Oxidation and Tests for Carbonyl Compounds
A concise revision guide to the oxidation of aldehydes and the tests that tell aldehydes from ketones: Fehling’s solution, Tollens’ reagent, acidified dichromate(VI).
- 3.3.8i
- 3.3.8ii-a
- 3.3.8ii-b
What these spec points say
- 3.3.8i know that aldehydes are readily oxidised to carboxylic acids
- 3.3.8ii-a use Fehling's solution to distinguish between aldehydes and ketones
- 3.3.8ii-b use Tollens' reagent to distinguish between aldehydes and ketones
Before You Start
This page follows Aldehydes and Ketones: Structure, Naming and Properties and uses the oxidation of alcohols from 3.3.5 Alcohols.
- An aldehyde has a hydrogen atom on its C=O carbon; a ketone does not.
- Acidified potassium dichromate(VI) is an oxidising agent: it turns from orange to green when it is reduced.
- In equations the oxidising agent is written as [O].
Oxidation of Aldehydes
Aldehydes are easily oxidised to carboxylic acids. Ketones are not oxidised by the same reagents.
CH₃CHO + [O] → CH₃COOH
- The reagent is potassium dichromate(VI), K₂Cr₂O₇, acidified with dilute sulfuric acid, and the mixture is heated under reflux.
- The orange dichromate(VI) ion, Cr₂O₇²⁻, is reduced to the green chromium(III) ion, Cr³⁺.
- A ketone leaves the solution orange.
Why ketones resist oxidation
- Oxidising an aldehyde removes the hydrogen on the C=O carbon and puts an –OH in its place. Only a C–H bond is broken.
- A ketone has no hydrogen on its C=O carbon. Oxidation would need a strong C–C bond to break.
- Because they are so easily oxidised, aldehydes are reducing agents. Every test below uses this.
The ionic equation
3CH₃CHO + Cr₂O₇²⁻ + 8H⁺ → 3CH₃COOH + 2Cr³⁺ + 4H₂O
Oxidation routes from primary and secondary alcohols, and the colour change with acidified dichromate(VI): green with an aldehyde, still orange with a ketone.
Remember: Acidified dichromate(VI) also turns green with primary and secondary alcohols. It separates an aldehyde from a ketone, but it does not prove that a compound is an aldehyde.
Tollens’ Reagent: the Silver Mirror Test
Tollens’ reagent is a colourless solution of silver nitrate in aqueous ammonia. It contains the complex ion [Ag(NH₃)₂]⁺ and is a mild oxidising agent.
Method
- Add a few drops of NaOH(aq) to silver nitrate solution: a brown precipitate of silver oxide forms.
- Add dilute ammonia drop by drop until the precipitate just dissolves.
- Add the carbonyl compound and warm the tube in a water bath at about 60 °C.
| Compound | Observation | Explanation |
|---|---|---|
| aldehyde | a silver mirror forms on the inside of the tube | aldehyde oxidised; Ag⁺ reduced to Ag |
| ketone | no change: colourless solution | ketones are not oxidised |
- The aldehyde is oxidised to a carboxylic acid, which forms its carboxylate ion in the alkaline solution.
- The silver ions are reduced to silver: Ag⁺(aq) + e⁻ → Ag(s).
- Overall: CH₃CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → CH₃COO⁻ + 2Ag + 4NH₃ + 2H₂O.
- A water bath is used because carbonyl compounds are flammable. The reagent is made fresh and never stored.
Making Tollens’ reagent and using it: a silver mirror with an aldehyde, no change with a ketone, and silver ions gaining electrons.
Exam focus: State the reagent, the condition and both observations: Tollens’ reagent, warm; an aldehyde gives a silver mirror; a ketone gives no change.
Quick Check: Reasoning With Oxidation Tests
Pick the one accurate statement in each set, using compounds that are new to this page.
Fehling’s Solution
Fehling’s solution is a deep blue alkaline solution containing complexed copper(II) ions, Cu²⁺. Like Tollens’ reagent, it is a mild oxidising agent.
| Compound | Observation on warming | Explanation |
|---|---|---|
| aldehyde | blue solution gives a brick-red precipitate | aldehyde oxidised; Cu²⁺ reduced to copper(I) oxide, Cu₂O |
| ketone | solution stays blue | ketones are not oxidised |
- The aldehyde is oxidised to a carboxylic acid, present as its carboxylate ion in the alkaline solution.
- Copper(II) is reduced to copper(I): 2Cu²⁺ + 2OH⁻ + 2e⁻ → Cu₂O + H₂O.
- Overall: CH₃CHO + 2Cu²⁺ + 5OH⁻ → CH₃COO⁻ + Cu₂O + 3H₂O.
Fehling’s solution warmed with an aldehyde and with a ketone: a brick-red precipitate of copper(I) oxide, or a solution that stays blue.
Exam focus: Give the colour and the state: a brick-red precipitate, not “the solution turns red”. Name the solid as copper(I) oxide, Cu₂O.
Check: Telling Aldehydes From Ketones
Predict the result of oxidising tests and explain them as redox reactions, with compounds not used on this page.
Summary of the Tests
| Reagent | Aldehyde | Ketone | Distinguishes |
|---|---|---|---|
| Acidified K₂Cr₂O₇, warm | orange solution turns green | stays orange | aldehyde from ketone |
| Tollens’ reagent, warm | silver mirror | no change | aldehyde from ketone |
| Fehling’s solution, warm | blue solution gives a brick-red precipitate | stays blue | aldehyde from ketone |
- Each of these tests relies on the aldehyde being oxidised while the ketone is not.
- Then decide between aldehyde and ketone with Tollens’ reagent or Fehling’s solution.
- In every positive test the aldehyde becomes a carboxylic acid (or its ion).
Exam focus: A full answer to “describe a test” gives the reagent, the conditions, the observation with each compound and what the observation shows.
Check: Choosing and Reading the Tests
Match reagents to observations and decide what each result shows, using compounds not used on this page.
Worked Example: Identifying an Unknown
Compound X has the molecular formula C₄H₈O. It is known to be a carbonyl compound. It gives a silver mirror with Tollens’ reagent.
Step 1. The carbonyl isomers of C₄H₈O are butanal, 2-methylpropanal and butanone.
The oxidation product of butanal would be butanoic acid: CH₃CH₂CH₂CHO + [O] → CH₃CH₂CH₂COOH.
Common Exam Points
Say
- “The aldehyde is oxidised to a carboxylic acid.”
- “Silver ions are reduced to silver, forming a silver mirror.”
- “A brick-red precipitate of copper(I) oxide forms.”
Do not say
- “Goes clear” (say colourless) or “silver precipitate” (say silver mirror).
- “The ketone is oxidised slowly” (it is not oxidised by these reagents).
- “The dichromate turns from green to orange” (it is orange to green).
Watch for
- Acidified dichromate(VI) reacting with alcohols as well as aldehydes.
- Equations with [O]: one [O] per aldehyde, and no water formed.
- Alkaline reagents: the acid is formed as its carboxylate ion, RCOO⁻.
Check: Identifying Unknown Carbonyl Compounds
Combine the results of several tests to work out structures, with unknowns not used on this page.
FAQs
Use these quick answers to check the oxidation of aldehydes and the tests for carbonyl compounds.
Why can aldehydes be oxidised but ketones cannot?
An aldehyde has a hydrogen atom on its carbonyl carbon, which is replaced as the CHO group becomes COOH. A ketone has no such hydrogen, so oxidation would mean breaking a carbon-carbon bond, and mild oxidising agents cannot do that.
What happens in the silver mirror test?
An aldehyde, warmed gently with Tollens’ reagent, coats the tube with a silver mirror. The aldehyde is oxidised to a carboxylic acid, present as its salt, while [Ag(NH₃)₂]⁺ ions are reduced to silver. A ketone leaves the solution colourless.
What does Fehling’s solution show?
On warming, an aldehyde reduces the blue copper(II) complex to a brick-red precipitate of copper(I) oxide, Cu₂O, and is itself oxidised to a carboxylic acid. A ketone is not oxidised, so the solution stays blue.
What is seen when an aldehyde is warmed with acidified dichromate(VI)?
The solution turns from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺ and the aldehyde is oxidised to a carboxylic acid. A ketone leaves it orange. Primary and secondary alcohols also give this change, so the test is not specific.
How do I tell propanal from propanone?
Warm each with Tollens’ reagent or Fehling’s solution. Propanal gives a silver mirror or a brick-red precipitate because it is oxidised to propanoic acid. Propanone gives no change because ketones are not oxidised by these reagents.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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