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Kinetics of elementary steps in the reaction of atomic bromine with 2,3‐dimethyl‐2‐butene under atmospheric conditions
Author(s) -
Laine Patrick L.,
Sohn Yoon S.,
Nicovich J. Michael,
Mckee Michael L.,
Wine Paul H.
Publication year - 2012
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.20608
Subject(s) - chemistry , arrhenius equation , reaction rate constant , bromine , kinetics , flash photolysis , analytical chemistry (journal) , resonance fluorescence , chemical kinetics , kinetic energy , reaction rate , thermodynamics , activation energy , fluorescence , catalysis , organic chemistry , physics , quantum mechanics
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