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.
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.