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Review
. 2023 Mar 30;15(7):1729.
doi: 10.3390/polym15071729.

Mechanism of Heterogeneous Alkaline Deacetylation of Chitin: A Review

Vitaly Yu Novikov  1 , Svetlana R Derkach  2 , Irina N Konovalova  2 , Natalya V Dolgopyatova  2 , Yulya A Kuchina  2
Affiliations
Review

Mechanism of Heterogeneous Alkaline Deacetylation of Chitin: A Review

Vitaly Yu Novikov et al. Polymers (Basel). .

Abstract

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.

Keywords: chitin; chitosan; deacetylation; hydration; kinetics; reaction mechanism.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Figures

Figure 1
Figure 1
Chemical structure of (a) completely acetylated chitin [polyβ-(1→4)-N-acetyl-D-glucosamine], (b) completely deacetylated chitosan [polyβ-(1→4)-D-glucosamine], and (c) commercial chitin or chitosan, a copolymer characterized by its average degree of deacetylation (DD, %).
Figure 2
Figure 2
Reaction scheme for N-deacetylation of chitin.
Figure 3
Figure 3
Kinetic curves of deacetylation of chitin obtained from crab shell, shrimp, Antarctic krill, and red king crab gills. Reaction conditions: 50% NaOH; 95 ± 2 °C; DD0 15–20%. Original figure.
Figure 4
Figure 4
Kinetic curves for the deacetylation of chitin/chitosan with different DD0: 15.6% (1), 67% (2), 86% (3), 93% (4), and the “standard” curve (5). Reaction conditions: 50% NaOH; 95 ± 2 °C. Original figure.
Figure 5
Figure 5
Kinetic curves of chitin deacetylation in 50% NaOH (●) and in 50% NaOH with the addition of 5% CH3COONa (●). Original figure.
Figure 6
Figure 6
Kinetic curves of chitin deacetylation: 1—initial dry; 2—initial wet; 3—dry reprecipitated from solution in HCl; and 4—wet reprecipitated from HCl solution. Conditions: 50% NaOH, 100 °C. Original figure.
Figure 7
Figure 7
Diffraction patterns of samples of initial chitin (1) and chitin reprecipitated from HCl, dried in air (2), and wet (3) with different degrees of crystallinity (indicated in the figure). Original figure.
Figure 8
Figure 8
Indicators of pore volume, specific surface area, pore diameter, and crystallinity of chitin samples: original chitin dried in air (1) and in a freeze dryer (2); 3 and 4—reprecipitated chitin dried in air (3) and after freeze-drying (4). Original figure.
Figure 9
Figure 9
Kinetic curves of deacetylation of chitin samples in 50% NaOH at (95 ± 1) °C: initial chitin dried in air (1) and in a freeze dryer (2); reprecipitated chitin dried in air (3) and in a freeze dryer (4). Original figure.
Figure 10
Figure 10
Dependence of the degree of deacetylation on the duration of preliminary exposure of chitin in 50% NaOH solution (1) and in water (2) at 20 °C. Deacetylation: T = 100 °C, τ = 30 min, DD0 = 18.7%. Original figure.
Figure 11
Figure 11
(a) Deacetylation degree, DD, (1) and electrical conductivity of the NaOH solution, s, (2) as a function of alkali concentration at T = 100 °C. (b) Scheme describing the formation of existence of a limit of complete hydration (LCH). Original figure.
Figure 12
Figure 12
Kinetic curves of heterogeneous alkaline deacetylation at 100 °C. Solution concentrations (mol/dm3): NaOH, 13.48 (1), KOH, 13.48 (2), and NaOH, 19.34 (3). The mass ratio of chitin and solution is 1:55. Original figure.
Figure 13
Figure 13
Dependence of the rate of the deacetylation reaction in the first section of the kinetic curve during the deacetylation time of 0–10 min at 100 °C in a solution of NaOH (1) and KOH (2) on the number of water molecules per one ion of dissolved hydroxide. Original figure.
Figure 14
Figure 14
Scheme of the mechanism of cleavage of the acetamide bond in a concentrated alkali solution.
Figure 15
Figure 15
Competitive mechanism of hydration of reaction particles in the local region of the deacetylation reaction. (a) Reactions; (b), (c)—scheme of hydration of reaction particles. CN—chitin; CS—chitosan; Ac−—acetate ion; OH−—hydroxide ion. Original figure.
Figure 16
Figure 16
Scheme of (a) formation of the amide anion of chitosan NHR- and (b) its hydrolysis.
Figure 17
Figure 17
Scheme of the influence of the negative charge of the quasi-stable amide anion of chitosan on the electron density of the carbon of the acetamide bond of the neighboring chitin molecule.
Figure 18
Figure 18
The kinetic curve of chitin deacetylation in 50 wt.% NaOH solution at T = 100 °C without added water (1) and with the addition of water (2) 3 h after the start of the reaction to a NaOH concentration of 40 wt.%. Original figure.
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