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Author: Subject: Benzoin condensation using hydrogen cyanide?
SplendidAcylation
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shocked.gif posted on 31-5-2020 at 17:39
Benzoin condensation using hydrogen cyanide?


Hi,

Sorry about the title, I tried 10 others but none were any better.

I am planning to make benzoin by the benzoin condensation of benzaldehyde.
The reaction requires a catalytic amount of alkali cyanide, and, sadly, I have none.

It would definitely be worthwhile making a cyanide salt because of the huge number of fun things it can be used for, such as making nitriles from haloalkanes.

However in this case, since the cyanide is apparently a catalyst, would it be possible to use a trace of hydrogen cyanide dissolved in the ethanol instead of an alkali cyanide?

http://www.orgsyn.org/demo.aspx?prep=CV1P0094

I have P2O5, and owing to the excellent recent thread by Fery about making ethyl formate, I shall soon have formamide.

The dehydration of formamide with P2O5 should give dry hydrogen cyanide gas in good yields, which could be lead directly into the ethanol-benzaldehyde solution.

Alternatively, of course, the ethanol could be made alkaline with sodium hydroxide, and then the hydrogen cyanide lead in.

So really, the main issue in my mind is that I'm not exactly sure whether significant amounts of the cyanide catalyst are required
Surely, since it's a catalyst, only a few milligrams would be needed?

The orgsyn page says to use 50g of NaCN for 500g benzaldehyde, which is a huge amount, considering that it is a catalyst.

I'm aware of the thiamine catalysts for this reaction, but sadly I have none of those either.

If anyone could clear this up, I'd be very grateful.

Thank you
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[*] posted on 31-5-2020 at 17:53


I have been wanting to do this reaction as well but am still waiting to get some bitter almond oil. There is a video that says the potassium cyanide can be swapped with a diazonium salt, if you have access to any.

https://www.youtube.com/watch?v=l8zTcUUO7b4
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karlos³
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[*] posted on 31-5-2020 at 18:57


Just use thiamine instead, it substitutes the cyanide.
Thats vitamin B1.
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[*] posted on 31-5-2020 at 19:38


Quote: Originally posted by karlos³  
Just use thiamine instead, it substitutes the cyanide.
Thats vitamin B1.


That is very good to know. Will this work to replace cyanide in other reactions, or just the benzoin synthesis?
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Pumukli
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[*] posted on 31-5-2020 at 22:16


Potassium-ferrocyanide + Sodium carbonate heated to melting and blackening on open flame for a few minutes in a test tube. Dissolve in water, filter out the black residue. Cyanide solution is ready. It worked for me when I prepared benzoin.
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Boffis
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[*] posted on 31-5-2020 at 23:42


@SplendidAcylation; I have made numerous posts on this topic and very particularly the use of thiamine (B1) as the catalyst. No right minded amateur should try this Acylion condensation with cyanide, thiamine is easy to get, much safer and with some sensitive aldehydes like furfuraldehyde gives much better yields of purer product; bit of a no brainer really :).

Check out Prepublications:
http://www.sciencemadness.org/talk/viewthread.php?tid=68245#...
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Ubya
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[*] posted on 1-6-2020 at 02:40


Quote: Originally posted by karlos³  
Just use thiamine instead, it substitutes the cyanide.
Thats vitamin B1.


used this at uni for the same reaction, use thiamine





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[*] posted on 1-6-2020 at 03:39


When I did this reaction at uni, it was the first week of term. The benzaldehyde in the stock bottle on the shelf had had the whole vacation to oxidise/ cannizzaro react with itself.
So, what I actually added to 5 grams of potassium cyanide was a mixture that contained a lot of benzoic acid.

I can confirm that the reaction did not work with HCN.
It took a while to work out what the problem was.
It was eventually solved when someone spotted crystals of benzoic acid in the benzaldehyde.

Just as well we were using fume cupboards...

Thiamine looks like a really good option.

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SplendidAcylation
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[*] posted on 1-6-2020 at 17:10


Thanks for all the replies.

Looks like thiamine is the way to go, although I'll probably end up trying both ways.

Interesting that HCN doesn't work, someone posted a reply on my accidental other thread saying that the CN- ion is required for the reaction mechanism.
I think HCN does dissociate in water, so one would think that that would be enough, even if its only very slightly dissociated.

@pumukli that's interesting!
I sadly have no ferrocyanide
I have read that sodium carbonate, mixed with charcoal, red hot, with ammonia gas passed over it, produces cyanide, but that feels rather like a long shot.

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[*] posted on 2-6-2020 at 02:00


Found online:

(From http://www.chem.latech.edu/~upali/chem254/BENZOINC.pdf)
Attachment: BENZOINC.pdf (4.3MB)
This file has been downloaded 49 times

(From https://www.unige.ch/asso-etud/aecb/rapports/2eme/chiorg/p38...)
Attachment: p38cbenzoine_pacho.pdf (317kB)
This file has been downloaded 46 times

(From http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organic...)
Attachment: Thiamine Catalyzed Benzoin Condensation.pdf (206kB)
This file has been downloaded 44 times

Re-uploaded rather than just linking in case these disappear from their original sources.

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SplendidAcylation
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[*] posted on 4-6-2020 at 18:02


Quote: Originally posted by DavidJR  
Found online:

(From http://www.chem.latech.edu/~upali/chem254/BENZOINC.pdf)


(From https://www.unige.ch/asso-etud/aecb/rapports/2eme/chiorg/p38...)


(From http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organic...)


Re-uploaded rather than just linking in case these disappear from their original sources.




Looks promising, I'm currently attempting to make a small amount of NaCN, and I'm getting some thiamine, so we'll see which method wins!

P.S. I am also located in Scotland :P
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[*] posted on 7-6-2020 at 14:03


Are you sure you want to make NaCN yourself? How much experience do you have in working with very toxic gases and do you have the equipment to do this safely? Keep in mind that if you go the route of making HCN and converting that to NaCN is VERY dangerous. Only 50 mg (!!!) of inhaled HCN can kill you. I ask this, because I have not seen many posts yet from you and it is hard to assess your experience and resources for working safely.

If you do not have a proper fumehood, then you should NOT attempt this reaction inside with more than small test tube quantities. If you perform the reaction outside, then you could use somewhat larger quantities, but even then, you must be very careful and do the reaction in an open area (e.g. a backyard) on a breezy day, with wind from behind.

I personally do some experiments with dangerous chemicals occasionally, but there are also quite some interesting experiments, which I do not perform, even if I have the chemicals, simply because I deem the risk too high. Not everything can be done in a domestic setting and the real good amateur also knows where his/her limits are.




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DavidJR
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[*] posted on 8-6-2020 at 06:38


Quote: Originally posted by SplendidAcylation  
Quote: Originally posted by DavidJR  
Found online:

(From http://www.chem.latech.edu/~upali/chem254/BENZOINC.pdf)


(From https://www.unige.ch/asso-etud/aecb/rapports/2eme/chiorg/p38...)


(From http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organic...)


Re-uploaded rather than just linking in case these disappear from their original sources.




Looks promising, I'm currently attempting to make a small amount of NaCN, and I'm getting some thiamine, so we'll see which method wins!

P.S. I am also located in Scotland :P


I tried the thiamine method, and it did work, but my yield wasn't great (44% after recrystallization of the product from MeOH/H2O and drying in desiccator). I think that this was just because the reaction wasn't complete, because the reaction mixture did smell a bit of almonds still.

I used the procedure from the second link (https://www.unige.ch/asso-etud/aecb/rapports/2eme/chiorg/p38...), except I didn't bother with chilling to 0c because that part seemed pointless. I think 90 mins at 60C was just not enough for the reaction to complete.
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[*] posted on 8-6-2020 at 06:51


@DavidJR; I have used the thiamine method numerous times on a wide variety of substrates. I have discovered a few things though, one point is that you have to let the thiamine salt stand with the alkali for a few minutes before adding the aldehyde, if not the yield can be very poor and most of the aldehyde recovered unaltered.

I have consistently obtained better yields that have been reported for the cyanide route and sometimes it works with substrates which are destroyed by the excessively alkaline. One issue I have had is that when the acyloin is sparingly soluble in most solvent eg pyridoin it is difficult to purify the product which often contains a little meaty smelling impurity. For some of the heterocyclic acyloins I have found it better sometimes to leach out the impurity which is much more soluble in methanol than the product. If you are oxidizing them to acylils this purification isn't so important. I discussed most of these points in my write, linked above.
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[*] posted on 8-6-2020 at 07:24


Quote: Originally posted by DavidJR  

I tried the thiamine method, and it did work, but my yield wasn't great (44% after recrystallization of the product from MeOH/H2O and drying in desiccator).


Well, that's almost twice as much as what they got in the paper. Doesn't seem too bad to me.




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[*] posted on 5-7-2020 at 17:24


I'm sorry, I thought I had responded to all of the posts here :o

@woelen, thanks for the advice, I have obtained some thiamine and I'm going to use that instead of cyanide

I have dealt with toxic gases such as chlorine and bromine vapour, but yeah, making NaCN by that route wouldn't really be safe, and I didn't really look forward to it anyway :P

Thanks for all the replies, I'm going to try the reaction tomorrow and I'll post my results!
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