woelen
Super Administrator
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Registered: 20-8-2005
Location: Netherlands
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Nitrosyl bromide??
I did a peculiar observation when I added NaNO2 to a strongly acidified solution of NaBr (e.g. saturated solution of NaBr in 20% H2SO4). Even more
spectacular results are obtained when NaNO2 is added to 40% HBr.
When NaNO2 is added to such a solution, then the liquid becomes VERY dark brown, more like dark chocolate. Not at all like the well-known dark red
color of bromine. The color is much darker, but it also is much more chocolate-like.
I know the reaction of NaNO2 with concentrated HCl. If that reaction is carried out, then the liquid becomes deep orange, due to formation of ONCl
(nitrosyl chloride):
NO2(-) + 2HCl --> H2O + ONCl + Cl(-)
Could it be that the dark brown material is ONBr, formed in a similar reaction:
NO2(-) + 2HBr --> H2O + ONBr + Br(-) ?
When I add a little water to the dark brown liquid, then the liquid looses its color. It becomes light orange/yellow, just like the well known color
of dilute bromine in water. It looks like the dark brown material hydrolyses and is converted to colorless material and as a side reaction a small
amount of bromide is oxidized to bromine.
I read about the existence of nitrosyl bromide, but this seems to be very unstable. My experiments with this show otherwise.
Here follows the page with the experiments, which I did some time ago, but I still have the impression that the explanation is not really good.
http://woelen.homescience.net/science/chem/riddles/nitrite+h...
If one of you has access to NaNO2 (or KNO2) and a bromide, then it is definitely worth the effort to do this simple but surprising experiment.
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not_important
International Hazard
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Registered: 21-7-2006
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The information I have indicates that pure dry NOBr at STP will disassociate to the extent of about 7-9 percent. It also says that it will quickly
react with water to give HNO2 and HBr.
However there were a number of studies done on amino acids, substituted carboxylic acids, and various optically active compounds where amino acids
were converted into their bromo equivalents by reaction a solution of NOBr in ether with one of the amino acid in 20% HCl, and possibly other
concentrations as well.
The normal means of producing NOBr were
2 NO + Br2 => 2 NOBr
2 NO2 + KBr => KNO3 + NOBr ( Inorg Synth 11, 195-200 )
KBr was used because it is easy to dry than NaBr, and they were also making NOCl and NOF by the same method, otherwise there's no reason NaBr could
not be used.
The descriptions of NOBr indicate that its colour is difficult to distinguish from NO2 using just the eye, instrumentation is just about a
requirement.
The reaction with alpha amino acids was of interest, as it seemed to be a reasonable route to alpha-bromo acids if phosphorous was not readily
attainable. The readily accessible and fairly inexpensive glycine and alanine are likely substrates, leading to reactions as the Reformatsky for
extending carbon chains.
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