7.85 +/- 0.26; 2-methyl-1-undecene, 7.85 +/- 0.21; 2-methyl-1-dodecene, 7.96 +/- 0.26; 2-methyl-1-tridecene, 8.06 +/- 0.37; trans-2-hexene, 6.08 +/- 0.26; trans-2-heptene, 6.76 +/- 0.32; trans-2-octene, 7.23 +/- 0.21; trans-2-nonene, 7.54 +/- 0.16; and trans-2-decene, 7.80 +/- 0.26, where the indicated errors are two least-squares standard deviations and do not include the uncertainty associated with the rate constant for the reference compound alpha-pinene. Our data show that the rate constants for the reactions of OH radicals with 2-methyl-1-alkenes and trans-2-alkenes increase with increasing carbon number, suggesting that this is in part due to H-atom abstraction from the C-H bonds of the alkyl substituent groups. Combined with previous literature data for the reactions of OH radicals with a series of 1-alkenes, we propose that the increase in rate constant with increasing carbon number is due to H-atom abstraction from the C-H bonds of the alkyl substituent groups and to enhancement of the rate constant for OH radical addition to the C=C bond, which increases with carbon number of a C(n)-alkyl substituent group up to a maximum at approximately C(8).