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

Near-critical water (liquid water between 200 and 374 °C) 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 °C 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 °C. The hydronium ion dissociated from water promotes the initial hydrolysis reaction without the additi...