Solvent effects on the kinetics of a Diels-Alder reaction in gas-expanded liquids

Gas-expanded liquids (GXLs) form a unique class of environmentally benign solvents that offer many of the benefits of both organic liquids and supercritical fluids. A more complete understanding of the interactions between the gas, the organic liquid, and solutes at the molecular level will enable the full exploitation of GXLs. Combining kinetic and solvatochromic studies, we have developed a comprehensive multiparameter approach to add insight into the molecular interactions related to a Diels-Alder reaction in CO2-expanded acetonitrile. We have studied the kinetics of the Diels-Alder reaction of anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione as a function of solvent composition using in situ high-pressure fluorescence spectroscopy. We have also measured the values of the Kamlet-Taft solvatochromic parameters pi* (dipolarity/polarizibility), alpha (hydrogen bonding acidity), and beta (hydrogen bonding basicity) in CO2-expanded acetonitrile using in situ high-pressure UV/vis spectroscopy. A linear solvation energy relationship (LSER) describes the reaction rate in terms of the measured solvatochromic parameters. We correlate the reaction rates with the solvatochromic parameters based on the LSER and yield satisfactory consistency with the experimental data.