EFFECT OF PRESSURE ON THE ACID-CATALYZED ENOLIZATION OF ACETONE AND ACETOPHENONE IN VARIOUS ETHANOL–WATER SOLVENTS. ORIGIN OF THE ENTHALPY–ENTROPY COMPENSATION EFFECT

The effect of pressure, temperature, and solvent composition on the rate of the acid-catalyzed enolization of acetone and acetophenone, and the solvent deuterium isotope effect for the enolization of acetophenone, have been measured by following the iodination. The solvent deuterium isotope effect [Formula: see text] for the enolization of acetophenone in 16.2% w/w ethanol–water is 2.50 ± ~0.05, which undoubtedly proves that there is a pre-equilibrium proton transfer. The effect of solvent in the range water to 33.4% w/w ethanol in water on the rate of enolization of both acetone and acetophenone is small at atmospheric pressure, but is about four times larger at 3 kbar. This cannot be explained on simple electrostatic grounds, and indicates that any simple electrostatic explanation of the solvent effect at atmospheric pressure is invalid. The volumes of activation for the enolizations are strongly dependent on the solvent, that for acetone varying from −2.l ± ~0.5 to −6.9 ± ~ 0.7 cm3 mole−1 between solvents water and 33.4% w/w ethanol in water.An examination has been made of the enthalpyentropy compensation effect. It is shown that in general if the rate or equilibrium constant of a reaction does not change with changing conditions (such as solvent, substituents, etc.) then either the quantities of activation at constant pressure, ΔHp≠ and ΔSp≠, or the corresponding quantities at constant volume, ΔUv≠ and ΔSv≠, must vary in a compensating manner, and the existence of an energy–entropy compensation effect is inevitable. For the enolization of acetone and acetophenone in ethanol–water mixtures, ΔUv≠ and ΔSv≠ vary only slightly with solvent, whereas ΔHp≠ and ΔSp≠ vary in a compensating manner. The main causes of the compensation effect in the constant-pressure parameters are, in a sense, the change with changing solvent of the thermal expansion of the solvent and of the volume of activation of the reaction. On the other hand, both the constant-pressure and the constant-volume parameters vary with substituent from acetone to acetophenone, and the constant-volume parameters vary the more.