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

The oxidation kinetics of NO by O 2 in aqueous solution was observed using a stopped flow apparatus. The kinetics follows a third order rate law of the form k · [NO] 2 · [O 2 ] in analogy to gas‐phase results. The rate constant at 296 K was measured as (6.4 ± 0.8) · 10 6 M −2 s −1 with an activation energy of 2.3 kcal/mol and a preexponential factor of (4.0 ± 0.5) · 10 8 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/O 2 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 NO 2 − . The absence of NO 3 − in the autoxidation of NO, the exclusive presence of NO 2 − as a product of the nitrosation reaction of phenol, and the kinetic results of the N 3 − trapping experiments point towards N 2 O 3 as the reactive intermediate.