Addendum: The Radiolysis of Acetone in Aqueous Solutions

Abstract

In 1960 Allan and Scholes (1) provided evidence to show that acetone can compete with hydrogen ions for hydrated electrons; however, the nature of the reaction products was not investigated. We have now studied the radiolysis of air-free aqueous acetone solutions, identified the more important products, and measured their G values as a function of acetone concentration and pH. 2,5-Hexanedione, isopropanol, hydroxyacetone, hydrogen, and hydrogen peroxide were found to be major products, and traces of diacetone alcohol, pinacol, and methane were also observed. The effect of acetone concentration on the yields of hydrogen and isopropanol in neutral and acid solutions are shown in Fig. 1. The G(AH202) value is equal to the difference between the molecular and the observed hydrogen peroxide yields and is a measure of the hydrogen peroxide reacting with isopropanol precursor radicals. In 10-2 M acetone at pH 1.2 the G value for hydrogen is 3.2, and no isopropanol can be observed. With increasing acetone concentration at pH 1.2, the H2 yield falls and the isopropanol yield rises, approaching the values for neutral solutions. This behavior can be understood if one assumes that hydrated electrons react with acetone to form the anion radical which leads to isopropanol. In acid solutions the hydrogen ions compete with acetone for reaction with the hydrated electrons, converting them to hydrogen atoms, which react principally to give H2 and acetonyl radicals. Not all the reducing radicals formed in acetone at pH 1.2 are accounted for by these reactions, however; and it may be inferred that some of the hydrogen atoms react with acetone by addition. This is consistent with the observation that hydrogen yields from neutral solutions are less than 1.0, the value to be expected from a molecular H2 yield of 0.45 and a direct H-atom yield of 0.55. It follows from the proposed mechanism for isopropanol formation that at a fixed acetone concentration the hydrogen yield should fall and that of isopropanol rise with increasing pH, as shown in Fig. 2. In neutral solutions of 0.1 M acetone all the reducing radicals (GH = 2.9) are accounted for by the sum G(H2) + 2G(iso-