Abstract
2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ) and 5,8-dihydroxy-[1,4]-naphthoquinone (DHNQ) are two key chromophores which are almost ubiquitous in cellulosic materials. Their fate under conditions of alkaline peroxide bleaching (P stage) has been established previously, but the intermediacy of cyclic peroxides, which so far had only been postulated, remained an open issue. By means of 17O NMR spectroscopy, additionally supported by other NMR techniques, it was demonstrated that both DHBQ and DHNQ form cyclic peroxides as primary intermediates in the reaction with hydrogen peroxide under alkaline conditions. These intermediates are subsequently further degraded to products already known. The experimental confirmation of the cyclic peroxides is an important step in the understanding of reaction mechanisms in pulp bleaching chemistry.Graphical
Highlights
Three compounds have been established as key chromophores in cellulosic matrices: 2,5-dihydroxy1,4-benzoquinone (1), 5,8-dihydroxy-naphthoquinone (2), and 2,5-dihydroxyacetophenone (Korntner et al 2015)
While 1 exhibits three 13C resonances, the keto and hydroxylated carbons are rendered magnetically equivalent in dianion 1a, which exhibits only two resonances. This structural change upon deprotonation is reflected in 17O NMR: the two resonances in neutral DHBQ become one resonance in the dianion 1a, in which all four oxygens are magnetically equivalent (Scheme 1)
Scheme 3 Non-symmetric cyclic peroxide intermediates 3b and 4b, formed from methyl derivatives of DHBQ and DHNQ. From these 17O NMR studies, the occurrence of the cyclic peroxides 1b and 2b as the primary intermediates in the oxidation of the cellulosic key chromophores DHBQ (1) and DHNQ (2), respectively, can be considered established, which expands our understanding of the chemistry of these compounds
Summary
Three compounds have been established as key chromophores in cellulosic matrices: 2,5-dihydroxy1,4-benzoquinone (1), 5,8-dihydroxy-naphthoquinone (2), and 2,5-dihydroxyacetophenone (Korntner et al 2015). The kinetics as well as the reaction mechanism, stable intermediates and final degradation products, mostly C1 to C4 acids and hydroxyacids and carbon dioxide, have been established
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