6.2.8Additional lessons
Oxidation of Alcohols — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 139 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Classification and Products
The class of alcohol determines which product forms.
Primary alcohol
- Primary alcohol Oxidation gives first an aldehyde and then, with more oxidant, a carboxylic acid.
Secondary alcohol
- Secondary alcohol Oxidation gives a ketone, and no further oxidation occurs under normal conditions.
Tertiary alcohol
- Tertiary alcohol These have no hydrogen on the carbon bearing the OH group, so they resist oxidation.
Oxidising agent
- Oxidising agent Acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid, is the usual reagent.
Section 2 of 4Primary Alcohol Oxidation
1
Ethanol is warmed with acidified dichromate(VI), and the orange Cr₂O₇²⁻ gradually turns green as Cr³⁺ forms.
2
To stop at the aldehyde the product ethanal is distilled off as it forms, because it boils at a lower temperature.
3
The equation using oxidant shorthand is CH₃CH₂OH + [O] → CH₃CHO + H₂O.
4
To obtain the carboxylic acid the mixture is heated under reflux with excess oxidant, so the aldehyde cannot escape.
5
The second step is CH₃CHO + [O] → CH₃COOH, giving ethanoic acid.
Section 3 of 4Secondary Alcohols and Tests
Secondary Alcohols and Tests
Secondary oxidation
- Secondary oxidation Propan-2-ol + [O] → propanone + H₂O, and the solution goes from orange to green.
Tertiary result
- Tertiary result The mixture stays orange because no reaction occurs with a tertiary alcohol such as 2-methylpropan-2-ol.
Distillation versus reflux
- Distillation versus reflux Distillation isolates the aldehyde, whereas reflux keeps volatile intermediates in the flask until fully oxidised.
Ionic equation
- Ionic equation 3CH₃CH₂OH + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CHO + 2Cr³⁺ + 7H₂O shows the redox change in full.
| Alcohol | Product | Colour change |
|---|---|---|
| Primary | Aldehyde then carboxylic acid | Orange to green |
| Secondary | Ketone | Orange to green |
| Tertiary | No reaction | Stays orange |
Section 4 of 4Exam Checklist & Tips
6.2.8 – Key Points for the Exam
- Primary alcohol An alcohol with the OH carbon bonded to only one other carbon atom.
- Reflux Heating with a condenser so that vapours return to the flask.
- Distillation Heating so that a lower-boiling product is collected as it forms.
- Oxidant [O] Shorthand used to balance organic oxidation equations.
Common Mistakes
- Forgetting reflux is needed for the carboxylic acid, and distillation for the aldehyde.
- Saying that tertiary alcohols turn the dichromate green.
- Writing the colour change as orange to blue instead of orange to green.
- Leaving water out of the product side when balancing [O] equations.
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