- Dissolve 76g (1 mol) of thiourea in 200 ml of warm water in a 1-litre round-bottomed flask.
- Dilute the solution with 135 ml of rectified spirit and add 126.5g (1 mol) of benzyl chloride.
- Heat the mixture under reflux on a water bath until the benzyl chloride dissolves (about 15 minutes) and for a further 30 minutes taking care that the mixture is well shaken from time to time.
- Cool the mixture in ice: there is a tendency to supersaturation so that it is advisable to stir (or shake) the cold solution vigorously, when the substance crystallises suddenly.
- Filter off the solid at the pump.
- Evaporate the filtrate to about half bulk in order to recover a further small quantity of product.
- Dry the compound upon filter paper in the air. The yield of S-benzylisothiouronium chloride, m.p. 174 °C, is 200g (99%).
- Recrystallise the salt from 400 ml of 0.2 m hydrochloric acid; filter off the solid which separates on cooling. The yield of recrystallised salt, m.p. 175 °C, is 185g (91%); some of the dimorphic form, m.p. 150 °C, may also separate.
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