title: 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.
authors: Institute of Chemistry, University of São Paulo, P.O. Box 26077; 05513-970 São Paulo, S.P., Brazil.
authors: Haq
authors: H
authors: Nawaz
authors: Paulo Augusto R
authors: PA
authors: Pires
authors: Omar A
authors: OA
authors: El Seoud
mesh_terms: D000081:Acetamides / Q000737:chemistry*; D000215:Acylation; D000812:Anhydrides / Q000737:chemistry; D002384:Catalysis; D002482:Cellulose / Q000737:chemistry*; D019277:Entropy; D007093:Imidazoles / Q000737:chemistry*; D007700:Kinetics; D018021:Lithium Chloride / Q000737:chemistry*; D014105:Tosyl Compounds / Q000737:chemistry
chemical_list: D000081:Acetamides; D000812:Anhydrides; D007093:Imidazoles; D014105:Tosyl Compounds; C025506:4-toluenesulfonyl chloride; C029899:imidazole; D002482:Cellulose; D018021:Lithium Chloride; C013959:dimethylacetamide