Different pyrolytic cleavage mechanisms of beta-ether bond depending on the side-chain structure of lignin dimers

Influence of C-gamma-OH on pyrolytic cleavage mechanism of beta-ether-linkage in lignin dimer was studied in N-2 with C-gamma-deoxy-type dimers, which have various p-substituents (-H, -Cl -OCH3) at their C-beta-phenoxy groups. Reactivities at 400 degrees C and the influence of radical scavenger (tetralin) indicated that C-gamma-deoxy types, especially in phenolic forms, are degraded through radical chain mechanism. Furthermore, substituent effects on their reactivities under excess amount of tetralin as discussed with Hammett's substituent constant (sigma(p)) and bond dissociation energy (BDE)-reducing parameter (Delta BDE) revealed that this chain reaction is initiated with the radical species formed through C-beta-O bond homolysis. These chain reactions were only effective in a closed-type reactor and not effective in an open-type reactor. Contrary to this, C-gamma-OH type in phenolic form was degraded at 250 degrees C, which is much lower than that (400 degrees C) of the C-gamma-deoxy types, and the reactivity was not influenced by tetralin. These results indicate that introducing OH at C-gamma, changes the P-ether cleavage mechanism in phenolic form from radical chain to quinone methide mechanism. (c) 2007 Elsevier B.V. All rights reserved.