Reaction dynamics in supercritical media
An in situ dielectric method is used to measure aromatic solubilities as a function of pressure and to follow the progress of an Esterification, Aldol and Friedel-Crafts reactions in supercritical (sc) difluoromethane (HFC 32). This is achieved by measuring the changes in the sc solution capacitance as the composition of the sc system changes. The solubility of p-hydroxybenzoic, p-toluic, p-aminobenzoic and p-chlorobenzoic acids and also p-chlorophenol and p-aminophenol is determined in sc HFC 32 at 363 K using the dielectric method, and presented as a function of pressure. The compounds are shown to be more soluble than in the commonly employed sc solvent, CO2 under comparative conditions. The enhancement factor is used as a way of expressing solubility to remove the effect of vapour pressure and the observed trend in solubility is found to be highly dependent on the polarity of the substituents present on the aromatic ring. The use of the in situ dielectric method for monitoring a reaction in a sc fluid is proposed. The Esterification. Aldol and Friedel-Crafts reactions in sc HFC 32 were used to test the applicability of this technique. These are the first reported reactions to be carried out in sc HFC 32. The rate/equilibrium constants for these reactions are determined and found to decrease with increasing pressure. It is proposed that the observed high reaction rates at low pressure are due to local composition enhancements. Determination of solvent-solute interactions is key to the understanding of solvent, properties in liquids and sc fluids. The Kamlet-Taft parameters were determined for HFC 32 and 1,1,1,2-tetrafluoroethane (HFC 134a) as a function of temperature and pressure in the range 313 to 403 K and 50 to 220 bar using three solvatochromic dyes. It is shown that both solvents exhibit considerable hydrogen bond donor and acceptor properties and these parameters also obey the three region density model often invoked when describing the polarisability/dipolarity parameter in sc fluids.