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Author: Subject: Pseudohalogen oxyacids
Pyrovus
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[*] posted on 16-2-2004 at 21:29
Pseudohalogen oxyacids


Cyanide can be oxidised to cyanate, for instance, but is it possible to oxidise it to higher states? For instance, chlorine forms the oxy-acids HOCl, HClO2, HClO3 and HClO4. While I've never heard of analogous compounds (apart from cyanate) for pseudohalogens, perhaps it is conceivable that they could exist, with structures such as
..........O
........|.....|
HO-C.-.N
........|.....|
..........O
[sorry about the dodgy ASCII diagram. Ignore the dots]


for HCNO3? If these compounds do exist, how stable would they be, and would they show similar oxidising power to that of the regular halogen oxyacids? Also, are pseudohalogens capable of forming polyhalide ions, analogous to I3[-], IBr2[-] etc?

[Edited on 17-2-2004 by Pyrovus]
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guaguanco
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[*] posted on 17-2-2004 at 00:13


Count the electrons around the carbon and nitrogen atoms. The only one that's even remotely legal by the octet rule would be H-O-CNO.
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[*] posted on 17-2-2004 at 11:54


Perhaps I'm missing something, but that structure looks legitimate... Not saying with any certainty that this is a potential oxy-acid for Cyanides, but that the Lewis structure is OK.

Nitrogen lacks 3 electrons. Thus, it is single-bonded to two Oxygens and one Carbon, thus filling its octet. The Carbon is short 4 electrons, and it is single-bonded to 3 Oxygens and a Nitrogen.

I did find this equation about the formation of the Cyanate ion by oxidation of the Cyanide ion, if it is at all helpful to you...

CN- + H2O2 --> NCO- + H2O

This can be done with with H2O2, Caros Acid, and Sodium Percarbonate ( Oxy-clean). The Cyanide is subsequently hydrolyzed to Sodium Carbonate and Ammonia. Notice where the Oxygen is going, though-- it is not breaking the CN bond, but instead attaching to the C itself and leaving the triple bond alone.

This might not be the route to go. However, let's not forget about the Isocyanate ion. The structure of that is -N=C=O . Don't know if that will be of much use to you either, but the triple bond is broken.
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guaguanco
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[*] posted on 17-2-2004 at 14:50


Ah, I see, he wants to form multiple 3-member rings. I assumed he was just hanging the oxygen atoms on the nitrogen.
I don't think any of these compounds exist; I've never run across reference to them in the literature I have access to.
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[*] posted on 17-2-2004 at 16:34


yes, basically it's a derivative of
......OR
R1..N
......OR

except that the 'R's are bonding to 'R1'.
Problem is, I am not awaer of a R1-N(OR)2 structure, I always thought that hydroxylamine H2NOH is as far as you can get. Never heard of a 3-valency nitrogen with two O-R's and one R, especially bonded to itself (R1). Come to think of it, even cyclopropanes are unstable and reactive, and I should think that the hetero ring cyclo [N-O-C-O] ring is even more unstable. H2C(OH2) (formaldehyde in aqueous solution) is already not isolatable in it's pure state, it condensates to H2CO.
On the other hand, I am sure compounds such as C(OCH3)4 exist - so what can I say?
If you synthesise it you get the MadScientist's award for sure!




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Pyrovus
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[*] posted on 19-2-2004 at 22:59


I've been thinking over this whole thing some more, and have had a few more thoughts. Cyanides form interhalogen compounds such as ClCN, and they should also form compounds in which the cyanide ion is oxidised further, as this doesn't involve forming those hideous heterocylic rings present in the proposed oxyacid. Indeed, some quick calculations showed that cyanogen should react exothermically with fluorine to give cyanogen pentafluoride ( (difluoro amino) trifluoro methane):
(CN)2 + 5F2 -> 2 CF3NF2 which gives delta H of about -492 kJ/mol. Now halogen halides hydrolyse to give the hydrogen halide of the negative halogen and the corresponding oxyacid of the positive one. I very much doubt that this would happen with this compound, but as this is a thought exercise we'll assume for not that it does, along the lines of
CF3NF2 + 5H2O -> C (OH)3 N(OH)2 +5HF. Quite a horrible structure with far too many hydroxy groups, but it is easy to see how by dehydrating this fictitious molecule we can arrive at a more reasonable structure for HCNO3:
C (OH)3 N(OH)2 -2H2O -> O=N.COOH!
This could presumably be prepared by something along these lines:
HCOOH + HNO2 -> ONCOOH + H2O
And percyanic acid, HCNO4, could be prepared by nitrating formic acid:
HCOOH + HNO3 -> O2NCOOH +H2O!
These compounds certainally look plausible to me, much more so than my original proposal. The question now is how do they compare in properties to their halogen oxyacid counterparts. They certainly would behave as oxidising agents, but I'm not terribly sure it would be that easy to reduce them to cyanide. I suspect the nitrogen in the molecule would prefer to form N2, rather than forming cyanide, but I may be wrong.
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smile.gif posted on 25-2-2004 at 19:23


what about HONCO? a oxime version of isocyanic acid. (replace the H-N= with H-O-N=)
it might form explosive salts like how fulminic acid( H-O-N=C: ) does.




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[*] posted on 10-3-2004 at 17:40


When it comes to pseudohalogens nitrogen compounds do the best.

Aside from the common nitrous acid, HNO2 and nitric acid, HNO3 there is also a pernitric acid, HNO4 although the nitrogen in that is not in a higher oxidation state, it really is a per-acid in terms of a -O-O- linkage. Also there is a hyponitrous acid HNO but it is fairly unstable.

In addition there are what would be considered the innerhalogen compounds such as BrNO3 BrNO2 BrNO along with the same for many of the other halogens.

Then of course there are always the oxides of nitrogen of which most are acidic. Many of which afford compounds with the halogens as seen above.

It's just weird that I never thought of the similarities between nitrogen and the halogens in terms of oxoacids before.




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