A. S-Ethylthiourea hydrobromide. A mixture of 150 g. of powdered thiourea (1.97 moles) (Note 1), 250 g. of ethyl bromide (2.29 moles), and 200 ml. of absolute ethanol is placed in a 1-l. round-bottomed flask equipped with an efficient condenser. The mixture is warmed on a water bath (bath temperature 55–65°) for 3 hours, with occasional shaking. During this time all the thiourea dissolves. The reflux condenser is replaced by one set for downward distillation, and the ethanol and excess ethyl bromide are removed under the vacuum of a water pump. During the distillation, the temperature of the bath is slowly raised to the boiling point (Note 2). The residual oil is poured into a 500-ml. beaker and allowed to crystallize (Note 3). The solid is pulverized and dried in a desiccator (Note 4) and (Note 5). The yield is 340–360 g. (93–99%).

B. Guanidoacetic acid. This reaction should be carried out in a well-ventilated hood, as considerable amounts of ethyl mercaptan are evolved. In a 1-l. Erlenmeyer flask is placed 92.5 g. (0.50) mole of S-ethylthiourea hydrobromide. The flask is immersed in an ice bath, and 252 ml. of 2 N sodium hydroxide solution is added. A hot (80+°) solution of 41 g. of glycine in 90 ml. of water is added rapidly. When the temperature reaches 25° (Note 6), the flask is removed from the ice water. After about 30 minutes crystallization begins; then approximately 100 ml. of ether is added, and the mixture is left in the hood overnight (Note 7). The mixture is then chilled for 2 hours in an ice bath, the ether layer is decanted, and the solid is filtered with suction. The crystals are washed on the funnel successively with two 20-ml. portions of ice water (Note 8), two 150-ml. portions of 95% ethanol, and two 150-ml. portions of ether. The yield of air-dried guanidoacetic acid is 47–53 g. (80–90%). This product is pure enough for most purposes (Note 9); it melts with decomposition at 280–284°.