walruslover69
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Cyanide synthesis by thermal decomposition of Thiocyanate
I recently came across the knowledge that sodium thiocyanate and potassium thiocyanate decompose at 307C and 500C respectively. I could not find a
reliable source but I do not see any other logical decomposition product besides elemental sulfur and the metal cyanide.
NaSCN ---> S + NaCN
I have previous synthesized sodium cyanide by the reduction of cyanuric acid with carbon and hydroxide but it is a pain to preform for a product that
is challenging to purify in large quantities.
If it is possibly to simply decompose sodium thiocyanate at 307C to sodium cyanide with limited byproducts that would be a much easier method for the
amateur chemist. Since Sodium thiocyanide is extremely cheap.
I am planning on purchasing some to experiment with, but does anyone have any reason why this method would not work? I was not able to find very much
on the top at all.
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Boffis
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On a preparative scale you will need to add something to take up the sulphur otherwise they simply recombine on cooling. Iron fillings are the
generally preferred material for this. The procedure is described in the book on Cyanogen compounds that as been posted before and may even be in the
SM library.
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clearly_not_atara
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It says here that solid carbon nitrides are produced:
https://www.researchgate.net/profile/Alexander_Semencha2/pub...
Given that the other products are CS2 and Na2S, the presumptive reaction looks like:
4 NaSCN >> C3N4 + Na2S + Na2CS3 (trithiocarbonate)
[Edited on 04-20-1969 by clearly_not_atara]
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walruslover69
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Boffis- I assume the sulfur would boil/distill off at that temperature.
That is very interesting that you actually get carbon nitride nanoparticles.
after doing some more research I believe there might be a way to generate HCN by the action of concentrated sulfuric acid on sodium thiocyanate to
generate the unstable thiocyanic acid.
https://sci-hub.se/https://pubs.acs.org/doi/abs/10.1021/ja01...
3HCNS ===> HCN + H2C2N2S3
The HCN could then be Bubbled through a NaOH solution. This could be promising as I do not know of another method for generating cyanides without
temperatures greater than 200-300C
[Edited on 11-11-2018 by walruslover69]
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S.C. Wack
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> I do not know of another method for generating cyanides without temperatures greater than 200-300C
It's not clear if you are only focused on SCN-, because there's ferrocyanide and sulfuric acid...it is said that Erlenmeyer said (haven't looked it
up) something about quantitative reaction of aq. KSCN with permanganate/sulfuric acid (it's a nice balanced reaction to KCN, water, and sulfates,
theoretically), but I'm unsure of the ease of quantitative recovery of KCN or HCN there.
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fusso
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Quote: Originally posted by S.C. Wack  | | It's not clear if you are only focused on SCN-, because there's ferrocyanide and sulfuric acid...it is said that Erlenmeyer said (haven't looked it
up) something about quantitative reaction of aq. KSCN with permanganate/sulfuric acid (it's a nice balanced reaction to KCN, water, and sulfates,
theoretically), but I'm unsure of the ease of quantitative recovery of KCN or HCN there. |
How could SCN- be
reduced to CN- in an oxidizing solution?
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S.C. Wack
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I would think that there is a way of doing things that makes chemistry an art; perhaps the exact theoretical quantity of dilute permanganate is added
dropwise to cold stirred KSCN in the needed or excess quantity of sulfuric acid.
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Tsjerk
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Ferrocyanide and sulfuric gives a quantitative yield of cyanide, the drawback is that it goes through HCN as intermediate.
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walruslover69
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I wasn't familiar with the ferrocyanide and sulfuric acid process, but it looks very promising, the key seems to be the use of dilute sulfuric acid
[Edited on 11-11-2018 by walruslover69]
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Boffis
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In the characterization of organic and inorganic compounds fused KSCN (I suspect NaSCN works too but not the ammonium salt) acts as a "nascent"
sulphur donor. At just above the melting point of the K salt the will liberate H2S from the inert OH in minerals for instance. So the S in KSCN is
reactive but not readily disengaged but simply heating, as several posters have already alluded to. So you need something to strip away the sulphur
before you end up with all the other side products. This process is described in the literature but it appears that neither of the books is in the SM
Library but as they are way out of copyright I can post them if you wish.
By the way I would always stick to the "hot melt" type cyanide processes rather than the "gas" type, they are much safer and more reliable.
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walruslover69
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Thanks for the suggestion, I bout about a pound of thiocyanate last night and am planning on trying out a few methods. I like you your idea for the
single displacement reaction to form a sulphide. I crunched some of the thermo numbers and the reduction of thiocyanates with metals isnt as favored
as I thought it would be. Here are the values for some common metals
Zn+KSCN---> KCN+ZnS ∆G=-120.1 Kj/mol
Pb+KSCN---> KCN Pb ∆G= -18.5 Kj/mol
Fe+KSCN---> KCN + FeS ∆G= -22.3 Kj/mol
This was using values with sold KSCN, I assume the values would be only slightly less favorable with molten KSCN due to the entropy penalty, but I
don't think there would be a huge difference, maybe 20 kj/mol or just enough to make the lead and Iron unfavorable.
I originally thought about using aluminum, but aluminum sulphide reacts with water or moist air to form hydrogen sulphide. I would preferably like to
avoid generating large amounts of H2S as I extract my cyanide.
It seems like this could be a very clean reactions with the products being easly seperable and minimal side products generated.
[Edited on 13-11-2018 by walruslover69]
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fusso
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@Walrus what about using C? CS2 has a low bp @46.24C. If that's feasible then one can make 2 useful chemicals in 1 rxn.
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walruslover69
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It looks like it doesn't become favorable until about 600C. at which point the KSCN would have already decomposed. That kind of surprised me. I
originally wanted to stay away from the carbon reduction because when I previously reduced sodium cyanate to cyanide with carbon, filtering out all of
the small carbon particles proved to be alittle harder than i expected with just coffee filters, but that isn't a huge problem if it worked well.
I agree that it would be great if i could make both at the same time. CS2 has been on my list for awhile.
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clearly_not_atara
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| Quote: | | I do not know of another method for generating cyanides without temperatures greater than 200-300C |
Alkaline decomposition of formaldoxime. No HCN, no high temperature/pressure.
https://www.sciencemadness.org/whisper/viewthread.php?tid=23...
[Edited on 13-11-2018 by clearly_not_atara]
[Edited on 04-20-1969 by clearly_not_atara]
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fusso
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Is there a mechanism for this rxn? I can't find it elsewhere.
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clearly_not_atara
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Consider e.g.
https://link.springer.com/article/10.1007%2FBF01106685?LI=tr...
https://pubs.acs.org/doi/abs/10.1021/ja01557a033?journalCode...
https://cloudfront.escholarship.org/dist/prd/content/qt3dz9b...
Oximes dehydrate into nitriles in strong alkali, but the nitriles themselves are hydrolysed rapidly except for CN-. I don't know the mechanism but I'd
guess that deprotonation at a carbon atom lets you arrow-push to lose a hydroxide anion.
[Edited on 04-20-1969 by clearly_not_atara]
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Tsjerk
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Very nice find! In the article they isolate the pure formaldoxime, but as far as I see it should be doable by just reacting free hydroxylamine and
formaldehyde in water and adding alkali hydroxide right? Or hydroxylamine hydrochloride with alkali base and formaldehyde to obtain an equimolar mix
of cyanide and chloride.
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clearly_not_atara
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The oxime formation kinetics should probably be considered, but I see no reason why in principle the formation and dehydration could not be performed
in situ. The reaction of alkali and unreacted formaldehyde will likely be of the Cannizzarro variety, so ensuring the formaldehyde is consumed before
adding NaOH seems prudent.
Isolating formaldoxime may be somewhat easier than isolating cyanide, though. Most cyanide salts are either highly soluble or highly insoluble, the
former being difficult to separate from formed NaCl and the latter not being useful as a source of CN- (except Zn(CN)2 in the Gatterman-Koch
reaction). Ba(CN)2 can be precipitated at low temperature and relatively pure sodium cyanide is then obtained by metathesis with Na2CO3 (cyanides
usually contain carbonate impurities anyway).
[Edited on 15-11-2018 by clearly_not_atara]
[Edited on 04-20-1969 by clearly_not_atara]
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Boffis
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As the thread seems to be wondering off coarse here is a excerpt from a book on the subject of the de-sulphurization of thiocyanates to cyanides.
Check out the top of page 210.
Attachment: Chemistry of cyanogen & cyanide p209-210.pdf (98kB)
This file has been downloaded 328 times
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