Oxidation of Benzoin Add conc. HNO, (5 mL) into a 125-ml conical flask, then benzoin (2 g) with gentle swirling and place on a water bath (in an efficient fume Hood) about 40 min until no more brown fume of nitrogen oxide evolve from the reaction. Caution: upon heating for a period of time, tremendous fume of nitrogen oxide will produce and gradually decrease and then finally disappeared.