Journal
CARBOHYDRATE POLYMERS
Volume 92, Issue 2, Pages 997-1005Publisher
ELSEVIER SCI LTD
DOI: 10.1016/j.carbpol.2012.10.009
Keywords
Kinetics; Imidazole-catalyzed homogeneous acylation; Cellulose; Anhydrides
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Funding
- TWAS (The Academy of Sciences for the Developing World)
- CNPq (National Council for Scientific and Technological Research)
- El Seoud
- Fapesp (Sao Paulo research Foundation)
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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 degrees 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. (C) 2012 Elsevier Ltd. All rights reserved.
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