Kinetic and mechanistic studies on the oxidation of hydrazine by halogens and interhalogens and general-acid catalyzed reactions of hypochlorous acid and acetyl hypochlorite with chlorite ion
Abstract
The kinetics of N2H5+ oxidation by ICl and IBr is investigated by following the formation of I2C l− and I2Br− with excess ICl 2− and IBr2−, respectively. The process of N2H5+ oxidation by Br 2, Cl2, and BrCl is followed by the loss of Br3 −, Cl3−, and BrCl2 −, respectively, with excess total hydrazine. In the proposed mechanisms, halogens and interhalogens (XY) react by an X+ transfer process to N2H5+ with loss of H+ to form a reactive intermediate (k 1), XN2H4+, that eliminates X − and H+ to give N2H3 + (k2). In the ICl and IBr reactions, XN2H4+ is a steady-state species. For the Br2, Cl2, and BrCl reactions, the k−1 step is not appreciable and k2 step is rapid. Subsequent reactions consume a second XY as diazine (N2H 2) is rapidly oxidized to N2. The values of k 1 (25.0°C, μ = 1.00 M) for the oxidation of N2H 5+ by IBr, ICl, Br2, Cl2, and BrCl are 7.6 × 104, 4.12 × 106, 1.49 × 107, 1.01 × 108, and 5.6 × 108 M−1 s−1, respectively. The relative rates (BrCl > Cl2 > Br2 > ICl >> IBr >> I2) are used to established an electrophilicity scale (EXY ). The rate of oxidation of ClO2− by HOCl is first-order in each reactant and is general-acid catalyzed. The yield of ClO2 decreases with increasing pH and total buffer concentrations and increases with increasing ClO2− concentrations. In the proposed mechanism, a steady-state intermediate, HOClOClO− , that forms in the first step (k1 = 1.6 M−1 s−1), undergoes general-acid (HA) catalyzed reactions (k2 HA) to generate a reactive intermediate, ClOClO. This intermediate reacts rapidly with OH− and H2O to form ClO 3− and with ClO2− to form ClO2. Values of k2HA /k−1 are 1.6 × 10 4 M−1 (H3O+), 20 M −1 (HOAc), and 8.5 M−1 (H2PO 4−). The apparent yield of ClO2, based on 2ClO2 per initial HOCl, reaches 140% at low pH and low buffer concentrations. This anomaly is attributed to the ClOClO-induced disproportionation of ClO2− to give ClO3− and additional HOCl. Acetyl hypochlorite is proposed as a steady-state intermediate that is much more reactive than HOCl in the transfer of Cl + to ClO2−.
Degree
Ph.D.
Advisors
Margerum, Purdue University.
Subject Area
Chemistry
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