Kinetics and mechanism of imidazole-catalyzed acylation of cellulose in LiCl/N,N-dimethylacetamide

Abstract

Cellulose acylation by anhydrides (ethanoic to hexanoic) plus tosyl chloride, TsCl, or imidazole in LiCl/N,N-dimethylacetamide solution has been studied. Contrary to a previous claim, TsCl does not catalyze acylation. For the diazole-catalyzed reaction, N-acylimidazole is the acylating agent. Third order rate constants (k3; 40–70°C) have been calculated from conductivity data and split, by using information from model compounds, into contributions from the primary- (k3;Prim(OH)) and secondary- (k3;Sec(OH)) hydroxyl groups of cellulose. Values of k3,Prim(OH)/k3,Sec(OH) are >1, and increase linearly as a function of increasing the number of carbon atoms of the acyl group. Rate constants and the degree of biopolymer substitution decrease on going from ethanoic- to butanoic-, then increase for pentanoic- and hexanoic anhydride, due to enthalpy/entropy compensation. Relative to the uncatalyzed reaction, the diazole-mediated one is associated with smaller enthalpy- and larger entropy of activation, due to difference of the acylating agent.