Question

13.) Redox chemistry isn’t only for chemists, it’s important in biology as well. Below are two different redox forms of Vitamin C. Oxygen lone pairs are omitted for clarity. a) Identify the oxidation states of the carbons that change oxidation state for both molecules. How many electrons are transferred when these forms interconvert? b) From a redox perspective (ignore acid/base aspects), would H2 or O2 be more likely to change Ascorbic Acid into Dehydroascorbic Acid? Briefly explain your choice.

Answer 1

The redox forms of L-ascorbic acid (vitamin C) are given below:

a) To measure the oxidation state of carbon, certain rules have to be followed:

1. If a carbon atom is bonded to another carbon, the oxidation state of the carbon atom is not altered
2. If the carbon is bonded to a hydrogen, the oxidation state is decreased by 1.
3. If the carbon is bonded to a more electronegative atom, the oxidation state is increased by 1.
4. Double and triple bonds are counted twice and thrice respectively.

The carbons labelled in the figure, change the oxidation states for both molecules:

In the case of ascorbic acid, the oxidation state of carbons 1 and 2 is +1, since it is bonded to an oxygen atom. Carbons 1 and 2 in dehydroascorbic acid have an oxidation state of +2 since the carbons are bonded to the oxygen atom via double bonds.

Hence, during interconversion between ascorbic acid and its oxidized form, oxidation states of two carbons change from +1 to +2 (loss of an electron each). Therefore, two electrons are transferred during the interconversion.

b) The conversion of ascorbic acid to dehydroascorbic acid occurs at the loss of two hydrogens, making it an oxidation process. According to Le Chatelier's principle, by adding H2, the equilibrium shifts towards the left impeding the oxidation. Hence, O2 would facilitate the interconversion.