Sonolysis of perhalomethane as studied by EPR and spin trapping

Abstract

Intense ultrasonic waves have been reported to produce chemical changes by the phenomenon of cavitation (1-3). Although some sonochemically induced reactions proceed undoubtedly by an ionic mechanism (4-7), others most probably involve free radical intermediates (8, 9) although no direct evidence has been provided. We report our attempts to generate free radicals at room temperature in fluid solutions by ultrasonic exposure and their detection by EPR. We found that sonolysis of pure carbon tetrachloride or trichlorobromomethane generates free radicals which could be spin-trapped by 2-methyl-2nitrosopropane and identified. A Bransonic 12 ultrasonic cleaner (frequency, 50 kHz) was used as a generator of ultrasonic waves. The reaction mixture (3 ml) contained in a Pyrex test tube was held in the middle of the bath, and the vertical position of the tube was adjusted to maintain the same level of liquid inside the tube as outside. Particular care was taken to insure the reproducibility, by positioning the tube in the same place, keeping the water level and the temperature in the bath constant (23”C), etc. (10). The intensity of ultrasonic irradiation under these experimental conditions was monitored before each experiment by a calorimetric procedure which measures the amount of chlorine released in CC&-saturated water by ortho-tolidine reagent (II). Thus, a mixture of 3 ml water saturated with CCL, and 0.15 ml o&o-tolidine reagent (0.1% solution, Fisher Scientific Co.) was sonicated for 30 sec. The intensity of the yellow color was measured in a l-cm cell at 436 nm to be 0.8 IC_ 0.1 under our experimental conditions. When CCL or CCl,Br containing 2-methyl-2-nitrosopropane (1.5 mg/ml) was sonicated and subsequently analyzed by EPR, a spectrum composed of a triplet of ON = 6.7 G could be recorded (Fig. 1). This splitting characteristic of an acyl spin adduct (12) is attributed to the nitroxone I