Sciencemadness Discussion Board

Why can't thionyl chloride be used to chlorinate oxalic acid to oxalyl chloride?

DavidJR - 16-5-2019 at 14:54

I've seen several mentions of the fact that SOCl2 cannot be used to chlorinate oxalic acid, including in papers, but I haven't been able to find a satisfactory explanation as to why it does not work at all when other chlorinating agents (like PCl5) do work.

I can confirm that it does, indeed, not work, because I tried it. I also tried addition of pyridine (as a base to mop up HCl), catalytic amount of DMF, or both, without success.

There have been some suggestions that the decomposition results from the instability of either the mono-chlorinated product, or of a cyclic anhydride.

However, thinking that these unstable products might still be able to react with alcohols etc, I tried the above mentioned prep (all four variants, with/without each of DMF/pyridine) in the presence of phenol, hoping to get at least a small amount of diphenyl oxalate. I got none.

Any ideas?

Keras - 16-5-2019 at 20:55

What do you get instead? Other products, or does nothing happen?

Did you have a look on this thread?

I wonder if the end result of oxalic acid + thionyl chloride is not phosgene. Did you test for this?

[Edited on 17-5-2019 by Keras]

Pumukli - 16-5-2019 at 21:01

First of all I don't know the answer to your question either.

Your negative results are still informative though, they may save someone to waste precious thionyl chloride to repeat them! No kidding.

On the other hand, if you don't mind wasting a few more milliliters I'd suggest something.
What if you tried the reaction with say anhydrous sodium-oxalate instead of the free acid? I remember (foggy) that S2Cl2 (or SCl2?) and a salt can be used to make acid-chlorides. Also, there is an article in JACS somewhere where they use benzoyl-chloride and a salt of a different acid to make the acid-chloride of that other acid.

What do you think?

DavidJR - 17-5-2019 at 02:44

Quote: Originally posted by Keras  
What do you get instead? Other products, or does nothing happen?

Seemed to decompose to gaseous products, probably including CO and CO2.
Quote: Originally posted by Keras  

I wonder if the end result of oxalic acid + thionyl chloride is not phosgene. Did you test for this?

I hope not.... How would I test for phosgene?
Quote: Originally posted by Pumukli  

On the other hand, if you don't mind wasting a few more milliliters I'd suggest something.
What if you tried the reaction with say anhydrous sodium-oxalate instead of the free acid? I remember (foggy) that S2Cl2 (or SCl2?) and a salt can be used to make acid-chlorides. Also, there is an article in JACS somewhere where they use benzoyl-chloride and a salt of a different acid to make the acid-chloride of that other acid.

Hmm that might be worth trying actually. I'm not worried about wasting SOCl2 because I have a 1 litre bottle (only ordered 500ml but they sent me 1l).

[Edited on 17-5-2019 by DavidJR]

Keras - 17-5-2019 at 03:13

Quote: Originally posted by DavidJR  

Seemed to decompose to gaseous products, probably including CO and CO2.

Seems standard for oxalic acid which is easily oxidised into CO and CO2
Quote: Originally posted by DavidJR  

I hope not.... How would I test for phosgene?


Have some old articles about that.

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[Edited on 17-5-2019 by Keras]

CuReUS - 17-5-2019 at 06:42

Quote: Originally posted by Pumukli  
What if you tried the reaction with say anhydrous sodium-oxalate instead of the free acid?
I second this idea.According to wiki - https://en.wikipedia.org/wiki/Oxalyl_chloride#Reactions
Quote:
Oxalyl chloride reacts with water giving off gaseous products only: hydrogen chloride (HCl), carbon dioxide (CO2), and carbon monoxide (CO)
Maybe your oxalyl chloride is decomposing as soon as it is formed.Did you use anhydrous oxalic acid or the dihydrate ?

Or maybe since oxalic acid is a reducing agent,its reducing SOCl2(and some of the formed oxalyl chloride) faster than the latter can chlorinate it.This problem can be solved by using oxalic ester.
Quote:
I tried the above mentioned prep (all four variants, with/without each of DMF/pyridine) in the presence of phenol, hoping to get at least a small amount of diphenyl oxalate. I got none.
Smart way to confirm a reaction mechanism.Try this again using aliphatic alcohol,since pheonls are less reactive-https://www.researchgate.net/post/why_phenol_is_less_reactiv...

[Edited on 17-5-2019 by CuReUS]

DavidJR - 17-5-2019 at 07:14

Quote: Originally posted by CuReUS  

Maybe your oxalyl chloride is decomposing as soon as it is formed.Did you use anhydrous oxalic acid or the dihydrate ?

At first I tried using the dihydrate, with the expectation that the SOCl2 would mop up the water. When that didn't work I moved to using (what I believe is) anhydrous oxalic acid (dried by dean stark trap w/ toluene - but I haven't done a water determination on it).
Quote: Originally posted by CuReUS  

Or maybe since oxalic acid is a reducing agent,its reducing SOCl2(and some of the formed oxalyl chloride) faster than the latter can chlorinate it.This problem can be solved by using oxalic ester.

Maybe, but then I would have to prepare an oxalate ester first, which would be a pain in the ass. I'd certainly rather try using an oxalate salt first. I have calcium oxalate on hand, maybe that's worth trying. If not I'll prepare some sodium oxalate.

clearly_not_atara - 17-5-2019 at 10:53

I would guess that SOCl2 reacts with phenol faster than it reacts with oxalate. Phenol is a much stronger nucleophile. I had personally been in the "cyclic anhydride" (2,1,3-thiadioxole-2,4,5-trione) camp.

It may however be possible to make oxalic ditosylate from oxalates and tosyl chloride, but mixed carboxylic-sulfonic anhydrides tend to disproportionate into the symmetric anhydrides, which in this case would lead to decomposition.

A more fanciful possibility would be to use N,N-diphenylcarbamoyl chloride or other similar N,N-dialkylcarbamoyl chloride to form mixed anhydrides with an oxalate salt. "DPCC" forms exceptionally stable mixed anhydrides with carboxylic acids that act as good acyl transfer reagents, cf Shepard et al (attached). Somewhat conveniently, diphenylcarbamoyl chloride is commercially available.

The at-home synthesis of DPCC however leaves a bit to be desired -- the only thing I can think of is the double Goldberg reaction of formamide with iodobenzene, followed by radical chlorination of the formamide. The literature route uses phosgene and diphenylamine, which I consider unreasonably dangerous.

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CharlieA - 17-5-2019 at 13:07

Test for phosgene from my prior laboratory experience: it smells like new-mown hay. THIS IS NOT A RECOMMENDED ANALYTICAL METHOD!!!

chemplayer... - 17-5-2019 at 21:09

Oxalic acid is a difficult one to chlorinate. We tried with PCl3 (apart from PCl5 it don't come much stronger) and the result was negative. Unfortunately if you want to go the route of using traditional chlorinating agents to prepare it, only PCl5 seems to work.

There was a SM contest / challenge a while back as to whether anyone could come up with an OTC way to prepare oxalyl chloride, and there were some good ideas floated around, but no final outcome or positive result as far as I am aware. Our suggestion would be bubbling anhydrous HCl gas through a mixture of acetonitrile with anhydrous oxalic acid dissolved in it (probably low yielding though if indeed it works at all).

CuReUS - 18-5-2019 at 04:06

Quote: Originally posted by DavidJR  
I'd certainly rather try using an oxalate salt first.
You could try it ,but if the reduction theory is correct,oxalate salts won't work because for oxalic acid to act as a reducing agent,it must get oxidised to CO2,which oxalate salts can do as well,but esters can't.
Quote: Originally posted by clearly_not_atara  
I had personally been in the "cyclic anhydride" (2,1,3-thiadioxole-2,4,5-trione) camp.
I think by "cyclic anhydride" he meant the reaction between oxalic acid and SOCl2,not phenol and SOCl2
Quote:
The at-home synthesis of DPCC however leaves a bit to be desired
You could make it by reacting triphosgene(safer than phosgene :D) with diphenylamine - https://www.sciencedirect.com/science/article/pii/S100184171...

triphosgene can be made from dimethyl carbonate - https://www.tandfonline.com/doi/abs/10.1080/0039791930801260...
maybe it could be done in 1 pot to reduce the risk
Quote: Originally posted by DavidJR  
How would I test for phosgene?
http://sciencemadness.org/library/books/the_war_gases.pdf ( read pg 65,pg 72 in pdf)
Quote: Originally posted by chemplayer...  
Unfortunately if you want to go the route of using traditional chlorinating agents to prepare it, only PCl5 seems to work.
http://www.sciencemadness.org/talk/viewthread.php?tid=8742#p...

[Edited on 18-5-2019 by CuReUS]

DavidJR - 18-5-2019 at 05:56

I've recently acquired some PCl5 and I'm using it right now to prepare some oxalyl chloride. So I'm not really looking for other practical ways to make oxalyl chloride, I'm really just interested in finding out why thionyl chloride doesn't work.

DavidJR - 18-5-2019 at 12:46

Quote: Originally posted by DavidJR  
I've recently acquired some PCl5 and I'm using it right now to prepare some oxalyl chloride. So I'm not really looking for other practical ways to make oxalyl chloride, I'm really just interested in finding out why thionyl chloride doesn't work.


Wanting to test my newly-prepared oxalyl chloride (made w/ PCl5) I tried diluting 100ul of product with about 2ml DCM, added a tiny bit of rhodamine B, then added about 2ml of 12% H2O2. Got a wonderful (but short lived) glow.

Later, it occurred to me that if this works for oxalyl chloride, then perhaps any kind of cyclic anhydride of oxalic acid (produced by reaction with thionyl chloride) could also do it in the same situation. So in a vial, I put a spatula of anhydrous oxalic acid, covered with about 2ml of DCM (doesn't dissolve), added a tiny bit of rhodamine B, and 200ul of thionyl chloride. Swirled it around for a few seconds, then added 2ml of 12% H2O2 on top. No glow, even with stirring (vigorous reaction between the water and thionyl chloride though).