Kinetics and Mechanism of the Reaction of Cl Atoms with 2-Methyl-1,3-butadiene (Isoprene) at 298 K

Abstract

The reaction of chlorine atoms with isoprene (2-methyl-1,3-butadiene) may be important in the marine boundary layer. The kinetics and mechanisms of this reaction have been studied at 760 Torr and 298 K using the photolysis of Cl2 as the chlorine atom source. Product studies in air were carried out using 40 m path length FTIR spectrometry. Rate constants were determined in either air or N2 using a relative rate technique and GC-FID to follow the decay of isoprene relative to n-butane. For comparison, the kinetics of the chlorine atom reactions with isoprene-d8, 1,3-butadiene, and 1,3-butadiene-d6 were also studied at 1 atm in air or N2 at 298 K. The rate constant ratios determined with n-butane as the reference compound are as follows: isoprene (2.16 ± 0.17); isoprene-d8 (1.79 ± 0.22), 1,3-butadiene (2.01 ± 0.11); 1,3-butadiene-d6 (1.85 ± 0.10) (±2σ). The corresponding absolute rate constants, based on a value for n-butane of 2.11 ± 0.18 × 10-10 cm3 molecule-1 s-1, are as follows: isoprene (4.6 ± 0.5); isoprene-d8 (3.8 ± 0.6); 1,3-butadiene (4.2 ± 0.4); 1,3-butadiene-d6 (3.9 ± 0.4) (±2σ), all in units of 10-10 cm3 molecule-1 s-1. Both the kinetics studies and the HCl yields determined using FTIR show that the net abstraction of an allylic hydrogen is a small but significant fraction, 15 ± 4% (±2σ), of the overall reaction at 1 atm. The major organic products appear to be unsaturated chlorine-containing carbonyl compounds which remain unidentified. The atmospheric implications are discussed.