Chemistry of 2,5-dihydroxyacetophenone, a Key Chromophore in Cellulosic Substrates

Abstract

2,5-Dihydroxyacetophenone (DHAP) is one of the three key chromophores found in aged cellulosics. Knowledge of the general reactivity and chemistry of this compound is helpful for a better understanding of cellulose aging and yellowing as well as bleaching of cellulosic materials - processes that also have considerable economic importance. This paper reviews the reactions reported for 2,5- dihydroxyacetophenone from the viewpoints of both synthesis and general reactivity. The compound exhibits a rich chemistry due to the superposition of the hydroquinone structure and the 2-hydroxyacetophenone structure with its α-hydroxy ortho-quinone methide tautomer. The 1-acetyl group exhibits high reactivity attributed to resonance of the carbonyl group with the π-electrons of the benzene ring and proton donation to the carbonyl group from the adjacent 2-hydroxy group. One of the prominent conversion paths for DHAP are aldol-type reactions that have been applied for the synthesis of a multitude of flavonoids. Other reaction pathways are nucleophilic substitution on the carbonyl group mainly with amines, electrophilic addition to the benzene ring, and oxidation into the corresponding benzoquinone. There are also quite a few reports of selective derivatization of the 5-hydroxy group of DHAP that has a higher reactivity than its 2-OH counterpart that is engaged in tight hydrogen bonding with the acetyl carbonyl moiety and thus largely blocked in esterifications and similar derivatizations. Keywords: 2-hydroxy-acetophenones, aging, aldol reaction, bleaching, cellulose, chromophore, reaction mechanism.