First report on the kinetics of the uncatalyzed esterification of cellulose under homogeneous reaction conditions: a rationale for the effect of carboxylic acid anhydride chain-length on the degree of biopolymer substitution

Abstract

The kinetics of the homogeneous acylation of microcrystalline cellulose, MCC, with carboxylic acid anhydrides with different acyl chain-length (Nc; ethanoic to hexanoic) in LiCl/N,N-dimethylacetamide have been studied by conductivity measurements from 65 to 85°C. We have employed cyclohexylmethanol, CHM, and trans-1,2-cyclohexanediol, CHD, as model compounds for the hydroxyl groups of the anhydroglucose unit of cellulose. The ratios of rate constants of acylation of primary (CHM; Prim-OH) and secondary (CHD; Sec-OH) groups have been employed, after correction, in order to split the overall rate constants of the reaction of MCC into contributions from the discrete OH groups. For the model compounds, we have found that k (Prim-OH) /k (Sec-OH) [ 1, akin to reactions of cellulose under heterogeneous conditions; this ratio increases as a function of increasing Nc. The overall, and partial rate constants of the acylation of MCC decrease from ethanoic-to butanoic-anhydride and then increase for pentanoic-and hexanoic anhydride, due to subtle changes in-and compensations of the enthalpy and entropy of activation.

Successful Application of an Ionic Liquid Carrying the Fluoride Counter‐ion in Biomacromolecular Chemistry: Microwave‐Assisted Acylation of Cellulose in the Presence of 1‐Allyl‐3‐methylimidazolium Fluoride/DMSO Mixtures

“…The dependence of DS on the anhydride employed is similar to that observed elsewhere, i.e., ethanoic ≈ hexanoic > butanoic 19, 55. As recently shown, this dependence of DS on the length of the acyl moiety of the anhydride is due to subtle changes in‐ and compensations of the enthalpy and entropy of activation 56…”

Effect of the Molecular Structure of Acylating Agents on the Regioselectivity of Cellulosic Hydroxyl Groups in Ionic Liquid

“…The reason for this result was that the higher steric hindrance of the two bulky substituents, BrBiB and CPAC, caused them to be unable to reach the secondary OH groups of cellulose, which had more steric hindrance. This result agreed with those of Nawaz et al (2012), who found that the preference for the C6 position increased as a function of increasing molecular volume of the reagent.…”

“…The low reactivity of cellulose with vinyl esters with long aliphatic chains is due to their stronger steric hindrance effect (Cao et al 2014). Nawaz et al (2012) concluded that the DS values of the cellulose esters depended on the carbon atoms of the acyl group of the derivatizing agent because of a complex dependence of the activation enthalpy and entropy terms on the structural variable. Moreover, according to Dicke (2004), the formation of regioselective derivatives only succeeds by reaction with bulky groups, which primarily react with sterically favored hydroxyl groups.…”

“…Thus, better control of the DS values and a uniform distribution of the functional groups along the polymer chain are possible (Nagel and Heinze 2010). Moreover, controlled substitution along the biopolymer backbone can be achieved in a homogeneous reaction system (Possidonio et al 2009;Nawaz et al 2012). The OH group at position C6 is the most reactive under homogeneous conditions because it is sterically less hindered compared to the secondary OH groups at positions C2 and C3 (Ramos et al 2005).…”

Practical Chemical Kinetics in Solution

“…Because these methods merely indicate the formation/disappearance of ions, an independent proof should be given, for example, by isolation of the reaction product under the conditions of the kinetic run. This approach was followed in acylation of cellulose in LiCl/N,N-dimethylacetamide where the cellulose esters formed were isolated at the end of the kinetic experiments [130].…”