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

Citation
L. Jurasek et al., Alkaline oxidative degradation of diphenylmethane structures - Activation energy and computational analysis of the reaction mechanism, CAN J CHEM, 79(9), 2001, pp. 1394-1401
Citations number
24
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry
Journal title
CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE
ISSN journal
0008-4042 → ACNP
Volume
79
Issue
9
Year of publication
2001
Pages
1394 - 1401
Database
ISI
SICI code
0008-4042(200109)79:9<1394:AODODS>2.0.ZU;2-X
Abstract
A diphenylmethane model compound (2,2'-methylenebis[6-methoxy-4-methylpheno l]) and residual kraft lignin were treated with alkaline hydrogen peroxide. Kinetic data for the disappearance of the model and the diphenylmethane st ructures in the residual lignin was collected. The activation energies for the degradation were found to be similar (54 +/- 11 kJ mol(-1) for the mode l and 58 +/- 5 kJ mol(-1) for the residual lignin). A comparison of the act ivation energies with the data of a previous study on a biphenyl model comp ound (3,3'-dimethoxy-5,5'-dimethyl-[1,1'-biphenyl]-2,2'-diol) showed a subs tantially higher activation energy for the degradation of the latter. Pathw ays for the degradation of 2,2'-methylenebis[6-methoxy-4-methylphenol] were proposed and the intermediates subjected to computational analysis using a semiempirical method (PM3). The results suggest that initially a common pa thway exists, resulting in 2-[2-hydroxy-3-methoxy-5-methyl-phenylmethyl]-4- methyl-2,4-hexadienedioic acid. Then the pathway branches into three, resul ting in three major degradation products. The main driving force of the rea ctions is the formation of radical sites after reaction with hydroxyl radic als and subsequent radical coupling with perhydroxyl radicals to form perox ides. All the reactions on the pathways are exothermic except for the trans formations of peroxides into dioxetanes. The dioxetanes cleave exothermical ly resulting in ring cleavage and fragmentation. The computed data permitte d for the rationalization as to why the diphenylmethane structures appear t o be more labile than biphenyl structures under alkaline oxidative conditio ns.