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: k 1a (T) = (3.84 ± 0.84) × 10 − 11 exp[(−169 ± 61)/ T ] cm 3 molecule −1 s −1 (uncertainties are 2 σ , precision only). At 203 K ⩽ T ⩽ 241 K, kinetic evidence for reversible addition, Br + TME ↔ Br−TME ( k 1b , k −1b ), is observed. Analysis of the approach to equilibrium data allows evaluation of the rate coefficients k 1b and k −1b . At atmospheric pressure addition of Br to TME occurs at a near gas kinetic rate. Equilibrium constants are obtained from k 1b / 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): Δ r H 0 = −47.1 ± 3.0 kJ mol −1 , Δ r H 298 =−47.3 ± 3.0 kJ mol −1 , and Δ r S 298 =−101.5 ± 10.0 J mol −1 K −1 . Examination of the effect of added O 2 on Br atom kinetics under conditions where reversible adduct formation is observed allows determination of the rate coefficient for the Br–TME + O 2 reaction ( k 2 ). At 223 K, we find that k 2 increases from 3.9 to 5.5 × 10 −12 cm 3 molecule −1 s −1 as pressure increases from 25 to 200 Torr. Our results suggest that under most atmospheric conditions the Br–TME reaction with O 2 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