Abstract
The ultrasonic relaxational absorption in methyl propionate solutions of propionic acid has been studied in the frequency range 0.5–95 Mc/sec at concentrations varying from 0 to 1 mole fraction. The relaxation frequencies are 1.8, 2.2, 2.8, and 3.4 Mc/sec independent of the concentration at 20°, 25°, 30°, and 35°C, respectively. The independence of fmax on the concentration indicates that the ultrasonic relaxational absorption is associated with the uni—unimolecular reaction. The kinetic values have been calculated for the uni—unimolecular reaction by Tabuchi's theoretical equations. The forward and backward rate constants are 2.3×105 and 1.06×107 sec−1 at 20°C for pure propionic acid, respectively, and the heat of reaction is 4.7 kcal/mole of dimer. The value of Cpr is equal to that of Cvr, since the volume change resulting from reaction is absent. The consideration for the type of reaction, the magnitude of heat of reaction, and no volume change associated with reaction have led us to the conclusion that the ultrasonic absorption in propionic acid is associated with a breakage of one of two hydrogen bonds. The solvent effect on the relaxation mechanism has also been studied in various solvent solutions of propionic acid. The relaxation frequency fmax does not change with increasing concentration in polar solvent solutions, whereas it decreases with increasing concentration in nonpolar solvent solutions.
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