Investigation of the mechanism of dissociation of glycolaldehyde dimer (2,5-dihydroxy-1,4-dioxane) by FTIR spectroscopy

Glycolaldehyde represents the simplest α-hydroxycarbonyl moiety–a common structural feature of reducing sugars. It exists in solid state, only in crystalline dimeric form as 2,5-dihydroxy-1,4-dioxane. However, in solution phase or during heating, it dissociates into different dimeric and monomeric forms. FTIR spectroscopy was used to study the effect of temperature, pH and solvent on the dissociation and chemical transformations of glycolaldehyde. The infrared spectra were recorded in different solvents as a function of time and temperature (both during heating and cooling cycles) between 30 and 85 °C. During heating, glycolaldehyde cyclic dimer generated two bands in the carbonyl region, one at 1744 cm−1 and the other at 1728 cm−1. These bands increased during the heating cycle and decreased during the cooling cycle. The data indicated that the glycolaldehyde cyclic dimer (2,5-dihydroxy-1,4-dioxane) undergoes a ring opening to form the acyclic dimer (1728 cm−1) that can recyclize into the 2-hydroxymethyl-4-hydroxy-1,3-dioxolane structure. The acyclic dimer can also dissociate into monomeric glycoladehyde (1744 cm−1) in equilibrium with the enediol form (1703 cm−1). There is evidence to indicate oxidation of glycolaldehyde into glycolic acid during heating, in either neutral or basic aqueous solutions.