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A diphenylmethane model compound (2,2¢-methylenebis(6-methoxy-4-methylphenol)) and residual kraft lignin were treated with alkaline hydrogen peroxide. Kinetic data for the disappearance of the model and the diphenylmethane structures in the residual lignin was collected. The activation energies for the degradation were found to be similar (54 ± 11 kJ mol -1 for the model and 5 8±5k J mol -1 for the residual lignin). A comparison of the acti- vation energies with the data of a previous study on a biphenyl model compound (3,3¢-dimethoxy-5,5¢-dimethyl-(1,1¢- biphenyl)-2,2¢-diol) showed a substantially higher activation energy for the degradation of the latter. Pathways for the degradation of 2,2¢-methylenebis(6-methoxy-4-methylphenol) were proposed and the intermediates subjected to compu- tational analysis using a semiempirical method (PM3). The results suggest that initially a common pathway exists, re- sulting in 2-(2-hydroxy-3-methoxy-5-methyl-phenylmethyl)-4-methyl-2,4-hexadienedioic acid. Then the pathway branches into three, resulting in three major degradation products. The main driving force of the reactions is the forma - tion of radical sites after reaction with hydroxyl radicals and subsequent radical coupling with perhydroxyl radicals to form peroxides. All the reactions on the pathways are exothermic except for the transformations of peroxides into dioxetanes. The dioxetanes cleave exothermically resulting in ring cleavage and fragmentation. The computed data per- mitted for the rationalization as to why the diphenylmethane structures appear to be more labile than biphenyl struc- tures under alkaline oxidative conditions.

Alkaline oxidative degradation of diphenylmethane structures  Activation energy and computational analysis of the reaction mechanism

Alkaline oxidative degradation of diphenylmethane structures Activation energy and computational analysis of the reaction mechanism