Beste Ariana, Buchanan A C
Joint Institute for Computational Sciences, University of Tennessee, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6367, USA.
J Org Chem. 2009 Apr 3;74(7):2837-41. doi: 10.1021/jo9001307.
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.
木质素是一种丰富的自然资源，是有价值化学品的潜在来源。通过对模型化合物的研究可以更好地理解木质素的热解过程，其中苯乙基苯基醚（PhCH(2)CH(2)OPh，PPE）是代表主要β-O-4醚键的最简单例子。PPE热分解的第一步是氧-碳键的均裂。这一关键步骤的速率取决于键离解焓，而键离解焓又取决于相关取代基的性质和位置。我们使用现代密度泛函方法计算了PPE和几种氧取代衍生物的氧-碳键离解焓。由于在高温热解条件下，PPE中的碳-碳键断裂可能是一个竞争性的初始反应，我们还计算了取代基对这些键离解焓的影响。我们发现，位于与醚氧相邻的苯环上的氧取代基会显著降低氧-碳键离解焓。另一方面，碳-碳键离解焓在两个苯环上不同的取代模式下变化不大。
