Thermally induced homolytic scissions of interunitary bonds in a softwood lignin solution: A spin‐trapping study

Abstract

Unstable chemical species, that is, radicals generated by the thermal treatment of a dimethyl sulfoxide (DMSO) solution of the lignin of a softwood, Yezo spruce (Picea jezoensis Carr.), were studied in detail with an electron spin resonance (ESR) method combined with a spin-trapping technique. An unstable secondary carbon radical (ϳCH ⅐ ) in the solution was trapped as a stable nitroxide spin adduct [RO(NOO ⅐ )OCHϳ (R ϭ tert-butyl benzene)] when the DMSO solution was heat-treated in the presence of a spin-trapping reagent [2,4,6-tri-tert-butylnitrosobenzene (BNB)] at about 40°C. This meant that alkyl phenyl ether bonds (ϳCHOO-phenyl), known as interunitary lignin bonds, were homolytically scissioned by the thermal treatment in the lignin solution. A detailed analysis of the ESR spectrum revealed that three kinds of radicals-primary (ϳCH 2 ⅐ ), secondary (ϳCH ⅐ ), and tertiary (ϳC ⅐ ) carbon radicals-were trapped as stable spin adducts at about 60°C, although the phenoxy radical (PhOO ⅐ ) was not trapped by the BNB spin trap as the counter radical of the secondary carbon radical. This suggested that a fairly large steric hindrance existed between the so-called guaiacoxy radical with a methoxy group in the ortho position and the BNB molecule bearing two butyl groups as bulky moieties in the ortho positions. However, the phenoxy radicals in the lignin solution were stable up to about 60°C.

Stable Organic Radicals in Lignin: A Review

“…This was attributed to large steric hindrance effects operating between the syringyl with two methoxy groups and/or guaiacyl with a single methoxy group at the ortho position, and the BNB trap with two large ortho ‐ tert ‐butyl groups . These rationalizations were in agreement with the thermally induced hemolytic cleavage of ether linkages and corresponding spin adducts with BNB …”

Electron spin resonance (some chemical applications)

“…At about 60 1C, three kinds of radicals were detected, primary (RCH 2 d ), secondary (R 2 CH d ) and tertiary (R 3 C d ). 58 I end this section with two papers, one of which reports an investigation of free radicals produced in the tar and gas phases of mainstream cigarette smoke using spin-trapping. Among the different kinds studied, it appears that oriental cigarettes give the highest yield of free radicals.…”