Laser-based studies of NO3 radical reactions with selected aromatic compounds in aqueous solution

Abstract

The reactivity of the nitrate radical towards selected aromatic compounds in aqueous solution has been investigated using a laser photolysis/long-path laser absorption technique (LP-LPLA). The reactions of NO₃ radicals with (1) phenol, (2) 4-nitrophenol, (3) 2,4-dinitrophenol, (4) ortho- and (5) para-cresol, (6) 2-nitro-p-cresol, (7) 2,6-dinitro-p-cresol, (8) benzoic acid, (9) 4-hydroxybenzoic acid, (10) 3-nitrobenzoic acid, (11) 4-nitrobenzoic acid, (12) 4-cyanobenzoic acid, (13) 4-methoxybenzoic acid, (14) 4-methylbenzoic acid and (15) 4-methyl-3-nitrobenzoic acid were investigated at T = 298 K. The following rate coefficients were obtained: k₁ = (1.9 ± 0.3) × 10⁹ M⁻¹ s⁻¹, k₂ = (7.1 ± 0.4) × 10⁷ M⁻¹ s⁻¹, k₃ = (5.3 ± 0.6) × 10⁷ M⁻¹ s⁻¹, k₄ = (1.1 ± 0.2) × 10⁹ M⁻¹ s⁻¹, k₅ = (8.2 ± 0.3) × 10⁸ M⁻¹ s⁻¹, k₆ = (1.0 ± 0.1) × 10⁸ M⁻¹ s⁻¹, k₇ = (1.4 ± 0.2) × 10⁸ M⁻¹ s⁻¹, k₈ = (6.5 ± 1.3) × 10⁷ M⁻¹ s⁻¹, k₉ = (5.0 ± 1.3) × 10⁸ M⁻¹ s⁻¹, k₁₀ = (2.0 ± 0.6) × 10⁷ M⁻¹ s⁻¹, k₁₁ = (2.0 ± 0.9) × 10⁷ M⁻¹ s⁻¹, k₁₂ = (2.7 ± 1.5) × 10⁷ M⁻¹ s⁻¹, k₁₃ = (6.9 ± 3.2) × 10⁸ M⁻¹ s⁻¹, k₁₄ = (6.0 ± 0.8) × 10⁸ M⁻¹ s⁻¹, k₁₅ = (3.3 ± 1.4) × 10⁷ M⁻¹ s⁻¹. Moreover, the effect of temperature on the reaction of NO₃ with (1) phenol, (5) para-cresol, (8) benzoic acid and (14) 4-methylbenzoic acid was studied leading to the Arrhenius expressions k₁(T) = (1.4 ± 0.2) × 10¹² exp [−(2100 ± 500) K/T] M⁻¹ s⁻¹, k₅(T) = (3.8 ± 0.2) × 10¹⁵ exp [−(4600 ± 1000) K/T] M⁻¹ s⁻¹, k₈(T) = (4.9 ± 0.2) × 10⁹ exp [−(1300 ± 400) K/T] M⁻¹ s⁻¹,and k₁₄(T) = (1.6 ± 0.3) × 10¹² exp [−(2400 ± 500) K/T] M⁻¹ s⁻¹, respectively. Based on the results the possible reaction pathways are discussed. The influence of ionic strength on the reaction rate for the reactions of NO₃ with phenol and benzoic acid was studied by varying the electrolyte concentration of the aqueous solution. From these experiments limiting rate constants for zero ionic strength and for I → ∞ could be determined. Additionally, the influence of pH on the reaction of NO₃ with selected aromatics was studied. From the derived data the limiting rate constants for the reaction of NO₃ with the undissociated aromatic compound and its anion were determined.