2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ) is one of the key chromophores occurring in all types of aged cellulosics.1,2 This work presents molecular mechanisms in H2O2 degradation of DHBQ by a combination of experimental and computational approaches, to provide a solid knowledge base for optimization of bleaching sequences aiming at DHBQ removal. Experimental investigation under neutral conditions (3.0 % H2O2) showed that DHBQ degraded into malonic acid, acetic acid, and carbon dioxide with the activation energy (Ea) being 20.4 kcal/mol. Theoretical calculations presented a plausible mechanism for the degradation of DHBQ (Figure 1A). The mechanism involves an intermediate IOH having an intramolecular O-O bridge. IOH then undergoes the homolysis of the O-O bond followed by b-fragmentation of the resulting biradical BROH, giving ketene and oxaloacetic acid. While ketene yields acetic acid, oxaloacetic acid eventually gives malonic acid and carbon dioxide.3a Under conditions more relevant to pulp bleaching (3.0% H2O2, NaOH, pH 10), DHBQ degraded quantitatively into malonic acid with an Ea of 16.1 kcal/mol. The theoretical computation indicated that this degradation formed a dianionic intermediate IO- (Figure 1B). The intermediate IO- undergoes O-O homolysis to form a biradical BRO-, which is fragmented into malonate anions. The O-O bond homolysis of the dianionic intermediate IO-, under alkaline conditions, is favored over that of the neutral counterpart IOH, with the unpaired electrons of BRO- being stabilized by the geminal anionic oxygens. 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 conditions.3b References 1. Rosenau, T.; Potthast, A.; Krainz, K.; Yoneda, Y.; Dietz, T.; Shields, Z. P.; French, A. D. Cellulose 2011, 18, 1623–1633. 2. Hosoya, T.; French, A. D.; Rosenau, T. Mini Rev. Org. Chem. 2013, 10, 309–315. (3. a) Hosoya, T.; Rosenau, T. J. Org. Chem. 2013, 78, 3176–3182. (b)Hosoya, T.; Rosenau, T. J. Org. Chem. 2013, 78, 11194–11203.

Reaction mechanisms in pulp bleaching: H2O2 degradation of 2,5-dihydroxy-[1,4]-benzoquinone as a key chromophore in aged cellulosics

249th National Meeting (2015)

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