Compare primary, secondary and tertiary alcohol oxidation
Predict aldehydes, carboxylic acids or ketones from alcohol class and interpret dichromate colour change.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a secondary alcohol form on oxidation?
What you will learn
- Predict primary alcohol oxidation products
- Predict secondary and tertiary outcomes
- Use orange-to-green dichromate as evidence
The OH-bearing carbon controls the oxidation product
A primary alcohol gives an aldehyde when distilled and can give a carboxylic acid under reflux. A secondary alcohol oxidises to a ketone; a tertiary alcohol cannot be oxidised under the syllabus conditions.
For example, propan-2-ol gives propanone. In a ketone, the former C–OH carbon becomes C=O and remains attached to two carbon groups.
Use a mild oxidation test carefully
Acidified potassium dichromate(VI) is orange and changes to green when it is reduced during oxidation of a primary or secondary alcohol. A tertiary alcohol shows no such oxidation under these conditions.
The colour change indicates a reaction with the oxidising reagent; it does not by itself distinguish a primary alcohol from a secondary alcohol. Product collection or another test is needed to tell those classes apart.
Pause and explain
What colour change indicates acidified dichromate has been reduced during alcohol oxidation?
Worked example
Predict the outcome when butan-2-ol and 2-methylpropan-2-ol are warmed with acidified dichromate.
Show the worked solution
- Butan-2-ol is secondary and can be oxidised to butanone.
- 2-methylpropan-2-ol is tertiary and cannot be oxidised in this syllabus model.
- Orange-to-green change is expected only for the oxidised sample.
Answer Butan-2-ol → butanone with dichromate orange to green; the tertiary alcohol shows no oxidation and no corresponding colour change.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict the product from cyclohexanol oxidation and state the class of alcohol.
Give me a hint
Identify the carbon neighbours at the OH-bearing carbon.
Compare my reasoning
- The OH-bearing ring carbon has two carbon neighbours.
- It is secondary.
- Oxidation gives cyclohexanone.
Cyclohexanol is secondary and oxidises to cyclohexanone.
Look for these in your work
- Class correct
- Product correct
- Reason stated
An unknown alcohol leaves acidified dichromate orange after warming. What class is consistent with this result, and what limitation remains?
Give me a hint
Check which alcohol class does not oxidise in the stated conditions.
Compare my reasoning
- Tertiary alcohols cannot be oxidised in this syllabus model.
- Primary and secondary alcohols would reduce dichromate, giving green.
- The result supports tertiary, but verify test conditions and sample identity.
A tertiary alcohol is consistent with no orange-to-green change; a positive colour change alone would not distinguish primary from secondary.
Look for these in your work
- No-change inference made
- Positive-test limitation stated
- Conditions bounded
How could a primary alcohol be oxidised selectively to an aldehyde rather than the acid?
Give me a hint
Use the product's volatility and the apparatus.
Compare my reasoning
- Use acidified dichromate or manganate(VII).
- Distil the aldehyde as it forms.
- Reflux would allow further oxidation to the acid.
Heat with acidified oxidant and distil the aldehyde from the mixture.
Look for these in your work
- Reagent stated
- Distillation chosen
- Further oxidation considered
Common mistakes
- Predicting a carboxylic acid from a secondary alcohol
- Claiming a tertiary alcohol gives a ketone
- Using orange-to-green colour alone to distinguish primary from secondary
What can you explain now?
What does a secondary alcohol form on oxidation?
Compare with the explanation
A ketone
The carbon carrying OH becomes a carbonyl carbon attached to two carbon groups.
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Make it stick
What do alcohol class and distillation or reflux together tell you about the oxidation product?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.