Evaluation of equilibrium constants for deprotonation and lactonisation of alpha-D-isosaccharinic acid

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  • アルファ-D-イソサッカリン酸の解離およびラクトン化の平衡定数の評価

Abstract

A great deal of disagreement exists in the literature regarding the intrinsic deprotonation and lactonisation constants of $\alpha$-D-isosaccharinic acid (ISA). Based on a combination of nuclear magnetic resonance (NMR) and extensive experimental Ca(ISA)$_{2}$(cr) solubility data involving $\alpha$-D-isosaccharinic acid, the reliable value of log K$^{\circ}$ for [HISA(aq) $\leftrightarrow $ ISA$^{-}$ + H$^{+}$] is -3.27 $\pm$ 0.01 and for [HISA(aq) $\leftrightarrow $ ISL($\alpha$-D-isosaccharinate-1,4-lactone)(aq) + H$_{2}$O] is 0.49 $\pm$ 0.09. These data also provide a composite log K$^{\circ}$ of -3.76 $\pm$ 0.09 for the reaction [ISL(aq) +H$_{2}$O $\leftrightarrow $ ISA$^{-}$ + H$^{+}$]. Reinterpretation of extensive Ca(ISA)$_{2}$(cr) solubility data using the SIT activity coefficient model provides log K$^{\circ}$ of -6.40 $\pm$ 0.09 for [Ca(ISA)$_{2}$(cr) $\leftrightarrow $ Ca$^{2+}$ + 2 (ISA)$^{-}$] and of 1.70 $\pm$ 0.09 for [Ca$^{2+}$ + ISA$^{-}$ $\leftrightarrow $ CaISA$^{+}$], based on the values that are consistent with the average of all of the available values.

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