A mixture of 50 g. (45 ml., 0.52 mole) of freshly distilled furfural and 500 ml. of water is stirred vigorously in a 2-l. three-necked round-bottomed flask equipped with a dropping funnel and a thermometer that dips into the liquid. The flask is immersed in an ice bath, and 450 g. (144 ml., 2.81 moles) of bromine is added, while the temperature of the reaction mixture is kept below 5° (Note 1). After the addition is complete, the thermometer is replaced by a reflux condenser, and the mixture is stirred and boiled for 30 minutes. The reflux condenser is replaced by a still head and condenser, and excess bromine is removed by distilling the liquid until the distillate is almost colorless (Note 2).

The reaction mixture is evaporated to dryness under reduced pressure at a water pump on a steam bath, using a trap cooled in ice and salt to condense the hydrobromic acid (Note 3). The solid residue is cooled in an ice bath and triturated with 30–50 ml. of ice water. A few grams of sodium bisulfite, dissolved in water, is added to discharge a slight yellow discoloration. The cold mixture is filtered with suction to separate crude mucobromic acid, which is washed with two small portions of ice water. The crude mucobromic acid weighs 125–132 g. (93–99%). It is dissolved in about 110 ml. of boiling water, 2–5 g. of decolorizing carbon is added, the hot mixture is stirred for 10 minutes and filtered, and the filtrate is cooled to 0–5°. Colorless crystals of mucobromic acid separate from the filtrate; weight 100–112 g. (75–83%); m.p. 124–125°.