3.6.1Organic Chemistry
Identification of Functional Groups by Test Tube Reactions — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 24 of 55
Part 1 of 6Alkenes and Halogenoalkanes
These two tests detect a C=C bond and a carbon–halogen bond.
Alkene test
- Shake the sample with bromine water;
- the orange-brown colour disappears as bromine adds across the double bond.
Halogenoalkane step 1
- Warm with aqueous sodium hydroxide so that nucleophilic substitution releases the halide ion into solution.
Halogenoalkane step 2
- Acidify with dilute nitric acid to remove carbonate and hydroxide ions that would give false precipitates.
Halogenoalkane step 3
- Add silver nitrate solutionAg⁺(aq) + X⁻(aq) → AgX(s), a white, cream or yellow precipitate for Cl, Br or I.
Part 2 of 6Aldehydes
Aldehydes are distinguished from ketones because only aldehydes are readily oxidised.
Aldehydes
Tollens’ reagent
- On gentle warming an aldehyde gives a silver mirror as Ag⁺ is reduced to Ag; a ketone gives no reaction.
Fehling’s solution
- On warming with an aldehyde the clear blue solution gives a brick-red precipitate as Cu²⁺ is reduced to copper(I) oxide.
Ketones
- Both tests are negative for ketones because a ketone cannot be oxidised by these mild oxidising agents.
Oxidation product
- In both tests the aldehyde is oxidised to a carboxylic acid, or its salt in alkaline solution.
Part 3 of 6Alcohols and Carboxylic Acids
Alcohols and Carboxylic Acids
Alcohol test
- Warm with acidified potassium dichromate(VI); a primary or secondary alcohol turns the orange solution green.
Tertiary alcohol
- No colour change occurs because a tertiary alcohol cannot be oxidised, so the mixture stays orange.
Acid test
- Add sodium carbonate or sodium hydrogencarbonate solution;
- effervescence of CO₂ shows an acid, confirmed by limewater turning milky.
Equation
- RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O, which works because carboxylic acids are stronger acids than carbonic acid.
Part 4 of 6Summary Table
Summary Table
Combining tests
- A single test rarely proves a structure, so several tests are used together and the results are compared.
Order of tests
- Tests on a small sample are done in separate tubes so one reagent does not interfere with another.
Unsaturation
- Only the alkene test identifies C=C; saturated alkanes do not decolourise bromine water in the dark.
Precaution
- Always state both the reagent and the observation, because marks are awarded for each.
| Group | Reagent | Positive result |
|---|---|---|
| Alkene | Bromine water | Orange-brown to colourless |
| Halogenoalkane | NaOH(aq), HNO₃, AgNO₃ | White, cream or yellow precipitate |
| Aldehyde | Tollens’ reagent | Silver mirror |
| Aldehyde | Fehling’s solution | Blue solution to brick-red precipitate |
| Primary or secondary alcohol | Acidified K₂Cr₂O₇ | Orange to green |
| Carboxylic acid | NaHCO₃(aq) | Bubbles of CO₂ |
Part 5 of 6Worked Example
Question
- An unknown organic compound turns acidified dichromate green, gives no silver mirror with Tollens’ reagent and gives no bubbles with sodium hydrogencarbonate.
- Which functional group is present?
- Green with dichromate shows oxidation, so a primary alcohol, secondary alcohol or aldehyde is present.
- No silver mirror rules out an aldehyde.
- No bubbles with NaHCO₃ rules out a carboxylic acid.
- The compound must therefore be a primary or secondary alcohol, and a tertiary alcohol is excluded because it would not turn green.
Answer The compound contains a primary or secondary alcohol group.
Part 6 of 6Exam Checklist & Tips
3.6.1 – Key Points for the Exam
- Tollens’ reagent Ammoniacal silver nitrate, which gives a silver mirror with aldehydes.
- Fehling’s solution An alkaline solution of copper(II) ions that gives a red precipitate with aldehydes.
- Decolourise To change from a coloured solution to a colourless one.
- Effervescence Bubbles of gas released from a liquid.
Common Mistakes
- Adding silver nitrate to a halogenoalkane without first hydrolysing it, which gives no precipitate.
- Omitting the nitric acid step, so that other ions could also give a precipitate.
- Saying bromine water goes colourless for an alkane; only unsaturated compounds react.
- Describing the Fehling’s positive result as a silver mirror instead of a brick-red precipitate.
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