EFFECT OF SOLVENT-SOLUTE AND SOLUTE-SOLUTE INTERACTIONS ON THE RATE OF A MICHAEL ADDITION IN SUPERCRITICAL FLUOROFORM AND ETHANE

The rate of the Michael addition of piperidine to methyl propiolate in supercritical fluoroform and ethane at 37 degrees C and pressures bet ween 48.3 and 213.8 bar depends on fluid density. In fluoroform, the r ate constant is linearly related to pressure above 82.8 bar, with a sm aller nonlinear change being observed at lower pressures and with a mi nimum at 82.8 bar. In ethane, the rate of reaction is linear with pres sure except near the critical point where the rate is significantly en hanced. We attribute the observed rate constant changes to a dependenc e on solvent dielectric at pressures higher than that at the critical point, as would be consistent with the stabilization of a highly polar transition state. The observed aberrations near the critical point ar e attributed to solvent-solute clustering in fluoroform and to solute- solute clustering in ethane.