The Pyridine-Catalysed Acylation of Pine Sapwood and Phenolic Model Compounds with Carboxylic Acid Anhydrides. Determination of Activation Energies and Entropy of Activation

Summary The reaction activation energies and entropies have been determined for the catalysed reaction of pyridine swollen pine sapwood samples and phenolic model compounds (in solution) with a homologous series of carboxylic acid anhydrides. With wood samples, the activation energy (Ea) was largest for the reaction of ethanoic anhydride (ca. 40 kJ mol−1), and decreased as the molecular weight of the anhydride increased (to ca. 10 kJ mol−1 with hexanoic anhydride). With the model compounds, there was no substantial evidence for such a relationship between molecular weight of anhydride and Ea. Values for the entropy of activation for reaction with wood were on the order of −300 J K−1 mol−1, whereas values for reaction with the model compounds were generally on the order of −−220 to −260 JK−1 mol−1. The activation energies of diffusion (Eadiff) for the anhydride series in Corsican and Scots pine have also been determined, these varied from ca. 7 to 19 kJ mol−1. No correlation was found between Eadiff and anhydride, nor between Eadiff and wood species. The results are interpreted by considering the void space created around the reactive hydroxyl groups through swelling of the cell wall with pyridine.