Mass-Spectrometric Characterization of Oligomeric Products from Hydroquinone Oxidation by Hydrogen Peroxide as an Analytical Problem of Particularly Complexity

Abstract

The oxidation of hydroquinone with hydrogen peroxide in the presence of catalytic amounts of FeSO₄ results in complex mixtures of oligomers. The average composition of these products is determined by the molar ratio of reagents and varies from (C₆H₄O₄)_n at the hydroquinone to hydrogen peroxide ratio 1 : 3 to (C₆H₄O₆)_n at the ratio 1 : 5. A characteristic feature of MALDI mass spectra for polymers is the periodicity of signals. However, no such periodicity is observed for the products of hydroquinone oxidation within the m/z range <1000 Da, indicating the absence of fixed sequences of structural fragments. This unique feature arises from the variable ratios and irregular positions of polyhydroxyphenylene and polyhydroxybenzoquinone fragments. Additionally, several factors contribute to the lack of mass-spectral periodicity, namely, the potential formation of peroxides, the presence of stable hydrates, various types of linkages, and possible interactions between hydroxylated phenylene and benzoquinone fragments. In contrast, for m/z values >1000 Da, MALDI mass spectra exhibit periodicity with a mass difference of Δ(m/z) = 74. This value likely corresponds to the fragment C₂H₂O₃, which is neither a hydroquinone nor a benzoquinone structural unit. Taking into account that an equal Δ(m/z) value is observed for fragment ions in the EI mass spectrum of tetrahydroxy-p-benzoquinone, this gives indirect evidence for the presence of polyhydroxybenzoquinone fragments within the oligomers. A key issue regarding the structure of hydroquinone oxidation products is the type of linkages between the polyhydroxyphenylene and/or polyhydroxybenzoquinone units, which could involve C–C or C–O–C bonds (or both). The available spectral data do not resolve this issue conclusively. However, the presence of a sample of the composition (C₆H₄O₆)_n, in which each unit contains six oxygen atoms, suggests that at least one oxygen atom forms a bond between the units, indicating a C–O–C connection. Overall, the unique nature of oligomeric products of hydroquinone oxidation explains their high structural variability and redox properties, which likely contribute to their unique pharmacological characteristics.