Test-tube reactions for functional groupsAQA A-Level Chemistry: Revision notes
Section 1
Alkenes: bromine water
An alkene decolourises bromine water quickly at room temperature. The orange solution turns colourless as bromine adds across the C=C double bond (electrophilic addition).
- CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
Alkanes and most other compounds with no C=C bond do not change the colour in the same way.
Say 'orange to colourless', not 'clear'. Colourless means no colour.
Section 2
Alcohols: acidified potassium dichromate(VI)
Warm the sample with acidified potassium dichromate(VI). A primary or secondary alcohol is oxidised and the mixture turns from orange to green (Cr₂O₇²⁻ reduced to Cr³⁺). A tertiary alcohol gives no change.
Aldehydes are also oxidised and give the same colour change, so a green result alone does not prove an alcohol.
Dichromate(VI) turns green with primary alcohols, secondary alcohols and aldehydes. It cannot tell an alcohol from an aldehyde by itself.
Section 3
Aldehydes: Tollens' reagent and Fehling's solution
Aldehydes are oxidised to carboxylic acids by mild oxidising agents; ketones are not.
- Tollens' reagent (warm): silver mirror, Ag⁺ + e⁻ → Ag
- Fehling's solution (warm): blue to a red precipitate of copper(I) oxide
Ketones give no change with either reagent. This distinguishes aldehydes from ketones.
Section 4
Carboxylic acids: carbonates
Carboxylic acids are strong enough acids to react with carbonates and hydrogencarbonates, giving effervescence of carbon dioxide.
- 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
The gas turns limewater milky. Alcohols, aldehydes and ketones do not react with sodium carbonate solution.
Section 5
Halogenoalkanes: hydrolysis and silver nitrate
Warm the halogenoalkane with aqueous sodium hydroxide so that the halogen is replaced by OH and the halide ion is released. Acidify with dilute nitric acid to remove excess OH⁻, then add aqueous silver nitrate.
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Chloride: white precipitate
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Bromide: cream precipitate
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Iodide: yellow precipitate
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Ag⁺ + Br⁻ → AgBr
Section 6
Identifying functional groups: required practical 6
In required practical 6 you carry out tests for an alcohol, an aldehyde, an alkene and a carboxylic acid on unknown samples.
- Test with all the reagents and record each observation.
- Use a hot water bath, not a naked flame, for flammable organic liquids.
- Combine results. A liquid that turns dichromate green and gives a silver mirror is an aldehyde; one that turns it green with no mirror is an alcohol (primary or secondary).
- A positive result for one group does not exclude another group in the same molecule.
Write the reagent, the conditions and the observation for every test, with the colour before and after.
Must Know
- Alkene: bromine water, orange to colourless
- Alcohol (primary, secondary): warm acidified dichromate(VI), orange to green
- Aldehyde: Tollens' silver mirror or Fehling's red precipitate; ketone: no change
- Carboxylic acid: effervescence of CO₂ with sodium carbonate
- Halogenoalkane: NaOH(aq), then HNO₃, then AgNO₃(aq): white, cream or yellow precipitate
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Test-tube reactions for functional groups
- A technician has four unlabelled bottles. Each contains one of hex-1-ene, hexan-1-ol, hexanal and hexanoic acid. She labels them W, X, Y and Z and plans simple test-tube reactions to identify each liquid.Write an equation for the reaction of hexanoic acid with aqueous sodium carbonate.2 marks
- A student is given an organic liquid that might be a halogenoalkane. She warms it with aqueous sodium hydroxide, cools the mixture, acidifies it with dilute nitric acid and then adds aqueous silver nitrate. A cream precipitate forms.Name the type of reaction between the halogenoalkane and aqueous sodium hydroxide, and explain how this reaction leads to the cream precipitate.2 marks
- An analyst has an unlabelled liquid with the molecular formula C₃H₆O. It could be propanal, propanone or prop-2-en-1-ol, CH₂=CHCH₂OH.Describe the observations when each of the three compounds is warmed separately with acidified potassium dichromate(VI) and with Tollens' reagent.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).