THE VISIBLE ABSORBENCY MAXIMUM OF 2-HYDROXY-1,4-NAPHTHOQUINONE AS A NOVEL PROBE FOR THE HYDROGEN-BOND DONOR ABILITIES OF SOLVENTS AND SOLVENT MIXTURES

The electronic absorption spectra of 2-hydroxy-1,4-naphthoquinone (HNQ ) in one- and two-component solvents are discussed. The visible absorp tion of this reagent has been shown to arise from a charge-transfer tr ansition of the zwitterionic tautomer stabilized by a hydrogen bond do nating solvent. The formation constant of the 1:1 hydrogen bonded comp lex with methanol, ethanol, and n-propanol has been determined at 25 d egrees C from the spectral behaviour in mixed solvents. The tautomeriz ation equilibrium of HNQ in aqueous ethanol is demonstrated and charac terized. The dependence of the absorbance maximum of HNQ on the hydrog en bond donor ability of the solvent in the visible spectrum, as measu red by the Taft-Kamlet alpha parameter, has been established. Experime ntally derived evidence is supplied to justify the recommendation of H NQ as a novel probe for hydrogen bond donor ability in pure solvents a nd in mixed aqueous solvents.