Some aspects of oxidation by nitric acid

Abstract

It was thought that the kinetics of oxidation of organic compounds by nitric acid would provide a suitable topic for research for two reasons: although nitric acid is commonly used as an oxidising agent in preparative work - for example in the preparation of adipic acid from cyclohexanol - very little is known about the kinetics and mechanism of these reactions, and further, oxidations by nitric acid might be inherently irreproducible. The kinetics of oxidation by strong aqueous nitric acid does not appear to have been investigated at all. Most references in the literature to the rate of oxidation of organic compounds by nitric acid are confined to statements that an induction period exists and is followed by violent reaction. The kinetics and mechanism of the oxidation of arsenious acid by dilute nitric acid has been investigated and discussed by Abel, Schmid and Weiss. They investigate the reaction in media up to 2.5N with respect to nitric acid, and 0.1N with respect to nitrous acid, and in an atmosphere of NO. The rate of reaction is proportional to the second power of the concentration of nitrous acid and they conclude that N2O3 is the oxidising entity. The concentration of HNO2 in the reaction is determined by the successive reactions. The intermediate N2O3 is formed from HNO2 and reacts with H3AsO3 according to the equations. The last equation represents the rate-determining step. The oxidation of hydrogen iodide by nitric acid has been investigated by Eckstadt in 1902. Although this author worked, under very rigorous conditions - i.e. exclusion of dust and impurities, and use of unscratched glassware -in order to obtain reproducibility, he was unable to determine the order of the reaction with respect to the reactants, possibly because he did not recognise the important role of nitrous acid in the reaction. Bobtelsky and Kaplan find that the oxidation of potassium iodide by nitrous acid is first order with respect to both reactants. The reaction between potassium iodide and nitric acid has recently been discussed by Abel, who concludes that the published data are compatible with attack by NO, NO2 or HNO2. The observed irreproducibility of many chemical reactions is due to failure to reproduce the conditions of experiments with sufficient accuracy. It has, however, been shown mathematically that reaction mechanisms are possible which lead to inherently irreproducible reaction rates. The essential feature of such a mechanism is the very slow formation of an active molecular species which initiates a branching chain reaction. The order of magnitude of velocity constants of the individual steps in a reaction mechanism which would lead to observable fluctuations has been calculated and found to be of the same order of magnitude as known velocity constants. The mechanisms of reactions with long induction periods, and of other antocatalytic reactions might be of the type leading to irreproducible reaction rates. The oxidation of formic acid by nitric acid, for example, has been reported irreproducible, and variable induction periods are a well-known feature of the oxidation of many organic compounds by nitric acid.