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Author: Subject: Iodomethane prep.
PainKilla
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[*] posted on 25-6-2009 at 11:49


I only needed MeI/DMS for methylations, and not on any particularly deactivated substrate either - methyl bromide suffices quite well for most reactions, and is far, far cheaper than iodides are, so I just use that, although it is nevertheless useful to know how to make the whole series of methyl/ethyl halides/sulfates... Thanks!
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[*] posted on 25-6-2009 at 12:41


Yes, I have maybe 5 Kg NaBr on hand but relately little KI. I could try MeBr and KSCN, easily enough, in a pressure bottle. MeI is easier to handle, just costs more.

Ammonium thiocyanate is actually cheaper (in lab grades) than KSCN and lower MW so more mols/Kg. I thought it was a lot more expensive. NH4SCN is c.$4/mol, 13 mols/Kg and KSCN is c.$6/mol, 10 mols/Kg.

So, I will make Ba(SCN)2 myself and react it with domethyl sulfate. If methylsulfuric acid will do the job, even better.

My interest in MeI as a result just plummeted.

Nicodem, both Merck and Brauer state that HSCN is a very strong acid, certainly not a weak one. So I must cibcurr with Painkilla and recommend you check that datum/


2 NH4SCN + Ba(OH)2 -> Ba(SCN)2 + 2 NH3 + 2 H2O

Ba(SCN)2 + 2 Me2SO4 -> 2 MeSCN + Ba(MeOSO3)2

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[Edited on 26-6-2009 by Sauron]




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[*] posted on 26-6-2009 at 00:40


PainKilla, you are correct in assuming that a methanolic solution of H2SO4 at equilibrium is more less methylsulfuric acid in methanol. The equilibrium constant of the H2SO4 + MeOH -> MeOSO3H + H2O reaction is 4.4 (Tetrahedron Letters, 28 (1987) 1627-1628).

Quote: Originally posted by Sauron  

Nicodem, both Merck and Brauer state that HSCN is a very strong acid, certainly not a weak one. So I must cibcurr with Painkilla and recommend you check that datum/

I assumed the pKa 4 given in pKa Data Compiled by R. Williams was correct. I now checked Beilstein and there are a few conflicting references with pKa's of 0.85, 4 and 5. If it is truly 0.85 then it is indeed a strong acid, even stronger than methylsulfuric acid of which pKa is from 2 to 3 (various references give a different value, but inside this interval).
Nevertheless, I would never choose to methylate thiocyanates in acidic media. There are simply too many possibilities for side reactions (in acidic in some cases you get the competing N-alkylation as well). Not only is HSCN unstable, even MeSCN can decompose in highly acidic media. When weighting the chances of success between using MeOSO3H and MeOSO3K, I would choose the potassium salt, since even though it is less reactive the reaction is way more predictable. Also, using KSCN gives way better chances of success since it is low melting and you can do the reaction solventless (while ammonium thiocyanate decomposes on heating).

PS: I did not get the point of using Ba(SCN)2. What is all that about?

[Edited on 26/6/2009 by Nicodem]




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[*] posted on 26-6-2009 at 00:57


Oh, I certainly agree, considering the paper I posted before that suggested reactions occured even in dilute acid, I wouldn't expect the reaction to work well... But it might be worth trying anyway since after all, the solution is not really highly acidic - bisulfate would be the most acidic species... If the thiocyanate salts are cheap, it's at least worth a try. After all, would you really have expected this way of making methyl iodide to work? I was skeptical until I tried it, and got a pleasant surprise... But the higher temperatures involved in this case may 'break' the reaction. But still, such experimentation is what makes chemistry so interesting!

BTW, do you have a reference for the pKa of methylsulfuric acid? A brief google search didn't yield any experimental calculations, only some theoretical ones (which implied it was ~0); I didn't realize it was actually that weak an acid.

[Edited on 26-6-2009 by PainKilla]
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[*] posted on 26-6-2009 at 02:24


Quote: Originally posted by PainKilla  

BTW, do you have a reference for the pKa of methylsulfuric acid? A brief google search didn't yield any experimental calculations, only some theoretical ones (which implied it was ~0); I didn't realize it was actually that weak an acid.

I was also surprised, because one would think it would be at least -1 or less, maybe even as strong as H2SO4. I went back to check the abstracts for those references given by Beilstein and it turned out that the pKa from 2 to 3 were actually the acidities of MeOSO3H in various alcohols (for example, in methanol its pKa is 2.16: Ukrainskii Khimicheskii Zhurnal, 44 (1978) 762-763). The only reference that I could find about its aqueous pKa is Canadian Journal of Chemistry, 56 (1978) 2342-2354. Now, this one gives the pKa of MeOSO3H in water as -3.4 which makes more sense, though it is still an "estimated value". Therefore, MeOSO3H appears to be a stronger acid than HSCN.




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[*] posted on 26-6-2009 at 03:24


Thanks, it's nice to see some real data finally!

I personally don't like negative pKa's at all, but according to http://www.springerlink.com/content/r3h103ru0l462mr5/ the pKa of thiocyanic acid is -1, elsewhere it's claimed -1 to -2 for thiocyanic acid. Though, it's a very confusing way to think about things in terms of negative pKa, as opposed to thinking of the entire system (configurational/phase space of solvent + ion molecules) and the relativistic measurement of pH. However, I've only recently started getting a better grasp on physics, so I'm still pretty new to thinking this way - still thinking about this sort of thing helps one practice. Studies that examine the water structure and hydration shells of the ions are good places to start:

According to http://www.pnas.org/content/100/8/4557.abstract?ck=nck and http://www.crystalresearch.com/crt/ab39/465_a.pdf which seems to support the previous study, the thiocyanate ion is but weakly hydrated (it is tempting to imagine that iodide is also not very well hydrated), while sulfate and bisulfate on the other hand, are extremely well hydrated... I had thought initially, because of these reasons, that an explanation for this reaction might be derived for the great thermodynamic favorability of solvating the incoming sulfate molecule as it enters into solution, which would simultaneously destabilize weakly hydrated ions (as iodide) and in that transient process increase their nucleophilicity (while the solvent could also increase the electrophilicity of the methyl group as the methylsulfate is solvated) such that the (wanted methylation) reaction occurs...

I found it quite interesting to see the extreme! solubility of potassium thiocyanate - 220g/100ml @ 20*C (and surely this is even higher at higher temperatures, or ~2.5 molecules of water per anion (which goes along well with the neutron diffraction study) - potassium iodide on the other hand, seems to have about 5 water molecules per iodide under the initial conditions used in the reaction...

Based on the crystal structure of the thiocyanate ion, it gives the impression that very slightly 'acidic' conditions (such that the nitrogen coordinates with a water molecule, although coordination with an ion would also work) would in principle promote S-alkylation, but that also assumes such conditions are attainable in the first place, in the sense that they do not cause side reactions to occur, which seems entirely likely if methylsulfuric acid is indeed stronger than thiocyanic acid. But as you said, use of an ion rather than a proton (as in, using a methylsulfate salt) most definitely ensures this is the case... I really doubt the reaction would work well under such hot and acidic conditions.

But it's fun to think about at the very least; I wonder what other sorts of anions could be tested alongside methylsulfuric acid, in order to test it's capacity as a methylating agent...

I wonder then, if methyl iodide could similarly be synthesized from a methylsulfate salt and an iodide... I imagine some sort of solvent would be necessary, since unlike the thiocyanates neither methylsulfates nor iodides have particularly low melting points - maybe a high-boiling point glycol would work, or perhaps the reaction done in a concentrated aqueous solution... In principle the yields would be even better (unless they really can be made consistently to 80-90% with MeHSO4, in which case going through the hassle of making the salts seems pretty pointless), unless this reaction really does only occur because of some sort of strange solution chemistry.

[Edited on 26-6-2009 by PainKilla]
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[*] posted on 26-6-2009 at 06:08


Nixodem, the point of using Ba(SCN)2 to prepare MeSCN from Me2SO4 is that Merck Index 12th Ed. states this as method for making same - unfortunately without reference.

And ammonium thiocyanate, from which Ba(SCN)2 is made, per Inorg.Sy. attachment above is substantially cheaper than KSCN. Ot's 2/3 the price on a molar basis and 10% less on a 2.5 Kg basis. The discrepency is because MW of NH4SCN is 75 while that of KSCN is 95.

Vogel's brief general statement that alkyl thiocyanates can be made by treating alkyl halide with KSCN in alcohol, is also unreferenced, so I am simply letting the economics drive the decision. I will save MeI for other duties.

Inorg.Syn. cited a number of other preps of Ba(SCN)2, including the original prep by Berzelius. None of them looked as attractive.

I have not found any other lit on prep of MeSCN, so f you run across any, please call them to my attention.




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[*] posted on 26-6-2009 at 07:00


Quote: Originally posted by Sauron  

I have not found any other lit on prep of MeSCN, so f you run across any, please call them to my attention.

Alkylation of KSCN with alkyl idodides, bromides or tosylates is the most standard method for preparing alkyl thiocyanates. I have no idea why would one want to use the barium salt. Maybe it used to be comercialy available in the ancient times. KSCN is well soluble in acetonitrile as solvent and the reaction with primary alkyl bromides/iodides/tosylates is rapid (with methyl iodide or dimethyl sulfate it should be over in a couple of hours at <40°C). Methanol, ethanol, DMF and other solvents where KSCN is soluble can also be used. There are plenty of references in the literature.
Here are a few outputs from Beilstein regarding the MeSCN synthesis:

Methylation of KSCN with (MeO)2SO2:
Walden, Chemische Berichte, 40 (1907) 3215 (and/or 4301?)
Kaufler, Pomeranz, Monatshefte fuer Chemie, 22 (1901) 495.
Rybin, Zil'berman, Trachenko, J. Gen. Chem. USSR (Engl. Transl.), 50 (1980) 1702-1705.

Methylation of KSCN with MeI:
Hassanein, Akelah, Selim, El-Hamshary, Indian Journal of Chemistry, Section B: Organic Chemistry Including Medicinal Chemistry, 29 (1990) 763-765.
Jaworski, International Journal of Chemical Kinetics, 35 (2003) 61-66 (DOI: 10.1002/kin.10105) nonpreparative, kinetics study

Methylation of KSCN with Ca(MeOSO3)2:
Muspratt, Justus Liebigs Annalen der Chemie, 65 (1848) 253,257.
Cahours, Justus Liebigs Annalen der Chemie, 61 (1847) 96.

MeSCN from S-methylisothiourea hydrochloride thermolysis at 160-200°C:
Arndt, Chemische Berichte, 54 (1921) 2242.




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[*] posted on 26-6-2009 at 07:23


Barium thiocyanate is still commercially available (S-A, Alfa Aesar) but not cheap, it is however readily prepared from NH4SCN which is very commercial and substantially cheaper than KSCN.

But thanks, the methyl tosylate is a very good idea. Methyl bromide has the disadvantage of low bp obviously.




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[*] posted on 26-6-2009 at 08:32


Sauron: You suggested a dewar condenser earlier to ensure no MeI escaped uncondensed during the distillation. Surely this would also provide complete condensation of methyl bromide too? Handling may be difficult but you could always set up the other reactants in the recieving flask to prevent this (would possibly save dirtying more glassware too).
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[*] posted on 26-6-2009 at 08:48


Sure it can be done, it's just that methyl tosylate has neither the cost of MeI nor the volatility of MeBr.

Perhaps the most interesting of the refs Nicodem posted is the one abour rearrangement of S-methylisothiourea hydrochloride. As it happems I have a Kg of the hemisulfate and this stuff is inexpensive, so if this reaction is efficient it is a short cheap route to MeSCN. Thiocyanates and thioureas have the same relationship as cyanates and ureas.




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[*] posted on 26-6-2009 at 11:36


Well it depends on what is available to you. As you have plenty of bromide (5kg right?), methanol and sulfuric acid (I'm assuming here, but its not like you do anything by halfs :D) then going by the methyl bromide may be more "convenient". However if you also have equal stock of methyl tosylate (or tosyl chloride) then this would be the most satisfactory way to proceed. The volatility/low bpt of MeBr is easily made negligable as I suggested above, supposing you have dry ice (which you can make from your tank of CO2).
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[*] posted on 26-6-2009 at 12:11


Quote: Originally posted by Sauron  
Barium thiocyanate is still commercially available (S-A, Alfa Aesar) but not cheap, it is however readily prepared from NH4SCN which is very commercial and substantially cheaper than KSCN.
That is because NH4SCN is made commercially in large quantities (technical grade) as an herbicide, particularly by the Dow Chemical Company. It is commonly mixed with other substances like aminotriazole, and used in weedkillers like Amitrole (which I have used myself on my property) and Radoxone 11, for weeds (especially some broadleaf weeds) which may be Roundup-resistant.
See e.g.:
http://www.freepatentsonline.com/4612034.html
http://www.freepatentsonline.com/WO2007147209.html
http://dictionary.infoplease.com/ammonium-thiocyanate
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[*] posted on 26-6-2009 at 12:29


True but I also have a Kg of tosyl chloride at hand, and I have the Org Syn prep of methyl tosylate in the notes to the prep attached, which just happens to be on a 1 Kg basis - how serendipitous. The prep calls for only TsCl, methanol and a little NaOH soln and yields 90%. The workup is somewhat simpler than that Klute used in his Me tosylate thread.

As I have some KSCN I can try this reagent, and also the rearrangement of S-methylisothiouronium hemisulfate, the latter once I obtain the Ber. paper by Arndt.

Tosyl chloride is very cheap, 2 Kg <$90, methyl tosylate from same supplier $240 for 2.5 Kg so this is surely a case where it pays to make your own ester.

These reactions do not demand a hood (though some may be run outside due to stench) so I hope to shortly hsve experimental results and photos to post so I can make some of my critics eat crow.

[Edited on 26-6-2009 by Sauron]

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[*] posted on 26-6-2009 at 20:31


While awaiting the Arndt paper from Refs I pulled up my Annalen archive and fortunately have both Bands concerned (61, 65) These two papers deal with alkylation of KSCN with calcium methyl sulfate Ca(MeOSO3)2. Obviously this salt can be easily prepared from methyl hydrogen sulfate (MeOSO3H, methylsulfuric acid) and calcium carbonate or hydroxide.

This reaction does work, but in the manner of this journal at that time (1847-48) no disclosure of yield is made, at least as far as I can tell with my clumsy German comprehension. (I have only looked at the Calhous paper, pertinent pages 95-96.)

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A quick look at the second paper also did not show any new details, but ib both cases further study is needed.



[Edited on 27-6-2009 by Sauron]

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[*] posted on 27-6-2009 at 01:48


Could it be possible that the calcium salt is used to facilitate an aqueous reaction? I remember reading somewhere that its soluble in aqueus solution, which would provide a good medium as the thiocyanate (as stated before) has excellent solubility in water (remember its an Sn2 reaction (?): rate = k[Ca(MeOSO3)2][SCN-] )
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[*] posted on 27-6-2009 at 02:16


Liebig earlier obtained MeSCN by mixing alcohol, KSCN and H2SO4 which if I understand correctly worked in poor yield. The use of the Ca salt was Calhous' improvement to the Liebig procedure, avoiding oxidation of the product I suppose.

A decent translation will doubtless clarify things further.

Indian J.Chem. is not available online, and neither is J.General Chem. USSR (English). I have requester the two Ber. papers.

[11] Morton, DE 1 295 549, 1966 (J. T. Venerable, J. Miyashiro, A. W. Seiling).
ï[12] Bayer, DE 1 183 903, 1965 (K. Goliasch, E. Grigat, R. Pütter).

These two German patent citations are from Ullmann's and the accompanying test states that methyl thiocyanate or isothiocyanate can be produced selectively by varying the solvent, temperature and thiocyanate concentration in the reaction of a methyl halide with a thiocyanate salt. The most common solvents are (as Nicodem stated) water, ethanol, and DMF. I am sure that Morton, and Bayer, would have taken out corresponding US patents to these so I will root aroundd for those rather than have to fight with the German patent texts. I think now we are getting down to the nitty gritty.

[Edited on 27-6-2009 by Sauron]




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[*] posted on 27-6-2009 at 05:06


I've been waltzing with patent SEs for hours and have gotten nowhere. Help!

Walden, Chemische Berichte, 40 (1907) 3215

I haveobtained this paper and in addition to the prep of MeSCN from DMS and KSCN which proceeds in excellent yield the paper also details

DMS + KCN -> 2 MeCN

DMS + KNO2 -> MeNO2 + MeOSO3K

Look again. Dimethyl sulfate reacts with 2 mols of the strong base KCN which implies that potassium methyl sulfate reacts with one mol. You may mot want to make acetobitrile from DME but from methanol and H2SO4 and potassium carbonate + KCN why not?

Potassium nitrite apparently will not react with potassium methyl sulfate but will with DMS. Therefor it may give nitromethane with ,ethylsulfuric acid, methyl tosylate - it is no surprise MeI would work but the others are cheaper.



[Edited on 28-6-2009 by Sauron]




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[*] posted on 28-6-2009 at 00:57


Sadly the German patents cae to little. The Moton Intl one concerns pressure reactions of MeCl with various inorganic thiocyanates, The Bayer patent, mercaptans amd cyanogen halides.

The Arndt paper does state that S-methylisothiorea hydrochloride thermolyzes efficiently at 160-200 C to MeSCN and NH4Cl, details of preparing the hydrochloride from the hemisulfate are provided.




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[*] posted on 28-6-2009 at 21:21
For Formatik


As per PM

This post may be deleted once I am sure formatic downloaded the paper.

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[Edited on 29-6-2009 by Sauron]




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[*] posted on 29-6-2009 at 01:37


The Arndt paper concerns a new (at that tine) reagent for generation of pure MeSH by reacting S-methylisothiourea hemisulfate with aq NaOH. This I am not interested in per se, but Arndy also observed that the hydrochloride salt thrermolizes betweeen 160 and 200 C to MeSCN and NH4Cl along with a little MeSH minor side product.

There is an Org,Syn. prep vased on Arndt's procedure. Two mols thiourea amd 70 ml water are reacted with 1,1 mol (10% excess) dimethyl sulfate; the first phase of the reaction usually starts without exterbak heating and as it proceeds to relux under its own exotherm may require cooling ro control. When this reaction ceases the methylation is half over and the DMS is now methylsulfuric acid. A futher hour;s reflux with external heating is required to drive methylation to completion. Yield is very high.

This tells me that DMS can be replaced entirely with preformed methyl hydrogen sulfate (methylsulfuric acid) made from methanp; and conc H2SO4. Just use 2 mols thuouewa to 2.2 mols MeOSO3H and reflux from the start, as there will be no exotherm. Otherwise procedure and workup will remain same. Hopefully yiels as well. Cheaper and safer!

To get to MeSCN, Arndt converts the hemisulfate to base and the base ti hydrochloride, I await translation for accurate details. From a kilo of the S-methylthiourea hydrochloride O estimate 500-600 g MeSCN will distill over leaving 600-500 g in the post mostly ammonium chloride, A scrubber is used to absorb the small amoun of methanethiol that forms.

(MeSC(=NH)-NH2).HCl

Easy to see how it breaks into NH4Cl and MeSCN

The S-meythylisothiourea hemisulfate is also used to prepare aminoguanidine bicarbinate without going through nitroguanidine - which ought to make the energetics lot happy.

From Org Syn:

"W. W. Hartman and Ross Philips have submitted a procedure suitable for the preparation of aminoguanidine bicarbonate on a larger scale. The sulfates of methylisothiourea and of hydrazine are allowed to react with the evolution of methyl mercaptan. In a 30-gal. crock are placed 10 l. of water and 5760 g. (20 moles) of methylisothiourea sulfate.2 In a 22-l. flask, 5.2 kg. (40 moles) of hydrazine sulfate3 is stirred with 12 l. of water, and 40% sodium hydroxide is added until all the hydrazine sulfate has dissolved and the solution is just neutral to Congo paper. The exact amount of alkali is noted and a duplicate amount added. The hydrazine solution is then added to the 30-gal. crock with stirring, as fast as possible, without allowing the foam to overflow the crock. The mixing is done out-of-doors, or in an efficient hood, since large volumes of methyl mercaptan are evolved. If the reaction is carried out on a smaller scale in 12- or 22-l. flasks, using appropriate amounts of material, the methyl mercaptan evolved may be absorbed in cold sodium hydroxide solution and isolated if desired. The solution is stirred until evolution of mercaptan stops, and then a few liters of water are distilled off under reduced pressure to free the solution entirely from mercaptan. The residual liquor is chilled in a crock, and a crop of hydrated sodium sulfate is filtered off, washed with ice water, and discarded. The filtrate is warmed to 20–25°, 25 ml. of glacial acetic acid is added, then 4 kg. of sodium bicarbonate, and the solution is stirred vigorously for 5 minutes and thereafter occasionally during an hour, or until the precipitate no longer increases. The precipitate is filtered with suction and washed with ice water and then with methanol, and is dried at a temperature not above 60–70°. The yield is 3760 g. (69% of the theoretical amount). Hydrazine sulfate may be recovered from the final filtrate, if the filtrate is strongly acidified with sulfuric acid and allowed to cool."



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[Edited on 29-6-2009 by Sauron]




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[*] posted on 29-6-2009 at 23:25


Here is a 1901 paper citrf by Nicodem whicg describes the reactions of MeI as well as DMS with KSCN.

It teaches that DMS reacts even at room temp with KSCN and that cooling may be required to control the reaction. I would expect them yo mean an equimolar reaction, because the reactivity of the methylsulfuric acid potassium salt will be much less, and if it reacts at all will certainly need heating.

Yn any case the yield is quantitative (again I am sure they mean 1:1).

Reactions of the two methylating agets with nitrite and cyanide are also given.

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[*] posted on 30-6-2009 at 14:15


They say the sulfate isn't suitable due to solubility. So when starting from the salts, they convert the sulfate to the chloride and then go from there to get the base.

S-Methyl-isothiourea: For the isolation of the base from its salts, it comes down to liberating them not in water, but in a more easily volatile solvent, namely methyl alcohol. The sulfate is not suitable though due to its insolubility in methyl alcohol, but the aforementioned hydrochloride, which can easily be obtained from the sulfate by conversion with barium chloride; its melting point we have found as in accordance with Dixon to be 59-60º, so conspicuously lower than that of the free base. This hydrochloride is solubilized in a small amount of absolute methyl alcohol, under ice cooling put into a solution of the calculated amount of sodium in a small amount of methyl alcohol, it is then filtered off from sodium chloride, and quickly evaporated in a vacuum desiccator. The residue is taken up in acetone, filtered and then treated as above. One obtains a compound identical to that described above.

0.1758 g subst.: 0.1712 g CO2, 0.1028g H2O, 0.4588 g BaSO4.
C2H6N2S.. Calc.: C 26.6, H 6.7, S 35.6.
Found: » 26.6, » 6.5, » 35.8
.

Base "treated as above": One can purify it again by taking it up in acetone at room temperature, filtering off unsolubilized dicyandiamide and cooling with ice. Here usually a part precipitates out in colorless, shiny leaflets, even more if ice cold petroleum ether is gradually added.

[Edited on 30-6-2009 by Formatik]
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[*] posted on 30-6-2009 at 16:06
Formatik's Translation of Arndt (Above)


Thanks. I was assuming incorrectly thyat one needed to go sulfate -> base -> hydrochloride. Not the case. All I need is to metathesize the sulfate in waterwith BaCl2 in same solvent, BaSO4 will ppt amd be filtered off, water is then rotavaped off.

The "sulfate" is hemisulfate, so I need 1 mol BaCl2 per mol S-methylisothiourea (C2H6N2S)2.H2SO4 to get BaSO4 + 2 C2H6N2S.HCl

That "sodium in methanol amd then filter off NaCl" step is clearly the liberattion of the base from the HCl salt with NaOMe. But I need not do that.

Dicyandiamide is the decomposition product of S-methylsiothiourea from moisture. Also known as cyanoguanidine, and is the dimer of cyanamide.

Thanks, formatik, alles in ordnung!

[Edited on 1-7-2009 by Sauron]




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[*] posted on 5-7-2009 at 19:24


I've finished the scale up to 400mmol. I'll post in the pre-publication section.
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