Oxidation reactions involve the addition of oxygen or the removal of hydrogen. First, we shall learn to identify structures that can undergo oxidation. Then, we shall learn the reagents that can oxidize the structures. Oxidation reactions require an “activated” carbon atom such as that shown in Figure 1 where the carbon is bonded to an oxygen atom and to at least one hydrogen atom. The carbon atom shown in the red square is bonded to oxygen and to a hydrogen atom. The hydrogen atoms are shown in blue. Oxidation involves the removal of the two blue hydrogen atoms. Figure 1 shows the oxidation of an alcohol and the formation of the carbonyl group. A mild oxidizing agent can accomplish this reaction.
Figure 1. Oxidation of an alcohol.
Aldehydes may be oxidized to carboxylic acids because the carbon atom of an aldehyde is bonded to oxygen and to hydrogen. Figure 2 shows the oxidation of an aldehyde to a carboxylic acid. Again, the oxidation involves a carbon atom that is bonded to oxygen and to hydrogen, but a stronger oxidizing agent is required for this reaction than the one shown in Figure 1.
Figure 2. Oxidation of an aldehyde. Alcohols are classified as primary (Io), secondary (IIo) or tertiary (IIIo), depending on how many hydrogen atoms share the carbon atom bearing a hydroxyl group. Figure 3 shows examples of methyl, Io, IIo and IIIo alcohols, with the carbon atom bearing the hydroxyl group shown in a red square.
Figure 3. Alcohols.
Look at the structures of the four alcohols in Figure 3. Which of these alcohols can be oxididized under normal lab conditions? Those with blue hydrogen atoms can be oxidized. The blue H atoms are bonded to carbon atoms that are also bonded to oxygen. What do we mean by normal lab conditions? We exclude combustion (oxidation) reactions that convert hydrocarbons and alcohols into carbon dioxide and water. We use common laboratory oxidizing reagents.
We are now ready to consider reagents that will oxidize “oxidizable” carbon atoms. Figure 4 shows the four kinds of alcohols and the various oxidized products that can be obtained by oxidizing them with specific oxidizing reagents.
Figure 4. Oxidation reactions.
Summary of Figure 4 Pyridinium chlorochromate (PCC) is a mild oxidizing agent and chromic acid (CrO3/H+) is a strong oxidizing agent. Mild oxidizing agents can oxidize oxidizable carbon atoms in alcohols to aldehydes. Strong oxidizing agents oxidize any oxidizable alcohol carbon until it is no longer oxidizable. PCC oxidizes only methanol and primary alcohols to aldehydes. Chromic acid oxidizes any compound that contains blue hydrogens bonded to a carbon bonded to oxygen and continues to oxidize (remove blue H’s) until no blue H’s are left bonded to carbon.
Table 1 shows mild and strong oxidizing agents.
Table 1. Oxidizing agents.
Mild Oxidizing Agents
Strong Oxidizing Agents
Pyridinium chlorochomate PCC
Chromic acid or
Potassium permanganate KMnO4/OH-
The reagents in Table 1 all contain a transition metal, either chromium(VI) or manganese(VII), that can be reduced. Pyridinium chlorochromate, PCC, is a modified form of chromic acid. PCC can be used in an organic solvent. The combination of an organic solvent and the presence of pyridine decreases the oxidizing power of Cr(VI), making PCC ideal for converting suitable alcohols to aldehydes or ketones. Chromic acid and potassium permanganate are both used in an aqueous medium. Because KMnO4 is used in a basic medium, any organic acid product is produced as a salt that must be acidified to obtain the organic acid.
Oxidation of Arene Side Chains
The strong oxidizing agents listed in Table 1 are capable of oxidizing side chains in arenes to the corresponding carboxylic