Fery
International Hazard
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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.
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Fery
International Hazard
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Registered: 27-8-2019
Location: Czechoslovakia
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Here some improvement by Boffis:
https://www.sciencemadness.org/whisper/viewthread.php?tid=76...
| Quote: | I have found out what is wrong with many of the preparations of Benzyl-isothiouronium chloride in Vogel and repeated elsewhere; they use too much
water to dissolve the thiourea. I have discovered that the resulting solution simply doesn't crystallize as described because the thiouronium chloride
is very soluble in water. In fact I evaporated a couple of ml to dryness on a water bath and then scaped the solid into the reaction mixture (chilled
to 4 C) and it dissolved showing the solution is nowhere near saturated. It is much less soluble in cold ethanol so the following preparation is both
simpler and easier.
Add 15.2g of thiourea (0.2 moles) to 50ml of 95% ethanol (rectified spirit) and add 25.3g (0.2 moles) of benzyl chloride. Warm until the benzyl
chloride disappears (very rapid 30 seconds) and then reflux for 40-45 minutes and cool, remove a few drops onto a small watchglass and dry. Pour the
remainder into a basin, when at room temperature chill in the fridge and scrape the dried material onto the liquid and stirr gently. Crystallisation
occurs fairly rapidly and the whole mass solidifies. Stirr up with a spatula and filter at the pump, stuck dry with strong suction, turn out the cake
and dry. The filtrate should be evaporated slowly to a small volume and chilled to recover a second crop. At 1 mole and larger quantities it might be
worth recovering the solvent but at this scale is isn't worth it. |
from orgsyn.org:
https://orgsyn.org/demo.aspx?prep=v91p0116
| Quote: | | S-Benzyl Isothiouronium Chloride (1). A 500-mL one-necked pear-shaped flask equipped with a 3-cm oval magnetic stir bar is sequentially charged with
thiourea (19.1 g, 250 mmol, 1 equiv) (Note 1), benzyl chloride (31.7 g, 250 mmol, 1 equiv) (Note 2) and ethanol (250 mL) (Note 3). Then the flask is
equipped with a condenser and the reaction mixture is heated to reflux with a 96 °C oil bath (Φ = 18 cm, h = 11 cm) and stirred for 1 h. After the
reaction mixture is cooled to ambient temperature, the ethanol is removed on a rotary evaporator (25 mmHg, 55 °C), and 57.9–65.7 g of a white solid
is obtained as the title product 1 in quantitative yield (Note 4) |
from prepchem:
https://prepchem.com/synthesis-of-s-benzylisothiouronium-chl...
| Quote: | Preparation of S-benzylisothiouronium chloride
To a round bottom flask fitted with a reflux condenser a mixture of 126 g (1.0 mole) of benzyl chloride, 76 g of thiourea, and 200 ml of ethanol is
refluxed carefully for 30 minutes. During reaction time a vigorous reaction takes place. The resulting homogeneous solution is cooled, filtered and
the precipitated S-benzylisothiouronium chloride is recrystallized from ethyl alcohol. The yield of S-benzylisothiouronium chloride is nearly
quantitative melting at 172-174° C. A lower-melting form (m.p. 146-148 °) is sometimes obtained. It is converted to the higher-melting form by
recrystallization from ethyl alcohol, during which the solution is seeded with the higher-melting product.
J. Am. Chem. Soc., 8, 1004, 1936 |
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Fery
International Hazard
Posts: 1010
Registered: 27-8-2019
Location: Czechoslovakia
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also look at this:
https://sci-hub.ee/10.1016/j.tetlet.2011.07.024
S-Benzyl isothiouronium chloride as a recoverable organocatalyst for the direct reductive amination of aldehydes
Attachment: j.tetlet.2011.07.024.pdf (1.1MB)
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