Primary alcohols oxidize first to aldehydes and then to carboxylic acids; secondary alcohols oxidize to ketones; tertiary alcohols do not normally oxidize under standard laboratory conditions. This is SL and HL content in Reactivity 3.2.9: Electron transfer reactions.
The Oxidation Process
In organic chemistry, oxidation can involve gaining oxygen, losing hydrogen, or increasing the oxidation state of carbon. Alcohols are commonly heated with acidified potassium dichromate(VI), which changes from orange to green as it is reduced.
At the particle level, oxidation removes two hydrogen atoms from the alcohol: one from the hydroxyl group and one from the carbon bonded to it. A carbontooxygen single bond becomes a carbontooxygen double bond. This explains both carbonyl formation and why tertiary alcohols resist oxidation without carbontocarbon bond cleavage.
The product depends on how many carbon atoms are bonded to the carbon carrying the hydroxyl group.
| Alcohol type | Oxidation product | Key condition or explanation |
|---|---|---|
| Primary | Aldehyde, then carboxylic acid | Distil the aldehyde as it forms to prevent further oxidation; heat under reflux with excess oxidizing agent to form the carboxylic acid |
| Secondary | Ketone | Heat under reflux; the ketone is not normally oxidized further under these conditions |
| Tertiary | No reaction under normal conditions | The carbon carrying the hydroxyl group has no hydrogen atom; oxidation would require breaking a carboncarbon bond |
For example, ethanol undergoes two oxidation stages:
Thus, ethanol forms ethanal and then ethanoic acid. By contrast, propan-2-ol forms propanone:
A common misconception is that tertiary alcohols cannot oxidize because they lack oxygen. They already contain oxygen; the actual issue is the absence of a hydrogen on the carbon bonded to the hydroxyl group.
IB Exam Technique
When asked to deduce an oxidation product, first classify the alcohol, then change only the relevant functional group. Distinguish carefully between distillation and reflux, and do not state that primary alcohols always stop at aldehydes.