Kinetics of elementary steps in the reaction of atomic bromine with 2,3-dimethyl-2-butene under atmospheric conditions



We have employed the laser flash photolysis–resonance fluorescence technique to investigate the gas-phase kinetics of elementary steps in the Br-initiated oxidation of 2,3-dimethyl-2-butene (TME) under atmospheric conditions. At T⩾ 274 K, measured rate coefficients are independent of pressure suggesting hydrogen abstraction as the dominant pathway. The following Arrhenius expression adequately describes all kinetic data at 274 K ⩽T⩽ 420 K: k1a(T) = (3.84 ± 0.84) × 10− 11 exp[(−169 ± 61)/T] cm3 molecule−1s−1 (uncertainties are 2σ, precision only). At 203 K ⩽T⩽ 241 K, kinetic evidence for reversible addition, Br + TME ↔ Br−TME (k1b, k−1b), is observed. Analysis of the approach to equilibrium data allows evaluation of the rate coefficients k1b and k−1b. At atmospheric pressure addition of Br to TME occurs at a near gas kinetic rate. Equilibrium constants are obtained from k1b/k−1b. Combining the experimental results with electronic structure calculations allows third-law analyses of the equilibrium data. The following thermochemical parameters for the addition reaction (1b) at 0 and 298 K are obtained (standard state = 1 atm): ΔrH0= −47.1 ± 3.0 kJ mol−1, ΔrH298=−47.3 ± 3.0 kJ mol−1, and ΔrS298=−101.5 ± 10.0 J mol−1 K−1. Examination of the effect of added O2 on Br atom kinetics under conditions where reversible adduct formation is observed allows determination of the rate coefficient for the Br–TME + O2 reaction (k2). At 223 K, we find that k2 increases from 3.9 to 5.5 × 10−12 cm3 molecule−1s−1 as pressure increases from 25 to 200 Torr. Our results suggest that under most atmospheric conditions the Br–TME reaction with O2 occurs more rapidly than the Br–TME unimolecular decomposition. Hence, the fast addition reaction appears to control the rate of TME loss by reaction with Br in the atmosphere. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 13–26, 2012