Kinetics of reaction of chlorine atoms with some biogenic organics

Abstract

The reaction kinetics of atomic chlorine with a series of biogenic hydrocarbons, including the two enantiomers of α-pinene, were studied at 298 K and 1 atm pressure using a relative rate technique. The simultaneous losses of the biogenic of interest and a reference compound, either n-nonane or n-butane, were followed using gas chromatography with flame ionization detection as a function of the extent of photolysis of a chlorine atom precursor. Thionyl chloride, trichloroacetyl chloride or in a few trials, acetyl chloride, were photolyzed at 254 nm to generate chlorine atoms, since molecular chlorine reacted in the dark with these organics. The relative rate constants for ethane and isoprene determined relative to n-butane using SOCl2 and CCl3COCl were compared to those determined using Cl2 to check for possible artifacts. The average relative rate constants for ethane and isoprene (both relative to n-butane) using these new sources are (0.281 ± 0.021) and (2.49 ± 0.39) (±2 σ) respectively, within experimental error of those measured using Cl2 as the chlorine atom source. The relative rate constants averaged over all sources including Cl2 are (0.277 ± 0.025) for ethane and (2.42 ± 0.45) for isoprene. The ratios of rate constants for the chlorine atom reactions with the biogenics with formula C10H16 relative to n-nonane were as follows: (R)-α-pinene (0.991 ± 0.264); (S)-α-pinene (0.946 ± 0.240); β-pinene (1.09 ± 0.30); (R)-limonene (1.33 ± 0.15); myrcene (1.36 ± 0.31); 3-carene (1.16 ± 0.23). That for p-cymene, C10H14, is (0.433 ± 0.072). Taking k(Cl + n-nonane) = (4.82 ± 0.14) × 10−10 cm3 molecule−1 s−1, the absolute rate constants (in units of 10−10 cm3 molecule−1 s−1) are: (R)-α-pinene (4.8 ± 1.3); (S)-α-pinene (4.6 ± 1.2); β-pinene (5.3 ± 1.5); limonene (6.4 ± 0.8); myrcene (6.6 ± 1.5); 3-carene (5.6 ± 1.3); p-cymene (2.1 ± 0.4). (All errors are ± 2 σ). Although abstraction was not measured directly in this study, it is likely a significant contributor to the overall reactions of the C10H16 biogenics. The rate constant for the reaction of the aromatic compound p-cymene is within experimental error of that predicted from the sum of reaction with toluene plus the isopropyl substituent. A limited number of experiments for methyl vinyl ketone in N2 using CCl3COCl as the chlorine atom source and nonane as the reference compound gave a relative rate constant of (0.422 ± 0.034), corresponding to an absolute rate constant of (2.0 ± 0.2) × 10−10 cm3 molecule−1 s−1. Based on these rate constants, the lifetimes of these biogenics at dawn with respect to reaction with chlorine atoms are expected to be comparable to reaction with OH. Thus, loss of these biogenics by reaction with atomic chlorine must be taken into account in coastal regions in addition to their reactions with OH, O3 and at night, NO3. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 491–499, 1999