title: Degradation of 2,5-dihydroxy-1,4-benzoquinone by hydrogen peroxide under moderately alkaline conditions resembling pulp bleaching: a combined kinetic and computational study.
abstract: 2,5-Dihydroxy-1,4-benzoquinone (DHBQ) is one of the key chromophores occurring in all types of aged cellulosics. This study investigates the mechanism of H2O2 degradation of DHBQ under conditions relevant to pulp bleaching (3.0% H2O2, NaOH, pH 10), to obtain insights useful for improved pulp processing. DHBQ is degraded quantitatively into malonic acid with an activation energy (E(a)) of 16.1 kcal/mol and activation entropy (Δ(‡)S°) of ~28 cal/mol·K. Higher concentrations of sodium cations increase the reaction rate. Theoretical computations indicate the formation of an intermediate I(O) having an O-O bridge between C-2 and C-5 of the 1,4-cyclohexadione structure. I(O) undergoes O-O homolysis to form a biradical Bt, which is fragmented into malonate anions. The calculated E(a) (17.8 kcal/mol) agrees well with the experimental one. Coordination of Na(+) to I(O) and Bt decreases their energies and enhances the O-O homolysis rate, which is consistent with the acceleration by sodium cation and the negative Δ(‡)S°. The homolysis of I(O) is much favored over that of the neutral counterpart, with the unpaired electrons of Bt being stabilized by the geminal anionic oxygen. This difference in the stability of the intermediates translates into significant variations in the reaction rate and the product distribution between pH 10 and neutral/acidic conditions.
authors.affiliation: Department of Chemistry, University of Natural Resources and Life Sciences , Muthgasse 18, A-1190 Vienna, Austria.
authors.forename: Takashi
authors.initials: T
authors.lastname: Hosoya
authors.forename: Thomas
authors.initials: T
authors.lastname: Rosenau