In a 3-l. three-necked, round-bottomed flask, fitted with a powerful glycerol-sealed stirrer, a dropping funnel, and an outlet tube for escaping gases, 1.6 kg. (17.4 moles) of glycerol is thoroughly mixed with 200 g. (6.5 gram atoms) of red phosphorus (Note 1). Nine hundred cubic centimeters (2808 g., 17.5 moles) of bromine (Note 2) is then gradually added, with effective stirring (Note 3), through the dropping funnel in the course of about eight hours. To minimize the escape of bromine, the end of the dropping funnel should reach almost to the bottom of the flask. The by-product gases, consisting mainly of hydrogen bromide and some bromine, are led over concentrated sodium hydroxide solution or to a gas trap. The reaction is exothermic, and the temperature quickly rises to 80–100°; then the addition of bromine is so regulated that this temperature is maintained. Toward the end of the period of addition of bromine, the flask is placed in a water bath at 70–75°. After all the bromine has been added, the mixture is allowed to stand overnight and is then warmed on the water bath until all the bromine is consumed (one to two hours).

The reaction mixture is transferred to a 3-l. round-bottomed flask provided with a two-holed rubber stopper carrying a wide delivery tube and a capillary tube. The flask is heated in an oil bath, and distillation is begun under the reduced pressure of a water pump. The receiver is cooled with water. At first a mixture of hydrobromic acid and water passes over; later the dibromohydrin distils. The temperature of the bath is raised as fast as the boiling of the mass permits, and is eventually brought to 180°. The distillation is carefully watched at the end and immediately interrupted at the first sign of decomposition. This is clearly indicated by gas formation, in consequence of which the vacuum cannot be maintained at the previous level. To the straw-yellow distillate a slight excess of solid sodium bicarbonate is added with continuous shaking until effervescence ceases. The inorganic salts are removed by filtration, and the aqueous layer of the filtrate is separated from the crude dibromohydrin. The latter is purified by fractional distillation under reduced pressure from a 2-l. Claisen flask. The distillation is continued until no more water passes over and the inside temperature reaches 100°. Then the dibromohydrin is separated from the water in the distillate, dried with anhydrous sodium sulfate, filtered, and poured back into the distilling flask. By this operation the water is largely removed (Note 4). Then the distillation is continued as before and, after a small fore-run, the dibromohydrin boils at 110–112° under 20 mm. pressure (Note 5). The yield is 2000–2050 g. (52–54 per cent of the theoretical amount) of a colorless product.

The dibromohydrin is a heavy, colorless liquid with a characteristic odor. On standing it gradually becomes yellow. Its specific gravity at 20° is about 2.14.