Oxidation of alcohols
Oxidising an alcohol removes hydrogen and adds a C=O. A primary alcohol gives an aldehyde, and with strong oxidant a carboxylic acid. A secondary alcohol gives a ketone. A tertiary alcohol does not oxidise easily because it has no hydrogen on the carbon.
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Analogy
Think of removing hydrogen as stripping layers off the carbon until it holds more bonds to oxygen.
Example
Primary ethanol → ethanal → ethanoic acidSecondary propan-2-ol → propanoneTertiary 2-methylpropan-2-ol → no reactionCr₂O₇²⁻ orange → Cr³⁺ green
In practice
The breathalyser test works because ethanol is oxidised by dichromate, which changes colour from orange to green.
Practice
Flashcards
- Oxidising a secondary alcohol gives what?A ketone.
Interview questions
- Describe how to convert ethanol into ethyl ethanoate, and say why the reaction is reversible.
- Oxidise ethanol to ethanoic acid
- Heat with ethanol and acid catalyst
- Esterification forms water
- Reaction reaches equilibrium, remove water for better yield
Questions people ask
How can I stop at the aldehyde and not go on to the acid?
Use a mild oxidant such as PCC in dry solvent, or distil the aldehyde out as it forms, since it boils lower than the alcohol. A strong oxidant in water, such as acidified dichromate with heating under reflux, carries on to the carboxylic acid.
Why can't tertiary alcohols be oxidised easily?
Oxidation needs a hydrogen on the carbon that holds the OH, so a C=O can form. A tertiary alcohol has three carbon groups on that carbon and no hydrogen. Oxidising it would mean breaking a carbon-carbon bond, which needs harsh conditions.