5- Benzylisothiouronium chloride

The preparation and isolation of S-benzylisothiouronium chloride by the interaction of benzyl chloride and thiourea is described in Expt 5.203. On recrystallisation the compound separates in either, or sometimes as both, of two dimorphic forms, m.p. 150 and 175 °C respectively. The former may be converted into the higher m.p. form by dissolving it in ethanol and seeding with crystals of the form, m.p. 175 °C the low m.p. form when warmed to 175 °C gives, after solidification, a m.p. of 175 °C. The compound is of particular interest as a reagent for the characterisation of carboxylic acids or sulphonic acids with which it forms the S-benzylisothiouronium carboxylate (or sulphonate) salts (Sections 9.6.15, p. 1264, and 9.6.26, p. 1285). Both dimorphic forms give identical derivatives. 

Dissolve 76 g (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 126.5 g (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 200 g (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 185 g (91%) some of the dimorphic form, m.p. 150 °C, may also separate. 

S-Benzylisothiouronium chloride reacts with the alkali metal salts of organic acids to produce crystalline S-benzylisothiouronium salts ... 

Dissolve (or suspend) 0.25 g of the acid in 5 ml of warm water, add a drop or two of phenolphthalein indicator and neutralise carefully with c. m sodium hydroxide solution. Then add 2-3 drops of c. 0.1 m hydrochloric acid to ensure that the solution is almost neutral (pale pink colour). (Under alkaline conditions the reagent tends to decompose to produce the evil-smelling phenylmethanethiol.) If the sodium salt is available, dissolve 0.25 g in 5 ml of water and add 2 drops of c. 0.1 m hydrochloric acid. Introduce a solution of 1 g of S-benzylisothiouronium chloride in 5 ml of water, and cool in ice until precipitation is complete. Recrystallise the crude derivative from dilute ethanol or from hot water. 

To a solution of 0.5 g of the salt in 5 ml of water and 2-3 drops of 0.1 m hydrochloric acid (or to a solution of the acid treated as above), add a slight excess of a cold, 15 per cent aqueous solution of S-benzylisothiouronium chloride (if the molecular weight of the compound is not known, use a solution of 1 g of the reagent in 5 ml of water), and cool in ice. Filter off the crystalline derivative and recrystallise it from 5 per cent ethanol. 

SYNS BENZYUSOTHIOURONIUM CHLORIDE 2-BENZYLISOTHIOURONIUM CHLORIDE O BENZYL THIOPSEUDOUREA HYDROCHLORIDE 2-BENZYL-2-THIO-PSEUDOUREA HYDROCHLORIDE BENZYL-THIURONIUM CHLORIDE S-BENZYLTHIURONIUM CHLORIDE BTKH ISOTHIOURONIUM CHLORIDE. BENZYL 2-THIO-2-BENZYL-PSEUDOUREA HYDROCHLORIDE TL944 USAF EK-2124... 

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