Solvatochromic characterization of near-critical water as a benign reaction medium

Near-critical water (liquid water between 200 and 374 degreesC) offers an environmentally benign alternative for the replacement of undesirable solvents and catalysts. This work characterizes the solvent strength of liquid water at saturation pressure from ambient temperature to 275 degreesC in terms of its Kamlet-Taft dipolarity/polarizability, hydrogen-bond-donating acidity, and hydrogen-bond-accepting basicity using in situ UV-vis spectroscopy. The results suggest that near-critical water exhibits a wide range of polarity and hydrogen-bond-donor ability for tailoring chemical reactions and separations. These Kamlet-Taft solvent parameters can be used to correlate kinetic properties for reactions in water. As model reactions, the temperature-dependent kinetics of the hydrolyses of two nitroaromatic compounds, 4-nitroaniline and N,N-dimethyl nitroaniline, were determined in NCW in the temperature range of 200-275 degreesC. The hydronium ion dissociated from water promotes the initial hydrolysis reaction without the addition of any acid. Solvent effects on the rate constant were correlated with Kamlet-Taft solvent parameters based on a linear solvation energy relationship (LSER).