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

2002; Royal Society of Chemistry; Issue: 7 Linguagem: Inglês

10.1039/b202620c

1472-779X

H.‐D. Brauer, Beate Eilers, Andreas Lange,

Chemical Reactions and Mechanisms

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 pKa(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.