About the assessment of the degree of oxidation of cellulose during periodate reaction: Comparison of different characterization techniques and their discrepancies

This study focuses on the side reactions responsible for the low efficiency of the periodate-chlorite oxidation sequence of cellulose. Discrepancies are systematically observed between different commonly-used characterization methods for assessing the degree of oxidation (DO) of cellulose. Indeed, the different titration methods of the aldehyde groups found in the dialdehyde cellulose (DAC) generated by the Malaprade reaction on cellulose, do not generally fit the titration of the carboxyl groups found in the dicarboxycellulose (DCC), generated after chlorite oxidation of the DAC. Possible side reactions affecting the Malaprade and chlorite reactions, or affecting the accuracy of the titrations, are presented here and discussed. Studying periodate consumption, iodate generation, organic acids release during the reactions, fiber mass yield, sodium hydroxide consumption during the β-alkoxy-elimination reaction of DAC, 13C NMR spectra of DAC and DCC, and carboxyl titration of DCC, allowed to conclude that chlorite did not fully oxidize the aldehyde groups in the DAC, but only about one aldehyde out of two. It was found that the non-oxidized aldehydes in the DAC were hindered by hemiacetal-type linkages. This study refutes several well-established hypotheses from the literature when applied to reactions under mild conditions.