Effects of Substituents on the SN2 Free Energy of Activation for α-O-4 Lignin Model Compounds

被引:7
|
作者
Pelzer, Adam W. [1 ]
Broadbelt, Linda J. [1 ]
机构
[1] Northwestern Univ, Dept Chem & Biol Engn, Evanston, IL 60208 USA
基金
美国国家科学基金会;
关键词
BENZYL PHENYL ETHER; BOND-DISSOCIATION ENTHALPIES; MOLECULAR-WEIGHT PHENOLS; ACID DEGRADATION; BIOFUEL PRODUCTION; MICROKINETIC MODEL; CORN STOVER; DEPOLYMERIZATION; CLEAVAGE; ACIDOLYSIS;
D O I
10.1021/acs.jpcc.6b12078
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Current methods for processing lignocelluloSic biomass fail to take full advantage of the phenolic chemicals available in lignin, the third most common polymer on the planet. The complexity of the lignin polymer structure, the lack of knowledge of the mechanisms by which it breaks down, and the difficulties in predicting and controlling the reaction product distribution make improvement in processing methods challenging. The use of model compounds, which contain only a few key features of the native polymer, allow the reactivity of the overall polymer to be investigated more simply by isolating key elements. In our previous work, we examined the mechanism of acidolysis for two alpha-O-4 lignin model compounds, benzylphenyl ether (BPE) and 1-(phenoxyethyl)benzene (PEB). In the present work, we examine the rate-limiting step of this mechanism, a nucleophilic attack on the alpha-carbon commensurate with protonation of the ether oxygen, for other alpha-O-4 model compounds that contain features more reminiscent of native lignin, including compounds based on native model dimers. The effects of individual substituents and combinations of substituents are also examined in order to determine their contributions to the reactivity of native lignin dimers. Simple relationships are examined between ground-state properties and reactivity. The results of these calculations show that the effects of substituents on the reactivity of BPE-based compounds are quite different from those of PEB-based compounds. While reasonable correlations can be found for BPE reactivity and properties across substituents, PEB shows much less predictable reactivity except among small subsets of substituents. Overall, the trends observed here provide useful information about the reactivity of alpha-O-4 bonds in lignin during acidolysis as a function of the local chemical environment.
引用
收藏
页码:7603 / 7614
页数:12
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