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 ( k 3 ; 40–70 °C) have been calculated from conductivity data and split, by using information from model compounds, into contributions from the primary- ( k 3;Prim(OH) ) and secondary- ( k 3;Sec(OH) ) hydroxyl groups of cellulose. Values of k 3,Prim(OH) / k 3,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.