6.2.8Additional lessons
Oxidation of Alcohols — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 91 of 98
Part 1 of 6Classifying the Alcohols
The oxidation product depends on the class of the alcohol and on the reaction conditions.
Primary alcohol
- The carbon bearing OH is attached to one other carbon, such as ethanol CH₃CH₂OH.
Secondary alcohol
- The carbon bearing OH is attached to two other carbons, such as propan-2-ol.
Tertiary alcohol
- The carbon bearing OH is attached to three carbons, so it has no hydrogen on that carbon and resists oxidation.
Oxidising agents
- Acidified potassium dichromate(VII) turns orange to green;
- acidified potassium manganate(VII) turns purple to colourless.
Part 2 of 6Oxidation Products
Oxidation Products
Primary to aldehyde
- Gentle heating with distillation removes the aldehyde as it forms: CH₃CH₂OH + [O] → CH₃CHO + H₂O.
Primary to acid
- Heating under reflux with excess oxidant gives the carboxylic acid: CH₃CHO + [O] → CH₃COOH.
Secondary to ketone
- CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O, and the ketone resists further oxidation.
Tertiary
- No reaction occurs under normal conditions with these oxidising agents.
| Alcohol type | Product | Conditions |
|---|---|---|
| Primary | Aldehyde | Distil immediately |
| Primary | Carboxylic acid | Reflux, excess oxidant |
| Secondary | Ketone | Reflux or heat |
| Tertiary | No reaction | – |
Part 3 of 6Why Reflux or Distil
1
Aldehydes have lower boiling points than alcohols because they cannot form hydrogen bonds with each other.
2
Distilling off the aldehyde as it forms prevents further oxidation to the acid.
3
Refluxing returns vapour to the flask so the volatile aldehyde is oxidised completely to the acid.
4
The orange dichromate colour changing to green shows that oxidation has occurred.
Part 4 of 6Observations and Practical Points
Observations and Practical Points
Colour change
- Orange dichromate(VI) turns green as Cr³⁺ forms, showing oxidation of a primary or secondary alcohol.
No change
- A tertiary alcohol leaves the mixture orange, which distinguishes it from the other two classes.
Apparatus
- Reflux uses a vertical condenser, while distillation uses a downward condenser to collect the product.
Safety
- Flammable alcohols should be heated using an electric heater or water bath and not a naked flame.
Part 5 of 6Worked Example
Question Write the balanced ionic equation for the oxidation of ethanol to ethanal by acidified dichromate(VII) and check the charge balance.
- Half equation for reductionCr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
- Half equation for oxidationCH₃CH₂OH → CH₃CHO + 2H⁺ + 2e⁻, multiplied by 3 to give 6 electrons.
- Add and cancel the 6H⁺ and electrons: 3CH₃CH₂OH + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CHO + 2Cr³⁺ + 7H₂O.
Answer Charges: left −2 + 8 = +6, right 2 × 3 = +6, so the equation is balanced.
Part 6 of 6Exam Checklist & Tips
6.2.8 – Key Points for the Exam
- Primary alcohol An alcohol with the OH group on a carbon bonded to one other carbon.
- Aldehyde A compound with the group –CHO at the end of a carbon chain.
- Ketone A compound with a C=O group inside a carbon chain.
- Reflux Heating with a condenser so vapours return to the flask.
- Distillation Separating a liquid by evaporating it and condensing the vapour.
Common Mistakes
- Saying tertiary alcohols give ketones.
- Forgetting that distillation versus reflux decides between aldehyde and acid.
- Writing the colour change of dichromate as green to orange.
- Giving ethanal as the product of refluxing with excess oxidant; the product is ethanoic acid.
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