Oxidation-Reduction Reactions in Your Lungs

     Equation 1 shows ascorbic acid (Vitamin C) from the epithelial lining fluid of the lungs reacting with an oxidizing agent (peroxide) to form harmless products, thus preventing lung damage. What is being reduced and what is being oxidized?  

     Recall, oxidized means that the substance is losing electrons and reduced means that it is gaining electrons.  In this example, hydrogen peroxide (H₂O₂) is being reduced as it is converted to water because the oxygen has a (-1) oxidation state in the reactants and a (-2) oxidation state in the products. 

If hydrogen peroxide is being reduced, then obviously ascorbic acid is being oxidized.  How can we verify that with oxidation numbers?  Since the carbons in ascorbic acid are not equivalent, and therefore have different oxidation numbers, we need to look at a Lewis structure to see the corresponding change in oxidation numbers between the species that is oxidized (ascorbic acid) and the products.  Lewis structures for the reduced form of Vitamin C (ascorbic acid) and the oxidized form (dehydroxyascorbic acid) are shown in Figure 4, below.  The atoms that participate in the oxidation are circled.    

Molecule Name Two-Dimensional Representation Three-Dimensional Representation Ascorbic Acid (reduced Vitamin C) Dehydroxyascorbic Acid (oxidized Vitamin C)

Figure 4 These  representations of the reduced and and oxidized forms of Vitamin C show how Vitamin C functions as an antioxidant.  The atoms and bonds that participate in oxidation are shown in blue in the two-dimensional representation, and the atoms involved are circled in the three-dimensional representation.  Note: The coordinates for the 3D representations were obtained from molecular-modeling calculations using ChemDraw 3D, and the images were rendered using SwissPDB Viewer and POV-Ray (see References).

Recall:  to determine oxidation numbers from Lewis structures, we begin by assigning electrons in bonds to the more electronegative element.  Lone-pair electrons are assigned to the atom on which they reside.  The oxidation number is equal to the number of valence electrons for that element minus the number of assigned electrons.

     Looking at the carbons highlighted in blue in the Lewis structures in Figure 4, we can see that the oxidation number for carbon changes from (+1) in ascorbic acid to (+2) in dehydroxyascorbic acid.