Computational Investigation of the Pyrolysis Product Selectivity for α-Hydroxy Phenethyl Phenyl Ether and Phenethyl Phenyl Ether: Analysis of Substituent Effects and Reactant Conformer Selection

Using computational methods, we determine product selectivities for the pyrolysis of two model compounds for the beta-O-4 linkage in lignin: phenethyl phenyl ether (PPE) and alpha-hydroxy phenethyl phenyl ether (alpha-hydroxy PPE). We investigate the dependence of the product selectivities on the number of reactant conformers included. Utilizing density functional theory in combination with transition state theory, we obtain rate constants for hydrogen abstraction reactions by the key chain-carrying radicals, which determine the product selectivity within a steady-state kinetic model. The inclusion of the energetically second lowest reactant conformer of PPE and alpha-hydroxy PPE has a large effect on the product selectivity. The final product selectivity computed for PPE agrees well with experiment. We find that the alpha-hydroxy substituent affects energetic as well as entropic contributions to the rate constant differently for alternative pathways of hydrogen abstraction and, thereby, significantly alters product distributions.