The acidity and self-catalyzed lactonization of l-gulonic acid: Thermodynamic, kinetic and computational study

Lactonization and proton dissociation of sugar acids take place simultaneously in acidic aqueous solutions. The protonation-deprotonation processes are always fast, whilst the formation and hydrolysis of γ- and δ-lactones are usually slower. Thus, both thermodynamic and kinetic information are required for the complete understanding of these reactions. The protonation constant (K_p) of l-gulonate (Gul^-) was determined from potentiometric and polarimetric measurements, while the individual lactonization constants (K_L,γ and K_L,δ) for l-gulonic acid (HGul) were obtained via¹³C NMR experiments. The applicability of this method was proven by measuring these well-known constants for d-gluconic acid (HGluc) and by comparing them to literature data. l-gulonic acid γ-lactone (γ-HGul) has remarkable stability in contrast with δ-HGul as well as γ- and δ-HGluc. The polarimetric measurement implies that the main factor responsible for the enhanced stability of γ-HGul is that its hydrolysis is much slower than that of δ-HGul. This higher stability of the γ-HGul ring over its δ-isomer was also confirmed by quantum chemical calculations. A new confirmed feature of the reaction is that in parallel to H₃O⁺, HGul also catalyzes the formation and reverse hydrolytic processes of γ-HGul, similarly to other general acid catalysts.