The Mechanism of Chitin and Chitosan Deacetylation during Long-Term Alkaline Treatment

The deacetylation of crab chitin in solutions of sodium hydroxide NaOH during the preparation of chitosan was studied, and assumptions were made about the reaction mechanism in two parts of the kinetic curve corresponding to fast and slow deacetylation. Rapid deacetylation is associated with the attack on chitin acetamide bonds by hydroxide ions (OH–). The deacetylation reaction begins at the full hydration boundary of OH– ions at a NaOH concentration of about 20%, and its rate increases with increases in the alkali concentration. At a NaOH concentration above 50%, the rate of rapid deacetylation decreased, which was apparently due to a decrease in the hydroxide ion concentration as a result of the ionic association of Na+ and OH–. With slow deacetylation, the reaction rate decreased with an increase in the alkali concentration, and deacetylation stopped at NaOH concentration of 60%. The main reason for the termination of the reaction is probably the formation of a kinetically stable chitosan anion in the reaction of the second-order bimolecular nucleophilic substitution SN2 with the participation of two hydroxide ions.