5.1 IR study of the effect of alkaline concentration on the chitin deacetylation

In this deacetylation reaction, it is an interrelation between the following variables: the concentration of the basic solution, the temperature, and the reaction time. The deacetylation reaction in basic medium is summarized in Eq. (5):

$$\text{R-NHCOCH}\_3 + \text{OH}^- \rightarrow \text{R-NH}\_2 + \text{CH}\_3\text{COO}^- \tag{5}$$

For CNaOH = 12 N, the DD values calculated from the FTIR shown in Table 5 do not exceed 55% and stabilize after a critical time t < 60 min. At a low concentration of NaOH, the equilibrium time became higher, which shows that the chitosan DD is low, which means that the deacetylation reaction is highly dependent on the concentration of NaOH, due to the inaccessibility of acetamide groups in the polymer chain. This type of behavior, observed by other authors, is explained firstly by the fact that the N-deacetylation occurs preferably at the level of the amorphous region of chitin and then passes from the edge to the interior of the crystalline region [26, 27]. The second reason concerns the equilibrium of the reaction and the degradation of chitosan. Other authors [10, 22, 28] have assumed that it can be controlled by both reaction and diffusion. The low deacetylation of chitin has also been attributed to the rearrangement of acetyl groups in the monomer unit with respect to the OH hydroxyl group [9]. Analysis of the results of the measurement of the DD (Table 5) shows that this increased by approximately 8% (66–74% DD), at a high concentration of NaOH (10 M) and with a thermal increase of 20°C (100–120°C). This variation of DD seems less important, only +2% (26–28% DD) at low concentration of NaOH (7.5 M). This increase in DD is particularly significant (13%) with a base concentration increase of 2.5 M (10 M to 12.5 M). Thus, the variation in NaOH concentration more significantly influences the reaction rate of DD than temperature.
