S-benzylisothiouronium chloride

A little of practical organosulfur chemistry. In this reaction the carbon of methyl group of benzyl binds to the sulfur of thiourea (C-Cl bond changes into C-S bond). The product could be hydrolyzed by 10% NaOH solution into sodium salt of benzyl mercaptan from which the benzyl mercaptan could be released by acidification with HCl. Only the first reaction was performed, not the mercaptan steps.

7,6 g of thiourea (100 mmol) and 20 ml water were heated in 100 ml RBF until it dissolved. 13,5 ml of 95% ethanol was added dropwise while stirring (be careful not to quickly add low boiling alcohol into too hot solution to prevent sudden boiling out). Then 12,7 grams of benzyl chloride (100 mmol) was added at once from a pipette while the flask was on a scale. Magnetic stir bar was inserted and Liebig reflux condenser attached. The reaction was stirred and refluxed. After about 15 minutes the 2 immiscible liquids changed into homogenous and later reflux temperature was reached (the heating mantle was set to somewhat middle power heating, not max, probably on max heating the reflux would be reached earlier than the mixture become homogenous, or maybe the same if not magnetic stirring).
The reflux was kept for 30 minutes.
The mixture was cooled down to room temperature and put into a fridge (+4 C). Nothing crytallized due to supersaturation. A glass rod was inserted into the flask and the surface of the supersaturated solution was touched to get a droplet of the liquid on the tip of the glass rod. The rod was let to dry and there was a glassy solid substance observed on it. The tip the glass rod was rubbed against inside wall of ground glass joint of the RBF which finally made something crystalline, these seeds fell into the RBF and finally started a crystallization process. A dusty like solid mass was obtained, not visible crystals by an eye sight (the size below human eye resolution limit).
The solid mass was vacuum filtered and mother liquor evaporated to about half of its volume. After cooling down it became again supersaturated so it was necessary to introduce a little of already available solid product to initiate the crystallization.
Both still wet crops were transferred back into the 100 ml RBF and then 40 ml of 0,2 N HCl (made from 0,6 ml of 37% HCl into 40 ml of water) was added. After heating the solid dissolved. It was gravity filtered while hot (no crystallization observed on filtration glass either filter paper), cooled down to lab temperature. Small amount of already available solid mass were repeatedly introduced (scraped by a spoon from walls of filter funnel) which always slowly dissolved, even when flask was about 30 C (just approximate temperature by touching by hand). The flask was put into a fridge for short time and then the crystal seeds finally started to work (the temp of the flask about 20 C just approx. by touching by hand). After seeing crystallization progressing successfully the flask was put back into the fridge (+4 C) overnight, crystals were vacuum filtered (now nice crystalline mass, crystals of size circa 0,5 mm, not more than 1 mm). The product was dried on air.
Yield 7,8 g (quite poor but at least more than the initial weight of the thiourea).
It is possible to synthesize thiols from these isothiouronium salts - hydrolysis with 10% solution of NaOH and then acidification with HCl to release free mercaptan from sodium salt. I do not have guts for making these mercaptans yet. I live in an inhabited area. A technician from the company distributing gas visited the whole area this year and checked gas systems on the street (connections, valves, volume/flow measuring devices). After detecting a small gas leak I had to call a repair service. Luckily the leak was very small and on the street, not inside my house / lab. The repair man made the job few weeks later. No hurry. 1st call result - call me tomorrow (I heard also a silent kid babblement so I knew he already became a father). The 2nd call - I will arrive the next week on Friday. The 3rd call the next week on Friday where I had to leave my house and won't be at home Friday afternoon - Cool so I will arrive on Monday. The next week - he just checked with detection device and said: OK I'll arrive the next week with necessary tools and material. The next week he finally repaired the leaking connection of gas tubes by replacing the packing material of the thread joints (2 joints were leaking but at almost undetectable level).

Some thoughts - the attached paper claims about twice as good yield (about 90 %). Maybe the reflux should be for longer time as chloride is less reactive than bromide (but I hardly believe it could help because the mixture was nicely homogenous and benzyl chloride is very little soluble in water (0,5 g per 1 liter) and even less in some salty solution. So the benzyl chloride seemed to react almost quantitatively. Maybe less water (0,2 N HCl) for the recrystallization could be used something like 30 ml or even 20 ml? Maybe the second mother liquor could be evaporated to dryness to increase the yield? Maybe it should be cooled to 0 C instead of +4 C. Also recrystallizing wet crude product introduced more water (especially not nice crystalline mass but dusty like with a lot of trapped water), perhaps better to dry it first to constant weight and only then recrystallize.

There could be 2 types of crystals (dimorphism), melting at 150 C and at 175 C. If you know how to distinguish them, please write a comment. Introducing only 1 seeding crystal into supersaturated solution should produce only this one form of crystals. Most of the crystals are the 175 C so introducing 1 crystal very likely produces only this one form. But you need at least 1 of such crystal (they are cca. 0,5 mm in size).

Vogel - thiols from bromides with thiourea
Attachment: vogel_thiols_from_bromides_with_thiourea.pdf (362kB)
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Beginning of the reaction





Magnetic stirring commenced which caused an emulsification of immiscible liquids.





After some time the reaction became homogenous due to rapid stirring, even before reaching reflux temperature.





After the reaction (the color could be slightly different due to already sunlight present, previous photos were made under lamp).





First filtration, the product in dusty form, not nice crystals.





Vacuum filtration of recrystallized product, now nice crystals.





Here some shiny walls of crystals reflecting light.