Institute of Chemistry, University of São Paulo, P.O. Box 26077; 05513-970 São Paulo, S.P., Brazil.
Carbohydr Polym. 2013 Feb 15;92(2):997-1005. doi: 10.1016/j.carbpol.2012.10.009. Epub 2012 Oct 12.
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.
已研究了在氯化锂/二甲基乙酰胺溶液中，酸酐（从乙酸酐到己酸酐）与对甲苯磺酰氯（TsCl）或咪唑共同作用下对纤维素的酰化反应。与之前的说法相反，TsCl 并不催化酰化反应。对于二唑催化的反应，N-酰基咪唑是酰化试剂。根据电导率数据计算了三级速率常数（k(3)；40-70°C），并通过使用模型化合物的信息进行了拆分，得出纤维素的伯（k(3;Prim(OH)))和仲（k(3;Sec(OH)))羟基的贡献。k(3,Prim(OH))/k(3,Sec(OH))的值大于 1，并随酰基碳原子数的增加呈线性增加。由于焓/熵补偿，从乙酸酐到丁酸酐，速率常数和生物聚合物取代度降低，然后戊酸酐和己酸酐增加。与未催化的反应相比，由于酰化试剂的不同，二唑介导的反应具有较小的活化焓和较大的活化熵。
