Kinetics of de-N-acetylation of the chitin disaccharide in aqueous sodium hydroxide solution

Chitosan is prepared from chitin, a process which is carried out at highly alkaline conditions, and that can be performed either on chitin in solution (homogeneous deacetylation) or heterogeneously with the chitin as a solid throughout the reaction. We report here a study of the de-N-acetylation reaction of the chitin dimer (GlcNAc–GlcNAc) in solution. The reaction was followed by 1H NMR spectroscopy in deuterated aqueous sodium hydroxide solution as a function of time, sodium-hydroxide concentration and temperature. The 1H NMR spectrum of GlcNAc–GlcNAc in 2.77 M deuterated aqueous sodium hydroxide solution was assigned. The interpretation of the 1H NMR spectra allowed us to determine the rates of de-N-acetylation of the reducing and non-reducing ends, showing that the reaction rate at the reducing end is twice the rate at the non-reducing end. The total deacetylation reaction rate was determined as a function of the hydroxide ion concentration, showing for the first time that this de-N-acetylation reaction is second order with respect to hydroxide ion concentration. No significant difference in the deacetylation rates in deuterated water compared to water was observed. The activation energy for the reaction (26–54 °C) was determined to 114.4 and 98.6 kJ/mol at 2.77 and 5.5 M in deuterated aqueous sodium hydroxide solution, respectively.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Deacetylation of the chitin dimer was followed in alkaline solution. ► The deacetylation reaction is second order with respect to the hydroxide ion. ► Rate of deacetylation on the reducing end is twice the rate at the non-reducing end.