Reaction between Copper nitrate and ascorbic acid

Question

I am confused on the products of reaction between copper nitrate and ascorbic acid. When I searched this online, one of the reactions I got was: $$\ce{2Cu(NO3)2 + C6H8O6 <=> 2CuO + 4H2O + CO2}$$

I am really confused as to how they determined that to be the product. My thinking was that the ascorbic acid breaks into $$\ce{HC6H7O6 <=> H+ + C6H6O6-}$$ and so the hydrogen ion, whose activity is higher than copper, replaces the copper to form nitric acid. I was thinking that this could be a double displacement reaction and so the copper ascorbate would become a precipitate. What am I thinking wrong here?

Answer 1

It is always good to cite the link when you mention that you saw a redox reaction online. It is wrong anyway.

Aqueous Cu(II) is not an strong enough oxidizing agent that it will convert organic matter into carbon dioxide. In other words, it is a weak oxidizing agent. Ascorbic acid is oxidized to dehydroascorbic acid and Cu(II) should be reduced to Cu(I). If you heat the solution strongly, Cu(I) may convert into Cu$_2$O.

Ascorbic acid is indeed an acid, but there is nothing which prevents its conjugate base to behave like a reducing agent. Many organic acids can behave like that.

I am really confused as to how they determined that to be the product.

By conducting the experiment. There is no other simple way to predict what will happen in a complex redox reaction.

Answer 2

In my assessment, an advanced reaction involving a cyclic REDOX reaction occurring with Copper ions, which I have previously discussed in this work 'Generation of Hydroxyl Radicals from Dissolved Transition Metals in Surrogate Lung Fluid Solutions' by Edgar Vidrio relating to disease (cancer) likely induced from progressive damage to DNA (in the new equation this is now an attack on the nitrate ion,...) arising from the transition metal copper interacting with oxygen (in the new reaction this is replaced by HNO3,...) leading to the powerful hydroxyl radical.

That reaction becomes cyclic (at least, to a limited degree depending on the respective concentration of Ascorbic acid) per Vidrio's to quote:

“Similar reactions can occur with Cu, Cr and Ni. Furthermore, biological chelators and reductants can greatly enhance the production of ROS (Burkitt et al., 1991; Engelmann et al., 2003; Wenk et al., 2001). For example, in the presence of ascorbate (Asc), a biological reductant, the oxidized form of the transition metal produced by the Fenton reaction can be reactivated (R2 and R3), thus allowing additional ROS to be produced.

$\ce{Fe(III) + Asc (n) → Fe(II) + Asc (n') (R2)}$

$\ce{Cu(II) + Asc (n) → Cu(I) + Asc (n') (R3)}$ “

which cycles the cupric back to cuprous to feed a REDOX, which, for our new reaction involves aqueous cupric nitrate and Ascorbic acid, that is detailed in this reference, "Fenton chemistry in biology and medicine" by Josef Prousek, to quote reaction (15) on page 2330, a general depiction of Fenton-type reactions, to quote:

"For Fe(II) and Cu(I), this situation can be generally depicted as follows [20,39],

$\ce{ Fe(II)/Cu(I) + HOX → Fe(III)/Cu(II) + ·OH + X- }$ (15)

where X = Cl, ONO, and SCN. "

In the current context, the hydrolysis of cupric nitrate introduces HNO3, which interacts with cuprous (created from the Ascorbic acid presence) reforming cupric and, importantly, always hydroxyl radicals. The latter radical can attack nitrogen oxides compounds which can further progressively be broken down. Some expected reactions:

$\ce{ .OH + NO3- -> OH- + ·NO2 }$

$\ce{ .NO2 + .NO2 -> N2O4 }$

$\ce{ N2O4 + 2 H2O <=> HNO2 + HNO3}$

$\ce{ 2 Cu(I) + HONO -> 2 Cu(II) + ·OH + N2O2^{2-} }$

etc., so the nitrate, nitrite,..., could be progressively decomposed (even to NH3), which may account for its notable absence in the cited text.

Lastly, the REDOX consumes H+ and select radical reactions could add OH-, all of which could eventually lead to either a basic copper salt, or Cu(OH)2. The latter on warming, forms CuO, as was claimed.