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3.6.1Organic Chemistry

Identification of Functional Groups by Test Tube Reactions

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Section 1 of 5Alkenes and Halogenoalkanes

3.6.1 Identification of Functional Groups by Test Tube Reactions

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.

Section 2 of 5Aldehydes

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.

Section 3 of 5Alcohols 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.

Section 4 of 5Worked 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?

  1. Green with dichromate shows oxidation, so a primary alcohol, secondary alcohol or aldehyde is present.
  2. No silver mirror rules out an aldehyde.
  3. No bubbles with NaHCO₃ rules out a carboxylic acid.
  4. 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.

Section 5 of 5Exam 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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