Kinetics and anomalous temperature dependence of the kinetic isotope effect of reactions of cycloalkanes C5H10, C6H12 and C6D12 with OH radicals in aqueous solutions

Abstract

in aqueous solution and in the gas phase has opposite signs. Values for the KIE and k 6 /k 5 decrease with increasing temperature from 5 to 55°C in the gas phase, while these values increase in solution. We propose that this phenomenon is a consequence of a solvent cage effect.The reactions of hydroxyl radicals with hydrocarbons play an important role in biochemical processes and the chemistry of the atmosphere and natural water supplies [1][2][3][4]. The kinetics of the reactions of OH radicals with saturated hydrocarbons (RH)including cycloalkanes, has been studied extensively in the gas phase [1]. The absolute rate constants for reaction (1) have been measured in aqueous solution only for methane [5,6], while relative rate constants in the methane series have been measured for butane [7-10] and cyclopentane [11,12].In previous work [13,14], we carried out a systematic study of the kinetics and selectivity of the reactions of OH radicals with a wide range of normal, iso, and cycloalkanes in the oxidative system containing H 2 O 2 , Fe 2+ , Fe 3+ , and water similar to Fenton's reagent [3]. We found unusually low substrate selectivity and hydrogen kinetic isotope effect (KIE) for reaction (1) in aqueous solution in comparison with the gas-phase reactions. These findings were qualitatively explained by diffusion complications for the reactions in solution related to the cage effect [13].A new approach toward the study of the mechanisms for activation of C-H bonds of saturated hydrocarbons by oxidizing agents, metal complexes, and electrophiles developed in previous work [13,15] involves study of the temperature dependence of the KIE for cyclohexane/cyclohexane-d 6 pairs and the ratios of the reaction rates for the cyclopentane/cyclohexane pair and analysis of the compensation ratios obtained. There have been no data on the temperature dependence of the rate constants of the reactions of alkanes with OH radicals in aqueous solution.

Kinetics and mechanism of the reactions of C6H12, C5H10 and C6D12 cycloalkanes with peroxynitrous acid in aqueous solution and the gas phase

“…The values for (KIE) l and (k A /k B ) l are the same for the reactions of cycloalkanes with HOONO and OH radicals in aqueous solution [13] in the temperature range studied (Table 1). An anomalous increase in KIE with increasing temperature is observed for both reactions.…”

“…The temperature dependence of the KIE for the c-C 6 H 12 /c-C 6 D 12 pair is described by the following equation According to our previous work [12], the value of KIE for all the reactions studied in solution with oxidizing agents, electrophiles, and metal complexes (except liquid-phase reactions with OH radicals [13]), is greater than unity and decreases with increasing temperature E D -E H > 0. The values of (KIE) g and (k 6 /k 5 ) g in the gas-phase reactions of hydrocarbons with PA studied in the present investigation decrease with increasing temperature (Table 1) in accord with Eqs.…”

“…E D -E H , kJ/mol A Í /A D E 5 -E 6 , kJ/mol A 6 /A 5 HOONO/gas 2.2 ± 0.3 1.0 ± 0.1 0.53 ± 0.16 1.1 ± 0.1 ⋅ ÎÍ/gas [15] 2.0 ± 0.1 1.2 ± 0.1 0.40 ± 0.14 1.12 HOONO/solution -1.9 ± 0.1 2.5 ± 0.1 -1.1 ± 0.1 1.8 ± 0.1 ⋅ ÎÍ/solution [13] -2.1 ± 0.1 2.8 ± 0.1 -1.4 ± 0.1 2.0 ± 0.1 k 6 /k 5 also increase. The activation parameters calculated from Eqs.…”

“…In recent work [13], we have shown that the temperature dependence for the KIE and k 6 /k 5 in the gas-phase and liquid-phase reactions with OH radicals at 5-55°C have different signs: the KIE and k 6 /k 5 decrease with increasing temperature in the gas phase, but these values increase in solution. These differences are presumably due to the cage effect of the solvent.…”

Temperature dependence of the relative rate constants of the reactions of alkanes with oh radicals in aqueous solution

“…Important additional information on mechanism can be obtained by studying the temperature dependence of the rate constant and substrate selectivity (rate constant ratios) for a broad range of hydrocarbons [13]. In recent work [14], we have shown that the temperature dependence of the KIE in gas-phase and liquid-phase reactions of cyclohexane (c-C 6 H 12 /C 6 D 12 ) with the OH radical at 5-55°C have opposite signs: the KIE values in the gas phase drop with increasing temperature, while these values increase in water. These discrepancies are presumably a consequence of the solvent cage effect.…”