Empirical Hydrogen Bonding Donor (HBD) Parameters of Organic Solvents Using Solvatochromic Probes – A Critical Evaluation
Stefan Spange, Nadine Weiß
Abstract
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 D HBD 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 D HBD 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.