The effect of pressure on the keto-enol equilibria of acetone and cyclohexanone

Abstract

The effect of pressure on the keto-enol equilibria of acetone and cyclohexanone in the solvents of carbon disulfide, toluene and n-hexane has been studied by the measurement of the infrared spectra at high pressure. Comparing the molal volume of the keto form with that of the enol form, it is expected that the enol from is favorable with increasing pressure. In this study, this expectation has been confirmed. That is, the keto-enol equilibria of acetone and cyclohexanone shift to the enol form with increasing pressure. However, the absolute value of the enol concentration is yet small. For example, the value of the keto-enol equilibrium constant, K=(enol)/(keto), for acetone in n-hexane is 1.6×10^-2 at a pressure of 8000kg/cm^2, and that for cyclohexanone in n-hexane is 3.6×10^-2 at the same pressure. Acetone and cyclohexanone are the monoketones of the aliphatic and the cyclic structure, respectively, and the pressure effect on the keto-enol equilibria is larger for the former than for the latter. This difference may be due to the structural differences between acetone and cyclohexanone. As for the solvent effect on the keto-enol equilibria, the concentration of the enol form of acetone and cyclohexanone in the solvent of n-hexane are much higher, and in general the enol form increases in n-hexane. This tendency is also confirmed is this experiment. The concentration of the enol form of acetone in carbon disulfide and in toluene are much the same, but for cyclohexanone the concentration of the enol form is carbon disulfide is higher than in toluene.