Trapping of Dioxygen Dissolved in Water by Alkylhydroxylamines: A Comparison of Hydroquinone, Gallic Acid and Aminophenols as Organocatalysts

One of the most usual methods used to deoxygenate water and prevent the corrosion of industrial boilers is based on the reduction of dioxygen in aqueous alkaline solution with an hydroxylamine in the presence of catalytic amounts of hydroquinone (H₂Q). In order to improve the effectiveness and the “greenness” of this process, the influence of the nature of the hydroxylamine and of the catalyst has been investigated. With H₂Q as a catalyst, the efficiency of various hydroxylamines was found to be in the following order: methylhydroxylamine > dimethylhydroxylamine > hydroxylamine > tert-butylhydroxylamine > diethylhydroxylamine (DEHA). To replace the potentially carcinogenic hydroquinone, a natural polyphenol, i.e. gallic acid, which was previously shown to be safer and more robust that H₂Q to trap O₂ in the presence of DEHA, was used. Methylhydroxylamine was still by far the most active partner, but DEHA was almost as efficient as the other hydroxylamines, free hydroxylamine being the least active oxygen scavenger. As only a catalytic amount of dioxygen scavenger is required in the presence of appropriate hydroxylamines, compounds having a bit more acute toxicity but higher reactivity than hydroquinone may also be alternative catalytic systems. Hence, five aminophenols, i.e. 4-aminophenol, 4-amino-2,5-dichlorophenol, 5-amino-salicylic acid, 4-N-methylaminophenol and 4-N,N-dimethylaminophenol, were also investigated. Among them, 4-N-methylaminophenol was found to be the most efficient although much slower than H₂Q.