Oxidation of alcohols is a central reaction pattern in organic chemistry because it converts simple alcohols into more reactive carbonyl compounds. The outcome depends strongly on whether the alcohol is primary, secondary, or tertiary. Primary alcohols can form aldehydes and then carboxylic acids, while secondary alcohols form ketones.
These transformations are widely used in synthesis, medicine, fragrance chemistry, and biochemical metabolism.
At the molecular level, oxidation usually removes hydrogen from the oxygen-bearing carbon and often increases the number of bonds from carbon to oxygen. A primary alcohol, R-CH2OH, can stop at R-CHO under mild conditions or continue to R-COOH under stronger aqueous conditions. A secondary alcohol, R2CHOH, oxidizes to a ketone, R2C=O.
Tertiary alcohols resist normal oxidation because the carbon bearing the OH group has no C-H bond needed for the usual oxidation pathway.
Understanding Chemistry: Oxidation of Alcohols
Oxidation is best understood as a change in electron sharing around carbon. In an alcohol, the carbon joined to the OH group has relatively many bonds to hydrogen. After oxidation, that carbon has a stronger connection to oxygen and fewer connections to hydrogen.
The carbonyl group that forms has a carbon oxygen double bond. This group is polar because oxygen attracts electrons strongly. That polarity makes the carbonyl carbon easier for other particles to attack.
For this reason, oxidation is often used early in a multistep synthesis. It turns a fairly unreactive alcohol into a useful starting point for later reactions.
The point at which a reaction stops depends on the reagent and the conditions. Aldehydes are especially easy to oxidise further. In water, an aldehyde can react with water molecules to form a hydrated structure with two OH groups on the same carbon.
This hydrated form is readily converted into a carboxylic acid by a strong oxidising agent. Dry conditions help prevent this extra step. Chemists may use PCC or DMP when they need to keep the aldehyde product.
In some practical preparations, the aldehyde is distilled away as it forms. Removing it from the reaction mixture reduces the chance of further oxidation.
Secondary alcohols give ketones, but ketones usually remain unchanged under these conditions. The reason is structural. Further oxidation would often require breaking a carbon to carbon bond, which is much harder than removing hydrogen from the alcohol carbon.
Tertiary alcohols show why checking the structure matters before predicting products. They lack the required hydrogen on the carbon carrying the OH group.
Strong reagents can still damage tertiary alcohols in other ways, especially with heat, but this is not the normal alcohol oxidation pathway. Students should first identify the carbon attached directly to oxygen, then count how many other carbon atoms are attached to it.
These reactions appear outside textbook reaction schemes. Ethanol in alcoholic drinks is processed by enzymes in the liver, first into ethanal and then into ethanoic acid related products. The first product contributes to harmful effects after drinking.
In a school laboratory, acidified potassium manganate may change from purple to colourless or pale brown as it reacts. Acidified dichromate changes from orange to green. These colour changes can indicate that oxidation is occurring, though they do not identify the organic product by themselves.
Chromium compounds are toxic and require careful handling and disposal. When writing equations, make sure atoms are balanced and state the conditions clearly. The reagent, water content, temperature, and whether the product is removed can all decide the final product.
Key Facts
- Primary alcohol mild oxidation: R-CH2OH + [O] -> R-CHO + H2O
- Primary alcohol strong oxidation: R-CH2OH + 2[O] -> R-COOH + H2O
- Secondary alcohol oxidation: R2CHOH + [O] -> R2C=O + H2O
- Tertiary alcohols usually do not oxidize under normal conditions because the C-OH carbon has no C-H bond.
- PCC or DMP can oxidize primary alcohols to aldehydes without overoxidation under dry conditions.
- Acidified KMnO4 or K2Cr2O7 can oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones.
Vocabulary
- Oxidation
- Oxidation is a reaction that increases bonding to oxygen, decreases bonding to hydrogen, or increases the oxidation state of an atom.
- Primary alcohol
- A primary alcohol has the carbon bonded to the OH group attached to only one other carbon atom.
- Aldehyde
- An aldehyde is a carbonyl compound with the general structure R-CHO, where the carbonyl carbon is bonded to at least one hydrogen.
- Ketone
- A ketone is a carbonyl compound with the general structure R2C=O, where the carbonyl carbon is bonded to two carbon groups.
- Oxidizing agent
- An oxidizing agent is a substance that causes another substance to be oxidized while it is reduced.
Common Mistakes to Avoid
- Treating all alcohols as if they oxidize the same way is wrong because primary, secondary, and tertiary alcohols give different products.
- Predicting an aldehyde from a secondary alcohol is wrong because secondary alcohols oxidize to ketones, not aldehydes.
- Using aqueous acidified dichromate and expecting a primary alcohol to stop at an aldehyde is wrong because water and strong oxidizing conditions usually continue oxidation to the carboxylic acid.
- Assuming tertiary alcohols easily form ketones is wrong because normal oxidation requires a hydrogen on the carbon that bears the OH group.
Practice Questions
- 1 Write the major organic product when butan-1-ol is treated with PCC. Then write the product when butan-1-ol is heated with acidified K2Cr2O7.
- 2 A 0.200 mol sample of ethanol is fully oxidized to ethanoic acid. Using CH3CH2OH + 2[O] -> CH3COOH + H2O, how many moles of oxygen equivalents, [O], are required?
- 3 Explain why 2-methylpropan-2-ol resists oxidation under normal laboratory conditions, but propan-2-ol oxidizes readily.