Abstract
Of the sugar carboxylates, D-gluconate is clearly the most significant representative: the world’s annual production of this organic compound is estimated to be in the order of 105 tonnes. The reason of its mass production is due to its outstandingly broad range of practical (medical, pharmaceutical, industrial, etc.) applications. D-gluconate is a well-known and exceptionally popular complexing agent; accordingly, it has been the subject of a large number of coordination chemical research investigations. Its complexation properties are specially remarkable in alkaline to hyperalkaline pH conditions, where the deprotonation of one or more of its alcoholic OH groups provides a favourable frame for the formation of very stable chelate complexes with a large variety of metal cations. With the aim to show the state of the art of some relevant issues in the aqueous chemistry of the D-gluconate ion, the current paper focusses on the acid-base properties and calcium(II) complexation of the compound encompassing the entire experimentally available pH-range in water. The accessible literature on the deprotonation of carboxylic and alcoholic OH groups is collected and critically evaluated. The lactonization equilibria of D-gluconic acid are also scrutinized. The available data on the calcium complexes forming in neutral and in (hyper)alkaline solutions (both in terms of composition, formation constants and solution structure) are also discussed. Where feasible, some of these properties are compared with those of D-glucose and its derivatives as well as some less common sugar carboxylates, structurally related to D-gluconate, (i.e., D-heptagluconate, L-gulonate and α-D-isosaccharinate). Special emphasis is laid on the relationship between complex stability and the type of metal-binding groups.
Highlights
D-gluconic acid ((2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanoic acid, (HGluc, Scheme 1) is a polyhydroxy carboxylic acid with many applications in the food, pharmaceutical, dye, metal and cement industries, among others [1,2]
Underalkaline conditions the formation of one or more alcoholate groups on gluconate gives rise to the formation of a variety of monomeric and polynuclear Ca2+ complexes
Acidic and neutral medium With careful experimentation and applying various experimental methods simultaneously, it proved to be possible to separate the experimental effects caused by carboxyl deprotonation and lactonization, and to accurately characterize the underlying multiple chemical equilibria and kinetic processes
Summary
D-gluconic acid ((2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanoic acid, (HGluc, Scheme 1) is a polyhydroxy carboxylic acid with many applications in the food, pharmaceutical, dye, metal and cement industries, among others [1,2] These applications are mostly related to its weak acidic character and the strong complexing capacity of its various deprotonated forms (including the Dgluconate anion, Gluc–, and those containing alcoholate moieties, GlucHÀn(n+1)–) forming under near-neutral to hyperalkaline (pH > 12) conditions. The present work aims at providing a comprehensive review of the literature available for the aqueous solutions containing gluconate, calcium ions and supporting electrolytes (from dilute to highly saline) and encompassing the entire experimentally available pH-range (from acidic to hyperalkaine), from the perspective of solution thermodynamics and coordination chemistry. An attempt will be made to summarize the literature findings relating to the protonation (both carboxylate and alcoholate), lactonization and Ca(II) complexation of gluconate (Scheme 1) For the latter, close-to-neutral and hyperalkaline systems will be discussed separately. The complexation properties of some gluconate-related ligands will be briefly assessed
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