Temperature and Ionic Strength Dependence of NO3-radical Reactions with Substituted Phenols in Aqueous Solution

Abstract

Abstract NO3 oxidation reactions contribute to the chemical conversion processes of organic compounds in the tropospheric multiphase system. Substituted phenols are known pollutants in cloud droplets, rain, fog and (deliquescent) particles. This study presents rate constants obtained from kinetic investigations of NO3 reactions with different substituted phenolic compounds in the aqueous solution. Effects of the temperature as well as the ionic strength on the reaction rates were measured using a laser flash photolysis – laser long path absorption (LP-LLPA) set-up. The following rate constants at T = 298 K in M-1s-1 were obtained for reactions of the NO3 radical with 4-nitrophenol (k = (8.8±4.6) · 108), 2-nitrophenol (k = (8.3±1.4) · 108), 4-hydroxybenzoic acid (k = (1.5±0.4) · 109), 4-methylphenol (k = (1.7±0.3) · 109), 4-methoxyphenol (k = (2.5±0.4) · 109), 4-aminophenol (k = (2.0±0.3) · 109), 2,6-dichlorophenol (k = (1.3±0.2) · 109), 2,6-dihydroxyphenol (k = (1.7±0.2) · 109), 2,6-dimethylphenol (k = (1.8±0.3) · 109), 2,6-dimethoxyphenol (k = (1.6±0.4) · 109), 2,6-dinitrophenol (k = (2.8±0.9) · 108), 4-hydroxy-3,5-dimethoxybenzaldehyde (k = (1.7±0.3) · 109), 4-hydroxy-3,5-dimethoxybenzoic acid (k = (1.4±0.6) · 109), 4-hydroxy-3-methoxybenzaldehyde (k = (1.1±0.2) · 109), 4-hydroxy-3-methoxybenzoic acid (k = (1.0±0.3) · 109), 3-hydroxy-4-methoxybenzoic acid (k = (1.3±0.4) · 109). Averaged activation energies of: para-substituted phenols are EAߙpara = 11±6 kJ mol-1; 2,6-substituted phenols EAߙ2,6 = 15±4 , poly-substituted phenols EAߙpoly = 16±4 kJ mol-1. From reactivity correlations it is concluded that 4-nitrophenol, 2,6-dinitrophenol and 4-hydroxybenzoic acid react via direct electron transfer. The majority of investigated compounds react in a mixed regime with contribution of both electron transfer and H-atom abstraction. Surprisingly, changes in ionic strength do not affect the reactions dominated by electron transfer but have an influence on the rate constants of reactions with a mixed mechanism.