Oxidation of a Water-Soluble Phosphine and Some Spectroscopic Probes with Nitric Oxide and Nitrous Acid in Aqueous Solutions

In acidic aqueous solutions, nitrogen monoxide oxidizes monosulfonated triphenylphosphine, TPPMS−, to the corresponding phosphine oxide. The NO-derived product is N2O. This chemistry parallels that reported for the reaction of NO with the unsubstituted triphenylphosphine in nonpolar organic solvents, but the rate constant measured in this work, 5.14 × 106 M−2 s−1, is greater by several orders of magnitude. This makes TPPMS− a useful analytical reagent for NO in aqueous solution. The increased rate constant in the present work appears to be a medium effect, and unrelated to the introduction of a single sulfonate group in the phosphine. The reaction between nitrous acid and TPPMS− has a 2:1 [TPPMS−]/[HNO2] stoichiometry and generates NH2OH quantitatively. The rate law, rate = 4k d[HNO2]2[TPPMS−], identifies the second-order self-reaction of HNO2 as the rate-limiting step that generates the active oxidant(s) for the fast subsequent reaction with TPPMS−. It appears that the active oxidant is N2O3, although the oxides NO and NO2 derived from it may be also involved. Bimolecular self-reaction of HNO2 also precedes the oxidations of ABTS2− and TMPD. Competing with this path are the acid-catalyzed oxidations of both reagents via NO+.