HCN question

[rvdzero]

hey i just had a thought and was wondering if it would be possible to remove HCN from benzaldehyde without distillation. benza is slightly soluble in water (i read 300mg / 100ml) and HCN is completely miscible in water. so could i add some h20 to the benz/hcn mixture to form two layers?

[big314mp]

It would be just like a liquid-liquid extraction. You can also add some sodium bicarbonate to the water, to neutralize the HCN, so you can get a better extraction.

[John Cuthber]

HCN may well be completely miscible with benzaldehyde in which case there's nothing much to force it into the water layer. It might work, or it might not.

Sodium bicarbonate isn't a strong enough base to deprotonate HCN- it's a very weak acid.

Sodium cyanide (if it's formed will react with benzaldehyde to form the cyanohydrin.

[rvdzero]

ah true. cheers. so basically id need to find something that will force the hcn into the water layer without any side reactions with benzaldehyde?

 

would use of a different solvent help? (like one that benza is soluble in, but hcn isnt?)

 

or is distillation the only way?

 

thanks for the help!

[YT2095]

how about "locking up" the cyanide by complexing it with silver?

the product being insoluble silver cyanide.

[rvdzero]

ah true, cheers! how would i do that? would any silver compound do? or does it have to be a solution of dissolved Ag+ ? would the reaction just go, or would i have to apply heat? (sorry for all the q's, had a look for info but couldnt find much. ill keep looking though)

[YT2095]

most reasonably soluble silver salts and even the metal itself will work, just make sure the leaving group isn`t going to affect your benzaldehyde, such as silver carbonate, but I would make elemental silver from a salt and use that.

[John Cuthber]

I'd like to see you balance the equation using silver metal. Don't forget that benzaldehyde is a classic example of auto-oxidation in air.

[rvdzero]

well, if i use silver nitrate as the silver salt, i could use a more reactive metal (aluminium) to precipitate elemental silver.

 

so, 3 AgNO3 + Al3+ => 3Ag+ + Al(NO3)3

 

(ive read that oxidation of benzaldehyde yields benzoic acid, im not sure about how auto oxidation plays a part here??)

 

HCN + Ag+ => H+ + AgCN

 

since the formation of AgCN liberates hydrogen gas, would i still need to include the bezaldehyde in the equation?

[John Cuthber]

I was asking YT2095 to explain how you could use elemental silver to strip HCN from the mixture. I don't think it's possible.

Silver will react with HCN to produce AgCN, but you need air to oxidise the silver or the reaction is far too slow. Air oxidises the aldehyde quite quickly so if there were enough air to get the silver to react the air would trash the benzaldehyde.

 

Distillation looks like a much easier option. Another possibillity would be to treat the mixture with precipitated "Fe(OH)3" and trap the cyanide as ferricyanide then steam distill the aldehyde out.

[YT2095]

what is there to explain? it`s perfectly straight forwards.

a silver salt (nitrate or halide) reacted with Hydroquinone, Metol, Amidol, Phenidone etc... to give micron sized silver particles, then you simply add this to your HCN.

OR as I said, you could use silver carbonate.

[John Cuthber]

Silver is a noble metal.

It doesn't react with acids (except oxidising ones like nitric).

The complexation of Ag+ is just strong enough to offset this, so silver just about dissolves in HCN, but only if there's enough HCH present to form [Ag(CN)2]+ ions.

In this case you simply don't have a lot of cyanide present and most of it will be trapped as a cyanohydrin.

To get the silver to dissolve you need lots of cyanide and an oxidant. Oxidants that will oxidise Ag to Ag+ will generally oxidise the aldehyde.

In the limit, the weakest oxidant that will oxidise Ag is Ag+.

Ag+ is noted for oxidising aldehydes.

[Tsadi]

Yes... well... John you are very knowledgable.

[rvdzero]

haha its like the battle of the super powers! you guys really know what your'e talking about! i appreciate being able to learn from both of you, thanks! (one day i hope to be at that level. if you dont mind my asking, what did you guys study / what are your professions? im studying to be a chemical engineer, still got a looong way to go though)

 

so back to the question. all ihave available is silver nitrate. i can react two aqueous solutions of silver nitrate and sodium carbonate, and should be left with a precipitate of silver carbonate (AgCO3 is insoluble in water, well i read 33mg/L) and an aqueous solution of sodium nitrate.

 

2 AgNO3 (aq) + Na2CO3 (aq) => Ag2CO3 (s) + 2 NaNO3 (aq)

 

Then, based on YT's answer, if i add the Ag2CO3 to the benzaldehyde and HCN, it should precipitate silver cyanide. Would the carbonate ion claim the hydrogen from the HCN to form HCO3-? if there was an excess of carbonate ions, would they try to break the benzaldehyde's bonds to form more HCO3-? just having trouble figuring out where the carbonate would go? oh i just had another idea, since CO3- would try and take hydrogen from water, couldnt i add a bit of water to the benza/hcn mix? because AgCN is insoluble in water, and so is benzaldehyde. the water would give the CO3 anion somewhere to go/something to react with?

 

thanks, rick

[John Cuthber]

Well, I studied chemistry at university, and I work as a chemist.

The silver carbonate should work OK the other product will be H2CO3 which will decompose to CO2 and water.

I might try shaking the aldehyde with a solution of silver acetate in water. Silver acetate is slightly soluble so enough would dissolve in water to react with the HCN but the solubillity is low enough that you can make it by precipitation.

In any event you will need to filter and dry the product. It's going to be easier to distill it.

[rvdzero]

thanks for all the help guys! sorry for lack of posting, have been over seas for the last 3 weeks. anyhoo, im back and just more more question about hcn.

 

how would one attempt to detect if a solution has hcn present? (without all fancy technology lol) a hint in the right direction would be great! ive done a few searches, but i havent found anything yet. just curious about how they detect certain chemicals in solutions.

 

thanks, rick

[YT2095]

Silver Nitrate titration, here:

"The preferred method for the analytical determination of free cyanide is silver nitrate titration. Silver ions are added to the solution to complex the free cyanide ions. When all free cyanide is consumed as silver cyanide complex, the excess silver ions indicate the endpoint of the titration. The analytical equipment used for the titration is rather simple. To accurately determine the cyanide concentration, a normalized silver nitrate solution is dosed with a manual or automatic burette, which should be capable of measuring volumes to an accuracy of better than 0.005 ml."

 

taken from:http://www.cyanidecode.org/cyanide_sampling.php

 

I`m pretty sure there`s also a method using Iron salts to give a blue color as well.

[John Cuthber]

That titration is great if you want to measure the HCN, if you just want to know if it's there scroll down this page

http://www.tutorvista.com/content/chemistry/chemistry-iii/organic-compounds/qualitative-analysis.php

to the bit that says "Detection of nitrogen"

[rvdzero]

hey all, i lost my internet for awhile there thanks to storms. anyways, thanks alot for the links! i did the test for nitrogen and it indeed went prussian blue. so my last question is, hcn has a boiling point of 26 degrees C and benzaldehyde has a boiling point of 178 degrees C, so could one set up a distillation and heat the solution to release hcn gas? (the gas would be bubbled into a cooled, agitated solution of sodium hydroxide to form sodium cyanide. the sodium cyanide would then be added to a hydrogen peroxide solution to form sodium cyanate, which is less toxic. any ideas?

[John Cuthber]

Distillation would work. Steam distillation might work better.

Add some water to the aldehyde and distill the "mixture" (they don't mix well) The steam will carry any HCN with it. It will also carry some of the aldehyde over. The solution of HCN in water is less hazardous to work with.

HCN does boil at a low temperature, but the amount of it in the benzaldehyde is small so you probably wouldn't get a proper low boiling fraction rich in HCN then a high boiling one rich in aldehyde.

 

If you plan to do this make sure you add some boiling chips or something to the flask you distill from. Also you might want to try adding about 5 or 10% salt to the water so it forms the bottom layer in the flask - that way the bubbles of steam are forced to go through the benzaldehyde (which would normally sit at the bottom of the flask because it's denser than water).

[Sedit]

correct me if im wrong here but would the use of a basic solution of NaOH washed over the benzylhyde HCN mixture convert the water layer to sodium cyanide with out messing with the benzylhyde?

 

And if NaOH would be to harsh the sodium carbonate would be a more gental approch

[UC]

Sedit- check out the Cannizzaro reaction. Base around benzaldehyde is a no-no

[amit]

i may be wrong but cant we use some under graduate method of presipitating some derivative of benzeldihyde (like oximes or something) and convert it back to benzaldehyde. also as far removal of CN- is concerned what about complexing them with some other metals using phase transfer method. which distillation would be better steam or reduced pressure?

[Sedit]

thank you UC i forgot all about that. I had a feeling so it must have been floating in the back of my brain some where.

 

I see your from NJ to ..lol im sorry to hear that