STEREO-PREFERENCE OF HYDROXYL AND OXYL ANION RADICALS IN THE REACTION WITH THE ERYTHRO AND THREO ISOMERS OF β-O-4 TYPE LIGNIN MODEL COMPOUNDS UNDER HYDROGEN PEROXIDE BLEACHING CONDITIONS

摘要

The erythro(E)and threo(T)isomers of a dimeric non-phenolic β-O-4 type lignin model compound,2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propa ne-1.3-diol(1),2-(4-hydroxymethyl-2-methoxy-phe noxy)-l-(3,4-dimethoxyphenyl)propane-1.3-diol(2),or 2-(4-methoxymethyl-2-methoxyphenoxy)-1-(3,4-dimet hoxyphenyl)propane-1,3-diol(3),was reacted with active oxygen species(AOS)under hydrogen peroxide bleaching conditions employed various pH levels to observe the stereo-preferential attack of AOS on the E or T isomer of the β-O-4 type lignin model compound.The results showed that the stereo-preference of O;(oxyl anion radical)is intrinsically the E isomer while HO.(hydroxyl radical)has almost no stereo-preference or a tiny preference of E isomer.These stereo-preferences were rationally explained by our previous findings that O;(oxyl anion radical)preferentially attacks the side-chain of the β-O-4 type lignin model compounds rather than the aromatic nucleus while HO*(hydroxyl radical)reacts with the aromatic nucleus as well as the side-chain.The degradation of the β-O-4 type lignin model compounds became less great with decreasing the initial pH from 13.3 to about 12,but conversely and drastically greater with the further decrease.This result suggests that hydrogen peroxide decomposes mainly following the bimolecular mechanism,by which the radicals are not generated,in the former pH range and that the main mechanism switches from the biomolecular to unimolecular,which generates the radicals,at an initial pH of around 12.