2.9.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Oxidation of Alcohols — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 53 of 53 Unit quiz
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.
Part 1 of 5Oxidising Alcohols
Acidified potassium dichromate(VI) is the standard oxidising agent.
Reagent
- Reagent K₂Cr₂O₇ in dilute sulfuric acid is orange because it contains the dichromate(VI) ion Cr₂O₇²⁻.
Colour change
- Colour change When the alcohol is oxidised, Cr₂O₇²⁻ is reduced and the solution turns from orange to green as Cr³⁺ ions form.
Oxygen symbol
- Oxygen symbol In organic equations the oxidising agent can be shown as [O], which stands for oxygen supplied by the oxidant.
Tertiary alcohols
- Tertiary alcohols These cannot be oxidised under these conditions, so the solution stays orange.
Part 2 of 5Products of Oxidation
Products of Oxidation
Primary alcohol
- Primary alcohol Oxidises first to an aldehyde, CH₃CH₂OH + [O] → CH₃CHO + H₂O, and then to a carboxylic acid, CH₃CHO + [O] → CH₃COOH.
Secondary alcohol
- Secondary alcohol Oxidises to a ketone only: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O.
Tertiary alcohol
- Tertiary alcohol Resists oxidation because the carbon bearing the –OH has no hydrogen atom to lose.
Ketones
- Ketones Ketones have no hydrogen on the carbonyl carbon, so further oxidation to an acid does not occur.
| Alcohol class | Oxidation product | Colour change in dichromate |
|---|---|---|
| Primary | Aldehyde, then carboxylic acid | Orange to green |
| Secondary | Ketone | Orange to green |
| Tertiary | No reaction | Stays orange |
Part 3 of 5Reflux or Distillation
The apparatus decides whether the aldehyde or the acid is obtained.
1
To make the aldehyde, add the alcohol slowly to warm acidified dichromate and distil the product out immediately.
2
The aldehyde boils at a lower temperature than the alcohol because it cannot form hydrogen bonds with itself.
3
Removing the aldehyde from the flask as it forms stops it meeting more oxidising agent and becoming an acid.
4
To make the carboxylic acid, heat the alcohol with excess acidified dichromate under reflux so nothing escapes.
5
Reflux with a vertical condenser keeps the aldehyde in the flask long enough to be oxidised completely to the acid.
Part 4 of 5Testing for Aldehydes
Aldehydes are oxidised easily, ketones are not, so mild oxidants tell them apart.
Fehling’s solution
- Fehling’s solution Warming an aldehyde with blue alkaline copper(II) solution gives a red-brown precipitate of copper(I) oxide.
Tollens’ reagent
- Tollens’ reagent Warming an aldehyde with ammoniacal silver nitrate deposits a silver mirror as Ag⁺ is reduced to Ag.
Ketones
- Ketones Ketones give no change with either reagent, so a negative result identifies a ketone.
Why it works
- Why it works The oxidising agent is reduced while the aldehyde group is oxidised to a carboxylate ion in alkaline conditions.
Part 5 of 5Exam Checklist & Tips
2.9.3 – Key Points for the Exam
- Oxidation
- Loss of electrons;
- in organic chemistry usually a gain of oxygen or a loss of hydrogen.
- Loss of electrons;
- Aldehyde A compound with the –CHO group at the end of a carbon chain.
- Ketone A compound with a C=O group bonded to two carbon atoms within the chain.
- Reflux Continuous boiling and condensing of the mixture to allow complete reaction without loss of volatile material.
Common Mistakes
- Stating that a tertiary alcohol turns dichromate green; it stays orange because no oxidation occurs.
- Using distillation when the question asks for the carboxylic acid, or reflux when the aldehyde is required.
- Writing the aldehyde as CH₃COH instead of CH₃CHO, which suggests the wrong structure.
- Saying that Tollens’ reagent forms a silver precipitate with ketones.
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