Formation of singlet molecular oxygen by the Radziszewski reaction between acetonitrile and hydrogen peroxide in the absence and presence of ketones

Abstract

Measurements of the infrared phosphorescence of singlet molecular oxygen (¹O₂) at 1270 nm have been used to demonstrate the formation of ¹O₂ by the Radziszewski reaction between acetonitrile and hydrogen peroxide. The kinetics of the Radziszewski reaction either alone or in the presence of ketones have been studied by this technique. The rate-determining step of the ¹O₂ formation of the reaction in the absence of ketones was found to be independent of both the concentration of acetonitrile and that of hydrogen peroxide. The kinetic data, the results of the volumetric measurements of the oxygen liberated and the results of the determination of the amount of ¹O₂ generated by the reaction are consistent with the assumption that the reaction between acetonitrile and hydrogen peroxide occurs via the heterolytic decomposition of the intermediate, peroxyacetimidic acid (PAIA), forming 1 mol acetamide and 0.5 mol ¹O₂ according to the stoichiometric equation: CH₃CN + H₂O₂ → CH₃C(O)NH₂ + 0.5 ¹O₂. The rate constant of the heterolytic decomposition of PAIA was determined to be k₈ = 1.2 × 10⁻³ dm³ mol⁻¹ s⁻¹ at T = 30 °C. From the measurements at different pH values in the range 9.1 < pH < 12.0 the pK_a(PAIA) value was estimated to be 11.1 at T = 30 °C. The investigation of the reaction between acetonitrile and hydrogen peroxide by using N,N-dimethyl-4-oxopiperidinium nitrate as catalyst, has unequivocally shown that the rate of ¹O₂ formation is considerably enhanced by this ketone. For the ketone-catalysed decomposition of PAIA a rate law can be derived showing a first order dependence on the concentration of acetonitrile and hydrogen peroxide at a given pH. In accordance with the observed rate law are the results with acetonitrile in 50% acetone containing a tenfold excess of hydrogen peroxide at pH 8.2 and T = 60 °C.