Kinetics and mechanistic studies on the catalytic disproportionation of chlorine dioxide by bromite and hypohalite ions and the reduction of chlorine dioxide by aqueous iron(II)

Lu Wang, Purdue University

Abstract

The rate of ClO2 conversion to ClO2− and ClO3− is accelerated by BrO 2− and OX− (X = Br −, Cl−) and it is repressed by ClO 2−. The kinetics show that the first steps of the mechanisms are an electron transfer between ClO2 and BrO2 − (k1 = 36 M−1s −1) to give ClO2− and BrO 2, or between ClO2 and OX− ([special characters omitted] = 2.05 ± 0.03 M−1s−1 for OBr−/ClO2 and [special characters omitted] = 0.91 ± 0.04 M−1s−1 for OCl−/ClO2) to give ClO2 − and OX. These highly reversible reactions account for the observed inhibition by ClO2−. The second step for the BrO2−/ClO2 reaction is an electron transfer between ClO2 and BrO2 to regenerate BrO2− and form ClO3− . This step is greatly assisted by many nucleophiles (Br− > PO43− > HPO4 2− > CO32− > Cl− ∼ OH− > CH3COO− ∼ SO42− ∼ C5H 5N >> H2O). A novel aspect of the reaction between BrO 2 and ClO2 is the large kinetic contribution from nucleophiles (kNu) that assist the electron transfer between ClO2 and BrO2. The kNu (M−2s −1) values at 25.0°C vary from 2.89 × 108 for Br− to 2.0 × 104 for H 2O. The second step in the OX/ClO2 mechanisms is the reaction between two free radicals (XO and ClO2) to form XOClO2. These rate constants are [special characters omitted] = 1.0 × 108 M−1s −1 for OBr/ClO2 and [special characters omitted] = 7 × 109 M−1s−1 for OCl/ClO2. The XOClO2 adduct hydrolyzes rapidly in the basic solution to give ClO3− and to regenerate OX−. The activation parameters for the electron transfer between OX− and ClO2 are: ΔHl‡ = 55 ± 1 kJ mol −1, ΔSl‡ = −49 ± 2 J mol−1 K−1 for the OBr −/ClO2 reaction and ΔHl‡ = 61 ± 3 kJ mol−1, ΔS1 ‡ = −43 ± 2 J mol−1 K −1 for the OCl−/ClO2 reaction. The reduction of ClO2 by Fe(aq)2+ in 0.50 M HClO 4 solution to give ClO2− and Fe(aq) 3+ proceeds via both inner-sphere (ki = (1.74 ± 0.07) × 105 M−1s −1) and outer-sphere (ko = (1.28 ± 0.05) × 106 M−1s−1) electron-transfer paths. The activation parameters for the combination of both paths are: ΔH r‡ = 40 ± 3 kJ mol−1, ΔS r‡ = 1.7 ± 0.1 J mol−1 K−1. The activation parameters for the dissociation of the intermediate (FeClO22+) are: ΔHd ‡ = 76 ± 6 KJ mol−1, ΔS d‡ = 32 ± 4 J K−1 mol−1.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Chemistry

COinS