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

Abstract

The 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 solvents DHBD as a physical parameter in comparison to the pKa, the chemical benchmark for acidity. Reichardt's dye B30, Kosowers Z-indicator 1-ethyl-4-(methoxycarbonyl) pyridinium iodide (K), Kamlet-Tafts α, Dragos S parameter, Catalans SA scale, the cis-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 of DHBD for several alcoholic solvent families requires a ompletely new physicochemical understanding of these established HBD parameters. Only the AN scale (or Fe) is able to bridge the gap between global polarity and acidity, provided the values are interpreted correctly and applied accordingly.