S-Benzyl -iso-thiuronium chloride

Benzyl chloride reacts with thiourea in dilute alcoholic solution to give f-benzyl-tso-thiuronium chloride  

The compound separates in either, sometimes as both, of two dimorphic forms, m.p. 150° and 175° respectively. The former may be converted into the higher m.p. form by dissolving it in alcohol and seeding with crystals of the form, m.p. 175° the low m.p. form when warmed to 175° gives, after solidification, a m.p. of 175°. Both dimorphic forms give identical derivatives with carboxyUc acids and sulphonic a ds (see Sections 111,85 and IV,33). 

Method 1. Dissolve 76 g. of thiourea in 200 ml. of warm water in a 750 ml. or 1 litre round-bottomed flask. Dilute the solution with 135 ml. of rectified spirit and add 126 6 g. 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 oflF 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 -benzyl-Mo-thiuronium chloride m.p. 174°, is 205 g. RecrystaUise the salt from 400 ml. of 0-2N hydrochloric acid filter off the solid which separates on cooling. Concentrate the filtrate to recover a further small quantity. The yield of recrystallised salt, m.p. 175° is 185 g. some of the dimorphic form, m.p. 150°, may also separate. 

Method 2. Place a mixture of 126-5 g. of benzyl chloride, 76 g. of thiourea and 150 ml. of rectified spirit in a 500 ml. round-bottomed flask fitted with a reflux condenser. Warm on a water bath. A sudden exothermic reaction soon occurs and all the thiourea passes into solution. Reflux the resulting yellow solution for 30 minutes and then cool in ice. Filter off the white crystals and dry in the air upon filter paper. Concentrate the filtrate to half its original volume and thus obtain a further small crop of crystals. The yield of crude (S-benzyl-iso-thiuronium chloride, m.p. 145°, is 236 g, RecrystaUise from 0-2M hydrochloric acid as in Method 1 the m.p. is raised to 150°, although on some occasions the form, m.p. 175°, separates. 

---

S-Benzyl-iso-thiuronium chloride (S-benzyl-iso-thiourea hydrochloride) reacts with the alkali metal salts of organic acids to produce crystalline S-benzyl-iao-thiuronium 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 ca. N sodium hydroxide solution. Then add 2-3 drops of co. 0-lAr 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 benzyl mercaptan.) If the sodium salt is available, dissolve 0-25 g. in 5 ml. of water, and add 2 drops of ca. 0-1N hydrochloric acid. Introduce a solution of 1 g. of S-benzyl-iso-thiuronium chloride in 5 ml. of water, and cool in ice until precipitation is Dibasic and tribasic acids will require 0-01 and 0-015 mol respectively. 

---