3.5.3Physical Chemistry
Oxidation of Alcohols — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 84 of 102
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 5Products of Oxidation
The symbol [O] represents oxygen supplied by the oxidising agent.
Primary alcohol
- Primary alcohol Oxidised first to an aldehyde, for example CH₃CH₂OH + [O] → CH₃CHO + H₂O.
Aldehyde
- Aldehyde Oxidised further to a carboxylic acid, for example CH₃CHO + [O] → CH₃COOH.
Secondary alcohol
- Secondary alcohol Oxidised to a ketone, for example CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O.
Tertiary alcohol
- Tertiary alcohol Not oxidised under these conditions, because the hydroxyl carbon carries no hydrogen atom.
Part 2 of 5Choosing the Conditions
Choosing the Conditions
Reagent
- Reagent Acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid, is the usual oxidising agent.
Distillation
- Distillation To stop at the aldehyde, warm the alcohol with limited oxidising agent and distil the aldehyde off as it forms.
Why distillation works
- Why distillation works The aldehyde cannot form hydrogen bonds with itself, so it boils at a lower temperature than the alcohol.
Reflux
- Reflux To reach the carboxylic acid, heat under reflux with excess oxidising agent so the aldehyde stays in the flask and is fully oxidised.
Part 3 of 5Colour Changes
1
Acidified dichromate(VI) starts as an orange solution containing Cr₂O₇²⁻ ions.
2
When an alcohol or aldehyde is oxidised, the dichromate ions are reduced to Cr³⁺ ions.
3
The solution therefore turns from orange to green for primary and secondary alcohols.
4
With a tertiary alcohol there is no reaction, so the solution stays orange.
5
Acidified potassium manganate(VII) is an alternative, changing from purple to colourless as Mn²⁺ forms.
Part 4 of 5Distinguishing Aldehydes from Ketones
Distinguishing Aldehydes from Ketones
Fehling's solution
- Fehling's solution Warmed with an aldehyde, the blue Cu²⁺ solution gives a brick-red precipitate of copper(I) oxide.
Tollens' reagent
- Tollens' reagent Warmed with an aldehyde, ammoniacal silver nitrate deposits a silver mirror as Ag⁺ is reduced to silver.
Ketones
- Ketones Ketones cannot be oxidised easily, so both reagents show no visible change with a ketone.
Why it works
- Why it works Aldehydes have a hydrogen on the carbonyl carbon, so they are oxidised to carboxylic acids while ketones are not.
| Test | Aldehyde | Ketone |
|---|---|---|
| Fehling's solution, warmed | Blue to brick-red precipitate | Stays blue |
| Tollens' reagent, warmed | Silver mirror | No visible change |
| Acidified dichromate, warmed | Orange to green | Stays orange |
Part 5 of 5Exam Checklist & Tips
3.5.3 – Key Points for the Exam
- Oxidation
- In organic chemistry, gain of oxygen or loss of hydrogen;
- the oxidation number of carbon increases.
- In organic chemistry, gain of oxygen or loss of hydrogen;
- Reflux Heating a mixture with a condenser fitted vertically so vapours condense and return to the flask.
- Distillation Heating a mixture and collecting the vapour that condenses, to separate a lower-boiling product.
- Fehling's solution An alkaline solution of copper(II) ions used to detect aldehydes.
- Tollens' reagent Ammoniacal silver nitrate, which gives a silver mirror with aldehydes.
Common Mistakes
- Using reflux when the aim is to make an aldehyde; the aldehyde is oxidised further.
- Saying a ketone can be oxidised to a carboxylic acid under these conditions.
- Writing the colour change for dichromate as green to orange instead of orange to green.
- Closing the top of the reflux condenser; it must stay open to the air to avoid pressure build-up.
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