Empirical Hydrogen Bonding Donor (HBD) Parameters of Organic Solvents Using Solvatochromic Probes – A Critical Evaluation

Abstract

AbstractThe solvatochromicity of established solvatochromic UV/Vis probes, which appear to be sensitive to the so‐called hydrogen bond donor (HBD) property of the solvent, is analysed using the hydroxyl group density of alcoholic solventsDHBDas a physical parameter in comparison to the pKa, the chemical benchmark for acidity. Reichardt's dyeB30, KosowersZ‐indicator 1‐ethyl‐4‐(methoxycarbonyl) pyridinium iodide (K), Kamlet‐Tafts α, DragosSparameter, CatalansSAscale, thecis‐dicyano‐bis (1,10‐phenanthroline) iron II complex (Schilt's Ferrocyphen dye,Fe) and Gutmann‘s acceptor number (AN) have been investigated. The observed dependencies of the empirical polarity parameters as a function ofDHBDfor several alcoholic solvent families requires a ompletely new physicochemical understanding of these established HBD parameters. Only theANscale (orFe) is able to bridge the gap between global polarity and acidity, provided the values are interpreted correctly and applied accordingly.