This review provides an analysis of experimental results on the study of alkaline heterogeneous deacetylation of chitin obtained by the authors and also published in the literature. A detailed analysis of the reaction kinetics was carried out considering the influence of numerous factors: reaction reversibility, crystallinity and porosity of chitin, changes in chitin morphology during washing, alkali concentration, diffusion of hydroxide ions, and hydration of reacting particles. A mechanism for the chitin deacetylation reaction is proposed, taking into account its kinetic features in which the decisive role is assigned to the effects of hydration. It has been shown that the rate of chitin deacetylation increases with a decrease in the degree of hydration of hydroxide ions in a concentrated alkali solution. When the alkali concentration is less than the limit of complete hydration, the reaction practically does not occur. Hypotheses have been put forward to explain the decrease in the rate of the reaction in the second flat portion of the kinetic curve. The first hypothesis is the formation of “free” water, leading to the hydration of chitin molecules and a decrease in the reaction rate. The second hypothesis postulates the formation of a stable amide anion of chitosan, which prevents the nucleophilic attack of the chitin macromolecule by hydroxide ions.

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 S_N2 with the participation of two hydroxide ions.

Depolymerization of chitin and chitosan in the course of deacetylation in an alkaline medium was studied, and the influence exerted on the process by the nature and concentration of the alkali (NaOH, KOH) and salts (NaCl, NaI, KI) was examined.