Reduced form of ascorbic acid?

[woelen]

It is well known that ascorbic acid (a.k.a. vitamin C) is a fairly good reductor and as such it is used by our body and also in many foods. In those places it acts as antioxidant, itself being oxidized, instead of the food or the material in our body.

 

Surprisingly I have found strong evidence, however, that ascorbid acid also can act as a reductor under very "normal" conditions. I would like to have a second opinion on that.

 

I did the following experiments:

1) Add some NaOH to an excess amount of a solution of ascorbic acid. This makes an ascorbate/ascorbic acid mix in solution:

OH(-) + HAsc --> H2O + Asc(-)

To this, add some copper (II) sulfate.

 

2) Add some metallic Mg to an excess amount of a solution of ascorbic acid. This also was supposed to make a mix of ascorbate/ascorbic acid, but now with some Mg(2+) ions in solution, instead of Na(+) ions:

Mg + 2 HAsc --> H2 + Mg(2+) + 2Asc(-)

To this, also add some copper (II) sulfate.

 

In the descriptions above, HAsc is ascorbic acid, Asc(-) is ascorbate ion.

 

 

In experiment (1) the copper (II) sulfate quickly is reduced to an orange/yellow suspension of hydrous copper (I) oxide. This is precisely what I expected. That is the normal mode of operation when copper (II) is reduced in mildly alkaline solution (c.f. Fehlings reagent). When some dilute HCl is added, all of the precipitate dissolves.

 

For experiment (2) I expected the same as in experiment (1), I do not expect a major difference because of the Mg(2+). But here, a dark red/brown complex is formed, which very quickly becomes dark green and then metallic copper is separated. The liquid has a beautiful red/brown metallic lustre, when viewed with reflected light. Also, quite some hydrous copper (I) oxide is formed. When some dilute HCl is added, the copper metal does not dissolve. After standing, it forms a nice brown/red layer at the bottom.

 

 

What I think is that in experiment (2) not only the reaction

Mg + 2 HAsc --> H2 + Mg(2+) + 2Asc(-)

occurred (which is a standard metal/acid reaction), but that a side reaction occurred, where the Mg reduces some of the HAsc to an even stronger reductor. I have been looking at the structure of ascorbic acid, but I'm not sure what the structure of this even more reducing agent could be. Is there someone out there, who could shed some light on this?

[Tartaglia]

Oxidation of 1,2 dihydroxyalkenes, by transtion metals is not a particularly unusual reaction. You form a chelating 5 membered pi system which is easy to push electrons around

[woelen]

Yes, I know that oxidation by transition metals is quite common. Probably I was not clear with my post, but what I meant to say is that I think that the ascorbic acid is reduced by the magnesium metal.

 

The ascorbic acid in the test tube, with the NaOH only reduces copper (II) to copper (I), but the ascorbic acid in the test tube, with the magnesium reduces copper (II) to metallic copper. So, there is a difference in reducing power of both solutions.

 

I have done further experimenting and ascorbic acid, treated with NaOH simply is not capable of reducing copper (II) to metallic copper. Ascorbic acid, treated with magnesium (of course, any remains of metallic magnesium being removed before copper (II) is added), reduces copper (II) to metallic copper very quickly, it only takes seconds to do so.

 

I hope that my point is clear with this second post. What I think is very special is that ascorbic acid can be reduced.

[Tartaglia]

The tautomer of a 1,2 hydroxy alkene is an alpha hydroxy ketone. Magnesium is a strong enough metal to react with water to produce hydrogen (all be it slowly) and so should be able to act as a reducing agent to give a 1,2 dihydroxy alkane. Again no surprise there