Computational study of bond dissociation enthalpies for lignin model compounds. Substituent effects in phenethyl phenyl ethers.

Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH(2)CH(2)OPh, PPE) is the simplest example representing the dominant beta-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen-carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen-carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon-carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen. On the other hand, the carbon-carbon bond dissociation enthalpy shows little variation with different substitution patterns on either phenyl ring.