Kinetic and mechanistic aspects of the NO oxidation by O2 in aqueous phase

Abstract

AbstractThe oxidation kinetics of NO by O2 in aqueous solution was observed using a stopped flow apparatus. The kinetics follows a third order rate law of the form k · [NO]2 · [O2] in analogy to gas‐phase results. The rate constant at 296 K was measured as (6.4 ± 0.8) · 106 M−2 s−1 with an activation energy of 2.3 kcal/mol and a preexponential factor of (4.0 ± 0.5) · 108 M−2 s−1. The rate constant displays a very slight pH dependence corresponding to less than a factor of three over the range 0 to 12. The system NO/O2 in aqueous solution is an efficient nitrosating agent which has been tested using phenol as a substrate over the pH range 0 to 12. The rate limiting step leading to formation of 4‐nitrosophenol is the formation of the reactive intermediate whose competitive hydrolysis yields HONO or NO2−. The absence of NO3− in the autoxidation of NO, the exclusive presence of NO2− as a product of the nitrosation reaction of phenol, and the kinetic results of the N3− trapping experiments point towards N2O3 as the reactive intermediate. © 1994 John Wiley & Sons, Inc.