Kinetics and Mechanisms of the Oxidation of Hydrazine by Aqueous Iodine

Kinetics for the reactions of I 2 with excess N 2 H 5 + /N 2 H 4 and I - are measured by the loss of I 3 - over a wide range of acidity from pH 0.35 to 8.0 at 25.0 °C, μ = 0.50 M. Pseudo-first-order rate constants increase by factors of more than 10 7 with increase of pH, hydrazine, and buffer concentrations. Below pH 1, I 2 reacts directly with N 2 H 5 + which has a relative reactivity that is 2.4 × 10 7 times smaller than N 2 H 4 (the dominant reactant at pH ≥ 1). Kinetic evidence for IN 2 H 4 + as a steady-state species is found below pH 3. From pH 3.5 to 6.3, rate constants are measured by stopped-flow methods and at higher pH by pulsed−accelerated-flow methods. A multistep mechanism is proposed where I 2 reacts rapidly with N 2 H 4 to form an I 2 N 2 H 4 adduct ( K A = 2.0 × 10 4 M - 1 ) that is present in appreciable concentrations above pH 6. The adduct undergoes general base-assisted deprotonation accompanied by loss of I - in the rate-determining step. Subsequent intermediates react rapidly with another I 2 to form N 2 as a final product. At high pH, hydrazine acts as a general base as well as the initial nucleophile. Rate constants for various bases (H 2 O, CH 3 COO -, HPO 4 2-, N 2 H 4, and OH - ) fit a Brønsted β value of 0.46. Values for the second-order rate constants (M - 1 s - 1 ) for I 2 N 2 H 4 reactions with CH 3 COO -, HPO 4 2-, N 2 H 4, and OH - are 7.5 × 10 3, 2.0 × 10 5, 8.5 × 10 5, and 4.8 × 10 8, respectively.