The aerobic oxidation of bromide to dibromine catalyzed by homogeneous oxidation catalysts and initiated by nitrate in acetic acid

• A commercial source of cobalt(II) acetate tetrahydrate contained a nitrate impurity.
• In a mixture of Co(II) and Mn(II) acetates with bromide in acetic acid, the nitrate initiated the aerobic oxidation of bromide to dibromine.
• A number of variables were studied to decrease the induction and reaction times of the autocatalytic reaction.

For ∼50 years mixtures of cobalt(II) and manganese(II) acetates with sodium bromide or hydrobromic acid in acetic acid have been used as catalysts for the homogeneous aerobic oxidation of alkylaromatic compounds. They are known to be stable mixtures. While characterizing this mixture via EXAFS, it was thus surprising to observe an unexpected change in the color of the reaction mixture from deep blue to yellow or orange. Subsequent characterization of the reaction via UV–vis and EXAFS showed that the color change was due to the oxidation of bromide to dibromine. The reaction was found to require the presence of dioxygen. It was found via MALDI, IR and other methods, that the commercial source of cobalt(II) acetate contained a small amount of nitrate impurity. It is the presence of nitrate that caused the oxidation of bromide since intentional addition of various nitrate compounds causes the reaction to occur. Nitrate is likely initiating a chain reaction. The dibromine formed is an active bromination agent. Thus we show that an inexpensive, in-situ preparatory method for generating dibromine is a Mn(II) acetate/nitrate/bromide/air mixture. We show that the reaction, for T < 40 °C, is autocatalytic and its initiation and reaction times are dependent on a number of variables such as temperature, nitrate concentration, type of bromide and metal, pH, water concentration in acetic acid, and purity of the solvent. A cautionary note: the accidental addition of nitrate into a catalyst feed tank used for homogeneous oxidation of alkylaromatics in pilot plants or commercial plants could cause the bromide to oxidize to dibromine. This could be potentially dangerous and/or result in decreased yields.

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