Kinetics of equilibrium attainment between molecular glycolaldehyde structures in an aqueous solution
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Kinetics of Equilibrium Attainment between Molecular Glycolaldehyde Structures in an Aqueous Solution G. K. Glushonok, T. G. Glushonok, and O. I. Shadyro Belarussian State University, Minsk, 220080 Belarus ReceivedDecember7, 1998
Abstract--The kinetics of the isomerization and monomerization of the glycolaldehyde dimer in a D20 solution (pD = 4.3) at 25~ is studied by IH NMR spectroscopy (360 MHz). The dynamics of the concentrations of seven dimeric and two monomeric glycolaldehyde forms present in the solution is examined when the system attains equilibrium. A kinetic scheme of equilibrium attainment in an aqueous solution of glycolaldehydeis proposed. The apparent rate constants of the transformation of the molecular glycolaldehyde structures into each other are determined. INTRODUCTION Glycolaldehyde exhibits a complex behavior in solution. This implies the formation of a number of isomeric monomeric and dimeric structures of this compound in different solvents because of the presence of two functional (hydroxyl and carbonyl) groups that are mutually reactive. This explains the fact that glycolaldehyde in the crystalline state exists as dimeric 2,5-dihydroxy-l,4dioxane. However, poor information on the behavior of this compound in solutions results in the incorrect (if not speculative) interpretation of the experimental results obtained. Of the available scarce, contradictory data, one often chooses only those that are necessary to explain the form assuming that glycolaldehyde is either completely dimerized or completely monomerized in an aqueous solution [1, 2]. Earlier [3], we determined the conditions for complete monomerization of glycolaldehyde in an aqueous solution. The aim of this work was to study and interpret the kinetics of equilibrium attainment between different glycolaldehyde forms in an aqueous solution. This is important for the understanding of the reactivity of glycolaldehyde and related compounds in various biochemical reactions. EXPERIMENTAL Glycolaldehyde (>98%; reagent grade; m.p., 9596~ purchased from Switzerland was used. A crystalline glycolaldehyde sample placed into an ampule of an NMR spectrometer was dissolved in a known volume of D20 ([D] = 99.8 at. %) purchased from IZOTOP, so that the concentration of the resulting solution was 1.0 mol/l based on the monomer. The aldehyde sample completely dissolved in ~30 s under the condition of intensive shaking, and the time of recording of the first spectrum elapsed from the beginning of dissolution was ~2-3 min. IH NMR spectra were recorded with a
WM-360 spectrometer (Brucker) with a working frequency of 360 MHz at 25~ without a reference substance. The DHO signal was used as a reference. The concentrations of the individual molecular glycolaldehyde structures in the solution were calculated from the results of signal assignment [3]. The pD value of the solution was measured using a pH-121 pH-meter and calculated by the equation pD = pH + 0.4 [4]. A program [5] was used to describe the experimental kinetic curves and determine the rate co
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