Oxidation of Sodium Hypophosphite by Oxygen in Alcoholic Solutions of Ferrum (III)

G.S. Polimbetova, M.M. Aleshkova, G.O. Bugubaeva, Zh.U. Ibraimova, A.K. Borangazieva

D.V. Sokolsky Institute of Organic Catalysis and Electrochemistry, 142 Kunaev St., 480100 Almaty, Kazakhstan

Abstract

Hypophosphites are widely used as reducers in the metal protection coating, as reagents in the synthesis of various organophosphorus compounds, in analytical chemistry, and in many other fields. NaH2PO2 difficultly reacts with many oxidizers without catalysts despite of the significant reduction potential. The kinetics and the mechanism of hypophosphite oxidation in aqueous acid solution of the metal and nonmetal salts are studied in detail. The reactivity of hypophosphite in the organic solvents was not almost studied. In this work the basic possibility of synthesis dialkylphosphites from cheap, accessible and harmless NaH2PO2 and alcohols is shown. Sodium hypophosphite is oxidized by oxygen in alcoholic solutions of FeCl3 at 50-80°С to dialkylphosphites. Kinetic and mechanism of the reaction are investigated by methods of volumetry, redox-potentiometry, GC, IR-, UV-, EPR-, Mössbauer- and NMR 31Р-spectroscopy and X-ray powder diffraction analysis, optimum conditions are found, kinetic and activation parameters of the reaction are calculated. It is shown, that the process follows redox-mechanism and consists of two key stages: reduction of Fe (III) by hypophosphite with formation of dialkylphosphite and reoxidation of Fe (II) by oxygen. The coordination mechanism of reduction reaction of Fe (III) by hypophosphite is proposed. According to this mechanism the dialkylphosphite forms through innersphere redox-decomposition of intermediate alcoxyhypophosphite complex of Fe (III). The coordination mechanism of the process is confirmed by low values of Е¹ and negative activation entropies DS¹. The availability in an inner sphere of Fe (III) bromide, low-molecular alcohols, water, characterized by high acidity, increases the reaction rate of oxidative alcoxylation of hypophosphite and promotes the further transformation of dialkylphosphite to di- and trialkylphosphate.