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

Abstract

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 2σ , 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 14 LAINE ET AL.(standard state = 1 atm): r H 0 = −47.1 ± 3.0 kJ mol −1 , r H 298 = −47.3 ± 3.0 kJ mol −1 , and 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.

Extension of Structure–Reactivity Correlations for the Hydrogen Abstraction Reaction by Bromine Atom and Comparison to Chlorine Atom and Hydroxyl Radical

“…The database of experimental gas-phase rate constants k Br is less extensive and less internally consistent than those for k Cl or k OH , nor do we have the advantage of a compilation of evaluated and recommended values as we had for k Cl and k OH . In addition, because of the lower reactivity of Br•, Arrhenius parameters have typically been obtained at higher temperature and therefore require longer, and questionable, extrapolations to 298 K. A compilation is given in Table . − All formulas have the carbon from which hydrogen abstraction occurs at the beginning followed by the substituents attached, that is, CH 3 X, CH 2 XY, or CHXYZ. For compounds with multiple potential abstraction sites, the experimental rate constant is typically assumed to be dominantly that of the most reactive site, a relatively safe assumption (see below) given the known high selectivity of Br•, compared with, for example, Cl•.…”

“…More recent methods have followed the decay of Br• in real time in an excess of RH by resonance fluorescence spectroscopy or mass spectrometry after its generation by a short flash photolysis or discharge pulse. This approach has been applied to ethane, neopentane, propylene, E -2-butene, 2,3-dimethyl-2-butene, , methanol, dimethyl sulfide, dimethyl sulfoxide, propane, n -butane, bromochloromethane, and i -butane. ,, …”

“…However, this hypothesis is suspect because four of the five Δlog k values for alcohols with the very similar − OH substituent at the radical center are negative: CH 3 OH (−1.12 and −0.23), 33,34 CH 2 (CH 2 CH 3 )OH (−0.45), 25 and CH(CH 3 ) 2 OH (−1.44). 25 The 13 cases with Δlog k < − 1, that is, the correlation overpredicts the observed value, are CHF 3 (−2.04), 9 CH 3 CHCH 2 (−1.84), 20 CH 3 CHCHCH 3 (−1.82), 21 1 of the 3 values for CH 3 C(CH 3 ) 3 (−1.62), 9 CH 3 F (−1.56), 9 CH 2 FCl (−1.55), 47 CH 2 (CH 3 ) 2 OH (−1.44), 25 CH(CF 3 ) 2 F (−1.19), 52 CH 2 (CF 3 )F (−1.17), 9 both values for (CH 3 ) 2 C C(CH 3 ) 2 (−1.16) 22 and (−1.10), 23 1 of the 2 values for CH 3 OH (−1.12), 34 and CH(CH 3 )F 2 (−1.04). 9 The allylic hydrogens in all four values for olefins fall into this class, 1.…”

“…For 1,1-dichloroethane, , k exp is 230-fold greater than that for ethane, again indicative of dominant α attack. For olefins addition is more rapid than abstraction but the rate constant for abstraction has been separated out by measuring the real-time formation of HBr , or by working at high enough temperature and/or low enough pressure that the addition channel is totally reversed and hence does not consume Br•. (These approaches may still give only an upper limit because of a competing addition–elimination channel …”

Kinetic and Mechanistic Studies for the Gas-phase Reaction of Ozone with 2, 3-Dimethyl-2-Butene and 1, 3-Butadiene

“…Acrolein and formaldehyde have been noticed as the initial products from those reactions and considerably furan from the reaction with OH, and nitrates from the reaction with NO 3 [5,11]. There were some studies suggested that a possible reaction of 2,3-dimethyl 2butene with bromine may be participated in elimination of this compound from the atmosphere [13].…”