All subjects · Chemistry · WCH12 (Unit 2)

Oxidation of alcohols (Core Practical 7)

Acidified potassium dichromate(VI) oxidises primary and secondary alcohols. Whether you distil or reflux decides which product you get from a primary alcohol.

The oxidising agent

Potassium dichromate(VI) in dilute sulfuric acid. The orange Cr2O7²- is reduced to green Cr3+. In equations, write the oxidising agent as [O].

What forms

Primary alcohol -> aldehyde (distil off as it forms) -> carboxylic acid (heat under reflux with excess oxidising agent):

CH3CH2CH2OH + [O] -> CH3CH2CHO + H2O

CH3CH2CH2OH + 2[O] -> CH3CH2COOH + H2O

Secondary alcohol -> ketone (heat under reflux): CH3CH(OH)CH3 + [O] -> CH3COCH3 + H2O
Tertiary alcohols are not oxidised: the colour stays orange.

Core Practical 7

Propanal: add propan-1-ol slowly to warm acidified dichromate, with the apparatus set up for distillation, and distil off the propanal (bp about 49 °C) as soon as it forms, so it cannot be oxidised further.

Propanoic acid: heat propan-1-ol with excess acidified dichromate under reflux (a vertical condenser returns vapour to the flask), then distil off the acid. Reflux allows prolonged heating without losing volatile substances. Add anti-bumping granules; never heat a sealed system.

Worked example

Explain how you would make propanal, not propanoic acid, from propan-1-ol.

Use a limited amount of acidified dichromate.
Distil immediately: propanal has the lowest boiling point (no hydrogen bonding between its molecules) and leaves the flask before it is oxidised further.

Answer: Distil the aldehyde off as it forms.

Key idea: Primary: aldehyde (distil) or carboxylic acid (reflux). Secondary: ketone. Tertiary: no reaction. Orange Cr2O7²- turns green Cr3+.

Check yourself

What colour change shows that acidified potassium dichromate(VI) has oxidised an alcohol?
- Purple to colourless
- Green to orange
- Orange to green
- Colourless to orange
Show answer
Orange to green. Cr2O7²- (orange) is reduced to Cr3+ (green).
What is the product of oxidising butan-2-ol?
- Butanone
- Butanal
- Butanoic acid
- But-2-ene
Show answer
Butanone. Secondary alcohols give ketones.
Why is heating under reflux used to make propanoic acid?
- It distils the aldehyde off
- It cools the mixture
- The aldehyde formed is kept in the flask until fully oxidised
- It removes water
Show answer
The aldehyde formed is kept in the flask until fully oxidised. Vapours condense and return, so oxidation can go to completion.
Why is propanal distilled off as soon as it forms?
- Because it is a catalyst
- To stop it being oxidised further to propanoic acid
- To increase the temperature
- Because it is denser than water
Show answer
To stop it being oxidised further to propanoic acid. Removing it from the oxidising agent prevents further oxidation.
Which alcohol is not oxidised by acidified dichromate under normal conditions?
- Pentan-1-ol
- Pentan-2-ol
- Ethanol
- 2-methylbutan-2-ol
Show answer
2-methylbutan-2-ol. It is tertiary: there is no H on the C bearing the OH.
Which equation shows the oxidation of ethanol to ethanoic acid?
- CH3CH2OH + [O] -> CH3COOH + H2O
- CH3CH2OH + 2[O] -> CH3COOH + H2O
- CH3CH2OH + [O] -> CH3CHO + H2O
- CH3CH2OH -> CH2=CH2 + H2O
Show answer
CH3CH2OH + 2[O] -> CH3COOH + H2O. Full oxidation to the acid needs 2[O].
Why does propanal have a lower boiling point than propan-1-ol?
- Propanal has more electrons
- Propanal is ionic
- Propanal has a C=C bond
- Propanal molecules cannot form hydrogen bonds with each other
Show answer
Propanal molecules cannot form hydrogen bonds with each other. Propanal has no O-H group, so it has only London and dipole-dipole forces.

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Last updated 7 November 2026. Spotted a mistake? Tell us.