To 2750 g. (2 l., 13.6 moles) of 40 per cent hydrobromic acid (Note 1) in a 3-l. flask is added 317 g. (4.5 moles) of ethylene cyanohydrin (p. 256), and the mixture is boiled for two hours under a reflux condenser. The condenser is then arranged for downward distillation and a thermometer immersed in the reaction mixture; dilute hydrobromic acid is distilled off until the temperature in the flask reaches 121°, whereupon the receiver is changed and a fraction consisting of stronger hydrobromic acid is collected over the range 121–129°. When the temperature of the mixture reaches 129°, very little hydrobromic acid remains, and on cooling, the mass sets to an almost colorless solid. This is now dissolved in 2000 cc. of carbon tetrachloride (Note 2), and the ammonium bromide is filtered off and washed with 500 cc. more of the solvent; a thin aqueous layer is separated and 1500 cc. of the carbon tetrachloride distilled from the filtrate. On cooling, about 470 g. of β-bromopropionic acid crystallizes from the residue; on filtering and drying, this melts at 62.5–63.5°. With further concentration, the mother liquor yields a second crop of β-bromopropionic acid, amounting to 60–70 g.

The aqueous layer separated from the main carbon tetrachloride solution is shaken with 100 cc. of carbon tetrachloride, and thus yields about 10 g. of pure acid; when the dilute and the concentrated hydrobromic acid fractions are extracted in the same way, about 5 g. and 15 g. respectively of β-bromopropionic acid are obtained. The total yield is 560–570 g. (82–83 per cent of the theoretical amount).