Temperature Dependence of the Rate Coefficients for the Reactions of Atomic Bromine with Toluene, Tetrahydrofuran, and Tetrahydropyran

Abstract

The rate coefficients for the reactions of atomic bromine with toluene, tetrahydrofuran, and tetrahydropyran were measured from approximately 295 to 362 K using the relative rate method. Iso-octane was used as the reference compound for the reaction with toluene, and iso-octane and toluene were used as the reference compounds for the reaction with tetrahydrofuran; tetrahydrofuran was used as the reference compound for the reaction with tetrahydropyran. The rate coefficients were found to be unaffected by changes in pressure and oxygen concentration. The rate coefficient ratios were converted to absolute values using the absolute rate coefficient for the reaction of Br with the reference compound. The absolute rate coefficients, in the units cm(3) molecule(-1) s(-1), for the reaction of Br with toluene are given by k(T) = (3.7 +/- 1.7) x 10(-12) exp(-(1.63 +/- 0.15) x 10(3)/T), for the reaction of Br with tetrahydrofuran by k(T) = (3.7 +/- 2.7) x 10(-10) exp(-(2.20 +/- 0.22) x 10(3)/T), and for the reaction of Br with tetrahydropyran by k(T) = (3.6 +/- 1.8) x 10(-10) exp(-(2.35 +/- 0.16) x 10(3)/T). The uncertainties represent one standard deviation. The Arrhenius parameters for these reactions are compared with results in the literature for dimethyl ether, diethyl ether, and a series of saturated hydrocarbons, and the effects of structural variation on these parameters are identified.

A theoretical kinetic study on the reaction of atomic bromine with toluene

“…Again, we note that these adjustments of the barrier heights are within the chemical accuracy of the ab initio methods used here. The adjusted barrier height of 10.4 kJ/mol agrees closely with the enthalpy of activation () reported by Giri and Roscoe 15 who calculated the thermodynamic parameters from their experimental kinetic data. The calculated values of the equilibrium constant ( K 1a ), rate coefficients ( k 1b ), and overall rate coefficients [ k 1, ov ( T ) = K 1 a ( T ) × k 1 b ( T )] are provided in Table 3.…”

“…There was evidence in the literature 22 that the reaction of benzene with Br was unmeasurably slow, suggesting that the abstraction of a ring hydrogen was unimportant. In our earlier experimental work, 15 we found that the reaction of Br with toluene occurred with a conveniently measurable rate coefficient. This suggested that the bromine atom reacted primarily with the methyl hydrogens.…”

“…Temperature dependence of the rate coefficients for the reaction of Br atoms with toluene (reaction R 1 ). Blue and open circles are the experimental data from Giri and Roscoe 15 and Bierbach et al., 22 respectively. Lines represent the theoretical rate coefficients, which are obtained using the parameters from the various quantum chemical methods as specified in the figure legend.…”

An experimental and theoretical kinetic study of the reaction of OH radicals with tetrahydrofuran

“…The low-temperature reactions of cyclic ethers, such as THF, tetrahydropyran (THP) and 1,4-dioxane, with halogen atoms (Br and Cl) have recently been investigated using experimental and computational methods [26][27][28] . The reactions of these cyclic ethers with Cl showed similar reactivity ( k ≈ 10 14 cm 3 mol −1 s −1 ).…”

Halogens

“…The 6-311G standard basis set used in electronic structure calculations for bromine possesses a contraction error of 25.6 kcal/mol À1 for Br( 2 P), which is appreciably higher than the contraction errors of the other five p-block elements of the fourth period. Using the relative rate method, the temperature dependence of the rate coefficients for the reactions of atomic Br 2 with toluene, tetrahydrofuran, and tetrahydropyran were measured over the range of 295 to 362 K. 28 Based on their results the authors suggest that differences in the entropy of activation may be just as important as the enthalpy of activation to explain variations in reactivity. 26 The AM1* semiempirical molecular orbital technique now includes parameters for bromine and iodine.…”