Functional Group Identification Tests – Complete Guide | EverExams

Identification of Functional Groups

A comprehensive guide to organic chemistry tests for aldehydes, ketones, alkenes, and alcohols

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Fehling’s Reagent Test

Test Interpretation

Positive Result: Formation of an orange/red precipitate

Indicates: Presence of an aldehyde group (-CHO)

Mechanism: Fehling’s reagent oxidizes aldehydes to carboxylic acids, reducing Cu²⁺ to Cu⁺ and forming copper(I) oxide (Cu₂O) precipitate.

Materials & Procedure

Materials: Fehling’s A (aqueous CuSO₄), Fehling’s B (alkaline KNa tartrate), test tube, Bunsen burner/water bath

Procedure:

  1. Mix equal volumes of Fehling’s A and B solutions
  2. Add a few drops of organic compound
  3. Heat gently in water bath for a few minutes
  4. Observe color change/precipitate formation

Chemical Reaction

CH₃CHO + 2Cu²⁺ + 5OH⁻ → Cu₂O↓ (red ppt) + CH₃COO⁻ + 3H₂O

Aldehyde oxidized to carboxylate ion, Cu²⁺ reduced to Cu⁺

Memory Tip

Remember: “Fehling’s Finds Aldehydes Fast” – The red precipitate (like rust) confirms aldehydes. Only aliphatic aldehydes give positive test; aromatic aldehydes do not.

Trick: Think of Fehling’s reagent as a “copper mirror” test – if it turns brick red, aldehydes are in your compound!

Tollen’s Reagent Test (Silver Mirror Test)

Test Interpretation

Positive Result: Formation of a silver mirror or black precipitate

Indicates: Presence of an aldehyde group (-CHO)

Mechanism: Tollen’s reagent (ammoniacal Ag⁺) oxidizes aldehydes to carboxylic acids, reducing Ag⁺ to metallic Ag.

Materials & Procedure

Materials: AgNO₃ solution, NaOH solution, NH₄OH, test tube, water bath

Procedure:

  1. Add AgNO₃ solution to test tube
  2. Add NaOH dropwise (forms brown Ag₂O precipitate)
  3. Add NH₄OH dropwise until precipitate dissolves
  4. Add organic compound and heat gently
  5. Observe for silver mirror/black precipitate

Chemical Reaction

CH₃CHO(aq) + 2[Ag(NH₃)₂]⁺(aq) + 2OH⁻(aq) → CH₃COO⁻(aq) + 4NH₃(aq) + 2Ag(s)↓ + H₂O(l)

Silver mirror forms on test tube walls – a classic positive test!

Memory Tip

Remember: “Tollen’s Tells Aldehydes Too” – The silver mirror is unmistakable. Both aliphatic and aromatic aldehydes give positive test.

Trick: Think of Tollen’s as the “silver mirror” test – if you see your reflection in the test tube, you’ve got an aldehyde!

Caution: Tollen’s reagent can form explosive silver nitride if stored – always prepare fresh!

Alkaline Aqueous Iodine Test (Iodoform Test)

Test Interpretation

Positive Result: Formation of a yellow precipitate

Indicates: Methyl ketone (CH₃CO-) or ethanol/2° alcohol with CH₃CH(OH)- group

Mechanism: Iodoform (CHI₃) precipitates from reaction with compounds containing CH₃CO- or CH₃CH(OH)- groups.

Materials & Procedure

Materials: I₂ in KI solution, NaOH solution, test tube, warm water bath

Procedure:

  1. Add test compound to test tube
  2. Add iodine solution
  3. Add NaOH dropwise while shaking
  4. Place in warm water bath (~60°C) for a few minutes
  5. Observe for yellow precipitate

Chemical Reactions

CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ (yellow) + CH₃COONa + 3NaI + 3H₂O
CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃↓ (yellow) + HCOONa + 5NaI + 5H₂O

Both methyl ketones and ethanol/2° alcohols with CH₃CH(OH)- give positive test!

Memory Tip

Remember: “Iodoform Identifies Methyl Groups” – The yellow precipitate is iodoform (CHI₃), which has a characteristic antiseptic smell.

Trick: Think “CHI₃” – if you see a yellow precipitate with iodine in base, you likely have a methyl group attached to carbonyl or alcohol.

Note: Ethanol is the only primary alcohol that gives a positive iodoform test.

Acidified Potassium Permanganate(VI) Test

Test Interpretation

Positive Result: Decolorization of purple solution to colorless

Indicates: Oxidizable compounds: alkenes (C=C), primary alcohols, or aldehydes

Mechanism: KMnO₄ is reduced from Mn(VII) to Mn(II), losing its purple color as it oxidizes the compound.

Materials & Procedure

Materials: KMnO₄ solution, dilute H₂SO₄, test tube

Procedure:

  1. Add dilute H₂SO₄ to test tube
  2. Add KMnO₄ solution (purple color appears)
  3. Add test compound
  4. Shake gently and observe color change

Chemical Reactions

3CH₂=CH₂ + 2KMnO₄ + 4H₂O → 3CH₂OH-CH₂OH + 2MnO₂ + 2KOH
5R-CHO + 2MnO₄⁻ + 6H⁺ → 5R-COOH + 2Mn²⁺ + 3H₂O

Purple MnO₄⁻ reduced to colorless Mn²⁺ when it oxidizes alkenes, alcohols, or aldehydes

Memory Tip

Remember: “Purple Power” – If purple KMnO₄ loses color, something oxidizable is present. This is a general test for unsaturation or oxidizable groups.

Trick: Think of KMnO₄ as a “color thief” – it steals electrons from oxidizable compounds and loses its purple color in the process.

Note: Aldehydes give positive with Fehling’s, Tollen’s AND KMnO₄ tests. Alkenes give positive only with KMnO₄ and bromine water.

Practice Quiz – Functional Group Tests

Test your knowledge with these 20 multiple choice questions. Select your answer and click submit to check your score.

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Study Guidelines & Tips

How to Master Functional Group Tests

  • Understand the mechanism: Don’t just memorize results – understand why each test works.
  • Create comparison charts: Make a table comparing all tests, their results, and compounds that give positive results.
  • Practice with examples: Test yourself with common compounds like ethanol, propanal, acetone, etc.
  • Focus on exceptions: Note which compounds are exceptions (e.g., aromatic aldehydes with Fehling’s test).
  • Use mnemonics: Create memory aids like “Fehling’s Finds Aldehydes Fast” to remember key points.
  • Practice safety: Always remember safety precautions for each test (e.g., fresh Tollen’s reagent).
  • Connect to real-world: Relate tests to their applications (food industry, forensics, etc.).
  • Review regularly: Functional group identification is cumulative – regular review is essential.

Daily Life Applications

Fehling’s Test

Used in food industry to test for reducing sugars (glucose, fructose) in beverages and food products.

Tollen’s Test

Applied in forensic science to detect aldehydes in fingerprint analysis and crime scene investigations.

Iodoform Test

Used in pharmaceuticals to identify methyl ketones in drug formulations and quality control.

KMnO₄ Test

Utilized in environmental chemistry to test for oxidizable pollutants in water samples.

Short Answer Questions (1 mark each)

  1. Describe the procedure for performing Fehling’s reagent test.
  2. What is the chemical reaction involved in Tollen’s reagent test?
  3. Explain why the iodoform test is used to identify methyl ketones.
  4. How does acidified potassium permanganate act as an oxidizing agent?
  5. What is the significance of observing a silver mirror in Tollen’s test?
  6. Why is it necessary to heat the reaction mixture in Fehling’s test?
  7. What compounds other than aldehydes can be oxidized by acidified KMnO₄?
  8. Describe the visual difference between positive and negative Fehling’s test.
  9. What structural feature leads to a positive iodoform test?
  10. How can you distinguish between an aldehyde and a primary alcohol using chemical tests?