The tautomeric constants, Kt, of several 4-pyridones were determined by measurement of pKa values. For 2-substituted 5-methoxy-4-pyridones, the Kt values were correlated to the substituent constants σ, the equation logkt=2.98+2.93σm−6.18σo being obtained.

The dissociation constants of 6-substituted 3-hydroxy-4-pyrones and trans-6-(substituted styryl)-3-hydroxy-4-pyrones were measured spectrophotometrically in 50% aqueous methanol at 25 °C. The results are correlated to the substituent constants σ, the following equations being obtained: pKa=8.57–1.53 σ and pKa=8.54–0.23 σ. The transmission coefficient through the styryl group was found to be 0.150 for the 3-hydroxy-6-styryl-4-pyrone system.

The tautomeric constants Kt of several 4-pyridinethiones were determined by measurement of pKa values. For 2-substituted 5-methoxy-4-pyridinethiones, the Kt values are correlated to the substituent constants σ, by the equation logKt=4.00+3.99σm−7.33σ0.

Die Pyranthione (Ia) und (Ib) reagieren mit NH3 unter Bildung der Pyridinthione (IIa) bzw. (IIb), während (Ic) bei der gleichen Reaktion das Disulfid (III) gibt.

Die Tautomerie‐Konstanten Kt der Titelverbindungen (II) und (III), die aus den Pyranthionen (I) mit Ammoniak bzw. Methylamin erhältlich sind, werden durch Messung der pKa‐Werte bestimmt.