I need to prepare some cyanogen bromide and the preparation calls for dissolving sodium cyanide in a large amount of water first. I have dealt with
sodium cyanide (and I have quite a bit of it) but I have always used it in procedures I feel comfortable with and handled it in ways I feel
comfortable with. One of the main safety protocols I use is to always keep the sodium cyanide away from water because it decomposes to hydrogen
cyanide in water. I even have to open my containers of sodium cyanide outside because over time sodium cyanide decomposes to hydrogen cyanide from the
moisture in the air. Yet now I am being told to dump a lot of it in water? This goes against all my instincts.
Does anyone have experience dissolving sodium cyanide in water? I know NaCN is basic and that hydrogen cyanide is most quickly released in acidic
environments, but still this just makes me shiver a little.Methyl.Magic - 10-6-2020 at 17:34
No need to worry, you can add tiny crystal of NaOH. The main problem of cyanide are during work-up if you add bronsted acid and end up with the
auswchwitz effect !!! But be careful IIRC cyanogen bromid is more toxic than HCN !
Why do you need water for BrCN ? BEcause it decomposes in water : (CN)Br + H2O → HCN + HOBrGerman - 10-6-2020 at 17:44
Why do you need water for BrCN ? BEcause it decomposes in water : (CN)Br + H2O → HCN + HOBr
Most the synthesis' of cyanogen bromide require water all around (in starting reagent and addition reagent):
To the two-neck two-liter flask which is cooled with ice-salt bath slowly 1000 g (320 ml) bromine and 150 ml of water is added. To this bromine and
water mixture with vigorous stirring at a rate of 1 drop per second solution containing 420 g of sodium cyanide (sodium cyanide us used in 30% excess
than the theoretical) in 850 ml of water is slowly added. The temperature of the reaction mixture should be below 20 °C. Local excess of cyanide in
the reaction flask should be avoided as this could lead to the formation of side products such as (CN)x. The last 150 ml solution of sodium cyanide is
half diluted with additional water. During addition of sodium cyanide vigorous stirring should be applied. The reaction is complete and addition of
sodium cyanide should be stopped when the color turns to brow (brownish) and no longer disappears with addition of sodium cyanide. Cyanogen bromide is
slowly removed from the reaction flask by gently warming it. Cyanogen bromide is dried by passing vapors trough calcium chloride during distillation
process and collected in 750-1000 ml flask which is immersed in salt-ice cooled bath.
The product is a snow-white crystals (yield of cyanogen bromide is 590 g, or 90% of theory based on the bromine); in this form it can be stored for a
long time. Cyanogen bromide with brown color, can not be stored. When storing cyanogen bromide the bottle is necessary to close with cork stopper and
seal it well with paraffin.
A process could be scaled up according to following procedure (Org. Syn. Coll. 2. 150, 1948). 500 g of bromine in 50 ml of water was mixed with a
solution containing 170 g sodium cyanide (0.125 times more than necessary by the equation) in 1200 ml of water (2.5 times greater than in the method
above). The yield of cyanogen bromide is 73-85%.
Cyanogen bromide crystallizes in the form of colorless needles. Melting point 52° C; boiling point 61.4° C. Cyanogen bromide is soluble in ether,
alcohol and water. Extremely toxic so all synthesis should be performed in well ventilated hood! Furthermore it is recommended to wear a mask.German - 10-6-2020 at 17:47
This site actually has nice pictures to go with the standard synthesis listed above:
hXXps://thosci.com/synthesis-of-cyanogen-bromide/
(I don't know if this site has rules against links... I will go spend 10 min reading them again and edit if needed)Methyl.Magic - 10-6-2020 at 17:54
You are making cyanogen bromide and are afraind of dissolved NaCN ? for me its the opposite....
After the reaction and isolation of the compound dont forget to kill every trace of cyanide. With H2O2 for example...German - 10-6-2020 at 18:00
After the reaction and isolation of the compound dont forget to kill every trace of cyanide. With H2O2 for example...
Yes I spent quite some time learning the best neutralizing procedures of cyanide. Sodium hypochlorite solution at pH 9-10 is the best way to do it.
The right pH means a lot too.karlos³ - 10-6-2020 at 18:07
Alkali cyanides are usually "stabilised", which means they contain a slight excess of the respective hydroxide, pH 8 is usual, exactly to take care of
this fear of yours.
You can prepare the BrCN in methanol as well, probably other alcohols would work fine.
I wouldn't use that stuff made in-situ for your planned reaction though, you probably have to purify it consequently.
But this has its advantages, this way you can, as soon as you feel comfortable enough to work with it, some more in advance and bottle that up for
later, so you don't have to run the dangerous reaction plenty of times, but need just a few reactions instead and then can simply use your stock made
before.German - 10-6-2020 at 18:15
I actually have tried for the last 2 months to avoid having to use cyanogen bromide. I have been trying to use 2,2,2-trichloroethyl chloroformate
which is suppose to be much high yielding than cyanogen bromide for N-demethylation but no matter how much I purify and dehydrate my substance and
solvent the trichloroethyl chloroformate ALWAYS produces HCl which drives my substrate out of solution and depronates the nitrogen. Even with lots of
DMAP and pyridine I can't keep the hcl production from over whelming the reaction.... So now I have to use the cyanogen bromide I wanted to avoid.wg48temp9 - 11-6-2020 at 02:48
Does anyone have experience dissolving sodium cyanide in water? I know NaCN is basic and that hydrogen cyanide is most quickly released in acidic
environments, but still this just makes me shiver a little.
Hydrogen cyanide is a weal acid so sodium cyanide will be hydrolysed to some extent. In fact its a much weaker acid than carbonic acid so solution
open to the air produce hydrogen cyanide principally via displacement by CO2. mackolol - 11-6-2020 at 03:28
The procedure calls for dissolving NaCN in large amount of water. Even if there will be some hydrolisis I believe that most if not all of the HCN will
remain dissolved in water too as it is soluble. Even though, do the procedure outside. You can handle large amounts of dangerous gases safely outside.
Wear your gasmask, it should absorb the tiny amount evolved.
[Edited on 11-6-2020 by mackolol]ChemichaelRXN - 11-6-2020 at 08:45
I found this synthesis interesting as well...related topic. (I dont make this because chlorine is illegal in Canada.)
Preparation of cyanogen iodide
To the round bottom flask fitted with dropping funnel containing 100 ml 5% aqueous solution of sodium cyanide, stirrer, inlet and outlet tubes for gas
12 g iodine and 20 g water are placed. The reaction mixture is stirred and 50 ml of the sodium cyanide solution are added slowly drop-wise. The NaI
formed in the reaction dissolves the iodine and the liquid in the flask is continuously decolorized. When the 50 ml sodium cyanide solution have been
added, a slow current of chlorine is passed into the flask, and while continuing to stir the liquid the other 50 ml cyanide solution are added from
the dropping-funnel at such a rate that there is always an excess of iodine present. The current of chlorine is then stopped and a little more sodium
cyanide is added. Cyanogen iodide is then extracted with ether, the ethereal extract dried over calcium chloride and filtered. The ether distilled off
and a crystalline residue of cyanogen iodide collected yielding 80-85% of theoretical. Cyanogen iodide forms white crystals which gradually decompose.
It melts at 140-146° C. Cyanogen iodide dissolves slightly in cold water, soluble in alcohol and ether.
The war gases chemistry and analysis, by M. Sartory, 193-194, 1939.clearly_not_atara - 11-6-2020 at 10:11
Even with lots of DMAP and pyridine I can't keep the hcl production from over whelming the reaction.... So now I have to use the cyanogen bromide I
wanted to avoid.
One thing you should keep in mind is that BrCN will react to give hydrogen bromide just as EtOCOCl will react to give hydrogen chloride. Both are acyl
halides of a hypothetical but unstable acid.
A more likely problem is that neither DMAP (pKb = 4.3) nor pyridine (pKb = 8.75) is strong enough to deprotonate a secondary amine (Et2NH pKb = 2.9)
or tertiary amine (Et3N pKb = 3.25). You would need to use a stronger base.karlos³ - 11-6-2020 at 10:17
In the syntheses I've seen BrCN being used, they take care of the HBr byproduct simply with sodium acetate, and this keeps the otherwise as salt
precipitating substrate available for the reaction.
But I guess the stronger base in that case is the substrate not sodium acetate, but still only a pkb of IIRC 4,64?
Its not a tertiary but a secondary amine if that explains anything... to me, it doesn't?
