A Spectroscopic Study on p-Hydroxyphenylpyruvic Acid. Keto-Enol Tautomerism and Stability of Its Complex with Fe+3 Ions

¹H-, ¹³C -NMR , IR, UV-Vis, and MS spectra of p-hydroxyphenylpyruvic acid (pH PPA) have been recorded and fully interpreted.pHPPA exists in solution as a mixture of interconverting forms: keto, hydrated keto and only one enol tautomer to which the Z configuration has been assigned according to the value (3.7 Hz) of the vicinal IH-C = C-¹³COOH coupling constant.In organic solvents the enol isomer is far more stable whereas in aqueous solutions the keto form predominates. The tautomeric equilibrium is pH-dependent and the anion is present as keto form not only in aqueous solution but also in H₂O-DMSO mixtures with a high content of DMSO.The Z enol tautomer does form a coloured complex (λₘₐₓ = 680 nm) with Fe⁺³ ions. The complex decomposes rapidly in all considered solvents except DMSO.In view of a possible use of H₂O-DMSO mixtures for clinical analysis purpose, the enol fraction of pHPPA and the stability of the pH PPA -FeCl₃ com plex in H₂O-DMSO mixtures have been examined. Our results suggest that a solvent com position containing at least 80 vol.% in DM SO could be an appropriate solvent for pH PPA determination by FeCl₃ method.