Bulletin of the Chemical Society of Japan
Online ISSN : 1348-0634
Print ISSN : 0009-2673
ISSN-L : 0009-2673
Studies of Aliphatic Hydroxycarboxylic Acids as Antioxidant. V. Complexes of Copper(II) with erythro- and threo-2-Methyltartaric, and meso- and threo-2,3-Dimethyltartaric Acids
Kazuo HorikawaShinroku Masuyama
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1971 Volume 44 Issue 10 Pages 2697-2702

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Abstract

The dissociation constants of erythro- and threo-2-methyltartaric and meso- and threo-2,3-dimethyltartaric acids were determined by the potentiometric titration method at the ionic strength of μ=0.1 and at 22°C. The values of the dissociation constants were pK1=3.54 and pK2=5.03 for erythro-2-methyltartaric acid, and pK1=3.28 and pK2=4.70 for the threo-form. For meso-2,3-dimethyltartaric acid, the corresponding values were 3.52 and 4.98, while the threo-form had values of 3.04 and 5.30. The complex formation of copper(II) with these acids has been investigated using Job’s method of continuous variations at the ionic strength of μ=0.2 and at 22°C. Copper-(II) and erythro-2-methyltartaric acid react in a 1 : 1 ratio at pH 2.40 and in a 3 : 2 ratio at pH 5.00. On the other hand, the threo-form does not combine with copper(II) at pH 2.40, though it does react at pH 5.00 in a 1 : 1 ratio. As for meso-2,3-dimethyltartaric acid, the complex formation in a 3 : 2 ratio was observed at pH 5.00. However, the threo-form and copper(II) react in a 1 : 2 ratio at pH 5.00. The equilibrium constants (apparent stability constants) were calculated by the method of Turner and Anderson. The values of the equilibrium constant were log K=9.71 (pH 5.00) and log K=2.02 (pH 2.40) for erythro-2-methyltartaric acid, log K=3.78 (pH 5.00) for the threo-form, log K=9.70 (pH 5.00) for meso-2,3-dimethyltartaric acid, and log K=5.19 (pH 5.00) for the threo- form. On the basis of these data, the structures of these complexes are discussed.

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