Synthesis of Oxalic acid from Vitamin C

[Toothpick93]

Ascorbic Acid -> Oxalic Acid

 

So there was a thread about decomposition of ascorbic acid on ScienceMagness.org and a user said

 

Quote: Originally posted by Sriraman The path way of decomposition of Ascorbic acid is Ascorbic acid to Dehydroascorbic acid to 2,5 Diketogulonic acid and then to oxalic acid and L-threonic acid. you can guess which gives yellow colour-possibly 2,5 diketogulonic acid due to its two ketonic functions

 

 

Ascorbic Acid is readily valuable from health stores as Vitamin C but most of the time it isn't 100% Ascorbic Acid, my tablets that my family get has Sodium Ascorbate also in it so i could probably dissolve the whole tablet into water and acidify the water to turn Sodium Ascorbate into ascorbic acid and a sodium salt... and then cool the solution to crystallize out the acid.

 

Now im a noob to how to do organic chemistry so i don't know how things take place but i think i figured out the equation to what is needed.

 

 

C₆H₈O₆ = C₆H₆O₆ + H₂

 

The first equation is a reduction, KMnO4 can be used to reduce it to Dehydroascorbic acid and Hydrogen Gas but how would u then separate the contaminates from the chemicals we want?

 

C₆H₆O₆ + H₂O + C₆H₈O₇

 

Now because the first reaction will contain water it will most likely go straight to the second reaction which hydrates it to 2,5 diketogulonic acid which would then maybe follow on to the next reaction as 2,5 diketogulonic acid reacts with the water to form Oxalic Acid and L-threonic acid

 

5 C₇H₈O₇ + 7 H₂O = 3 H₂C₂O₄ + 6 C₄H₈O₅

 

So would the reaction play out on its own? id really like to know more about this

[Horza2002]

Ascorbic acid, as the name implies, is acid. This means that one of the protons is very acidic and can be deprotonated easily. All that has happened to get the sodium ascorbate, is that you have deptotonated and made the sodium salt. When you swallow the tablet, it will be reprotonated in your stomach to give you ascorbic acid back. This is an example of a tetronic acid...there is no carboxylic acid, but due to conjugation through the double bond, one fo the hydroxy groups is acidic.

 

 

Now onto your synthesis bits...No, potassium permanganate is not a reducing agent, it is an oxidising agent. And the first step is actually an oxidation as well, not a reduction. A general rule, if you are increasing the number of hydrogens in a molcule, then it is a reduction...if you are dereasing the number of hydrogen, it is an oxidation (this is not 100% all the time...but for a begininger it should be ok). Potassium permanganate is rarely used as a reagent because it is such a strong oxidiser, you will oxidise absolutely everything in your molcule (that is why it used as a stin for TLC). You very rarely get hydrogen gas given off as well.

 

Your second step is essentially an ester hydrolysis resulting in the opening of the 5-membered ring to give you the 2,5 diketogulonic acid. In theory, water could do this reaction...but in practise, sodium hydroxide is used (it is much mroe reactive). One potential problem here, is that 5-membered rings are pretty stable, and 5-memebred lactones (cyclic esters) are notoriously difficult to make them undergo hydrolysis. So this step is unlikely to work very well if at all.

 

That final step does not seem very likely to happen at all to me...im not even sure where to begin with that one.

[Toothpick93]

Thanks for a great answer so this wouldn't be a viable route to obtain Oxalic Acid? I only really use Potassium Permanganate becuase thats the only real good oxidizer i can get, i can get a few nitrates but thats about it... and by get i mean make.... i get short on money to buy new chemicals so i tend to make them instead... what would your best way to make Oxalic acid?

[Horza2002]

I would say no, this isn't a viable route. As I said, potassium permanganate is a very very strong oxidising agent. If you exposed ascorbic acid to it, then you would oxidise the secondary alcohol to the ketone, the primary alcohol to a carboxylic acid...maybe then the carbxoyclic acids to the orthoesters....epoxidise the double bond...in short, you would get an absolutly horrible mess!

[hypervalent_iodine]

Now im a noob to how to do organic chemistry so i don't know how things take place but i think i figured out the equation to what is needed.

 

 

C₆H₈O₆ = C₆H₆O₆ + H₂

 

The first equation is a reduction, KMnO4 can be used to reduce it to Dehydroascorbic acid and Hydrogen Gas but how would u then separate the contaminates from the chemicals we want?

 

Empirical formula aren't particularly informative in terms of well, anything. You're not really going to be able to predict how a reaction will proceed without the structures.

 

KMnO₄ is an oxidising agent, and a very strong one at that; there is no way it is going to reduce anything. Converting ascorbic acid to DHA is in fact an oxidative process, as Horza noted.

 

Isolation of compounds from a reaction mixture is typically done using silica flash-column chromatography (if you've ever used a TLC plate to separate pen ink pigments, it's essentially the same thing, but in a column). There are other ways such as distillation, prep TLC, etc., but columns are usually the first go-to. Occasionally you get lucky and remove your product with a simple liquid-liquid extraction work-up.

 

C₆H₆O₆ + H₂O + C₆H₈O₇

 

Now because the first reaction will contain water it will most likely go straight to the second reaction which hydrates it to 2,5 diketogulonic acid

 

The person who wrote that is slightly incorrect. It will not hydrolyse to give 2,5-diketogluonic acid, but 2,3-diketogluonic acid. At an educated guess, I would say that oxygen of the water will attack at the ester carbonyl, breaking the ring open and giving you your product. As Horza said, though, this would be a tough reaction to get to completion.

 

 

which would then maybe follow on to the next reaction as 2,5 diketogulonic acid reacts with the water to form Oxalic Acid and L-threonic acid

 

5 C₇H₈O₇ + 7 H₂O = 3 H₂C₂O₄ + 6 C₄H₈O₅

 

So would the reaction play out on its own? id really like to know more about this

 

A quick search the final step will happen, but the mechanism is evasive in the literature and it doesn't seem that water would do the job by itself. I found one paper that uses calcium hypochlorite to cleave alpha-ketones, which I suppose could work with your substrate.

 

A search of the literature suggests that non-enzymatic oxidative and non-oxidative decomposition of ascorbic acid is in fact quite convoluted in terms of the number of products you get. One paper I read isolated over 50 products, which would be somewhat difficult to separate properly without good equipment.