Transformations of the cellulosic key chromophore 2,5-dihydroxy-[1,4]-benzoquinone in aqueous media

Abstract 2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ, 1 ) is an almost ubiquitous key chromophore in cellulosic materials that underwent aging processes (yellowing/brightness reversion). While cellulosic fibers containing DHBQ kept under dry conditions over several weeks did not show any alteration, storage under moist conditions caused a change in their UV reflectance spectra. DHBQ was absent and transformed into different UV-absorbing compounds. In this work, the chemical structure of these transformation products of DHBQ has been clarified. Isolation from fibers and model reactions of DHBQ in water provided three condensation products, identified as bis -quinones 2 and 3 and benzofuranone derivative 4 by NMR spectroscopy and X-ray diffraction analysis. Mechanistic analyses showed DHBQ to be especially prone to condensation reactions in a pH range around pH = 4–5, while either strongly acidic conditions (pH < 3) or neutral/alkaline conditions (pH > 7) stabilized the chromophore. In the neutral range, a Michael addition of the monoanion of DHBQ to another molecule of DHBQ occurs, which is the first step in a sequence of condensation reactions leading to the products 2 – 4 . The involvement of ketohydrate structures and extensive tautomerism (sigmatropic proton shifts) explain the crucial role of water in this process. Mechanistic details of the transformations are presented and implications of these DHBQ transformations for the brightness stability of cellulosic materials are discussed.