Required Practical 5 • Unit 2 / Unit 5

RP5: Testing for Organic Functional Groups

Diagnostic qualitative test-tube reactions to identify alkenes, halogenoalkanes, primary/secondary/tertiary alcohols, aldehydes, ketones, and carboxylic acids with precise observation terminology.

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1. Master Diagnostic Matrix: Organic Functional Group Tests

In OxfordAQA Unit 5, candidates are given results of functional group tests and expected to deduce chemical structures, reagents, observations, and balanced equations:

Functional Group Diagnostic Reagent & Conditions Positive Observation Reaction Type / Equation
Alkene (C=C) Bromine water (Br₂(aq)), room temperature, shake. Orange/brown to colourless (decolourisation). Electrophilic addition: forms bromoalcohol or dibromoalkane.
Halogenoalkane (R-X)
(Extension beyond the four RP5 tests)		 1. Warm with NaOH(aq) in water bath.
2. Acidify with dilute HNO₃.
3. Add AgNO₃(aq).		 Precipitate of silver halide:
- R-Cl: White ppt (soluble in dilute NH₃)
- R-Br: Cream ppt (soluble in conc. NH₃)
- R-I: Yellow ppt (insoluble in conc. NH₃)		 Nucleophilic substitution (hydrolysis) releasing X⁻(aq), followed by precipitation with Ag⁺(aq).
Primary / Secondary Alcohol Acidified potassium dichromate(VI) (K₂Cr₂O₇ / H₂SO₄), warm gently. Orange solution turns green. Oxidation of alcohol: Cr₂O₇²⁻ reduced to green Cr³⁺(aq).
Tertiary Alcohol Acidified potassium dichromate(VI), warm gently. Solution remains orange (no reaction). No hydrogen atom on carbinol carbon; resistant to oxidation.
Aldehyde (R-CHO) Tollens' Reagent ([Ag(NH₃)₂]⁺), warm in water bath. Silver mirror coats inside of test tube. Aldehyde oxidised to carboxylate; Ag⁺ reduced to metallic Ag(s).
Aldehyde (R-CHO) Fehling's Solution (Cu²⁺ complex in alkali), warm in water bath. Deep blue solution → brick-red precipitate. Cu²⁺ reduced to Cu⁺ as copper(I) oxide (Cu₂O(s)).
Ketone (R-CO-R') Tollens' or Fehling's, warm in water bath. No visible change (remains colourless/blue). Ketones resist mild oxidation.
Carboxylic Acid (R-COOH) Solid sodium carbonate (Na₂CO₃) or NaHCO₃(aq). Effervescence (fizzing); gas turns limewater cloudy. Acid-base reaction producing CO₂ gas: 2RCOOH + CO₃²⁻ → 2RCOO⁻ + H₂O + CO₂.
Organic functional group diagnostic observations Alkene (C=C) bromine water, shake orange colourless Decolourised Br₂ adds across C=C Aldehyde Tollens' reagent, warm colourless silver mirror Silver mirror forms Ag⁺ reduced to Ag Aldehyde Fehling's solution, warm blue brick-red solid Brick-red precipitate Cu²⁺ reduced to Cu₂O 1° or 2° alcohol acidified K₂Cr₂O₇, warm orange green Orange to green Cr₂O₇²⁻ reduced to Cr³⁺

2. Test for Alkenes (Carbon-Carbon Double Bond)

Alkenes contain an electron-rich π-bond that undergoes rapid electrophilic addition at room temperature:

Bromine Water Test - Add approximately 1 cm³ of the organic sample to a clean test tube.
- Add 5 drops of aqueous bromine (Br₂ in water, orange-brown).
- Stopper and shake gently.
- Positive Result: The orange solution turns colourless.
- Examiner Tip: Never write "turns clear". Clear means transparent; water is clear and colourless. The mark requires "colourless".

3. Halogenoalkanes: Hydrolysis Rate & Precipitation (Extension)

Extension beyond the four RP5 tests: 9620 RP5 covers tests for alcohols, aldehydes, alkenes and carboxylic acids. The halogenoalkane test uses Unit 2 chemistry (hydrolysis) and the Group 7 silver nitrate test, but it is not one of the RP5 tests.

Halogenoalkanes do not contain free halide ions; they are covalent molecules. To test them, the halogen must first be converted into a halide ion by nucleophilic substitution with hydroxide:

  1. Place 5 drops of halogenoalkane into a test tube. Add 1 cm³ of ethanol (co-solvent to dissolve both the halogenoalkane and water).
  2. Add 1 cm³ of dilute aqueous sodium hydroxide (NaOH).
  3. Place the test tube in a hot water bath at 50 °C for 5 minutes (promotes alkaline hydrolysis).
  4. Cool and acidify with dilute nitric acid (HNO₃): This is mandatory to neutralise remaining OH⁻ ions; otherwise, OH⁻ would react with Ag⁺ to precipitate brown silver oxide (Ag₂O).
  5. Add 5 drops of aqueous silver nitrate (AgNO₃). Observe precipitate colour and confirm with ammonia.

Rate of Hydrolysis Trend (C-X Bond Enthalpy)

When comparing 1-chlorobutane, 1-bromobutane, and 1-iodobutane under identical conditions:

  • 1-Iodobutane: Precipitates yellow AgI fastest (~15 seconds).
  • 1-Bromobutane: Precipitates cream AgBr at a moderate rate (~2 minutes).
  • 1-Chlorobutane: Precipitates white AgCl slowest (>5 minutes).

Reasoning: The rate of hydrolysis is governed by bond enthalpy, NOT bond polarity. The C-I bond is the weakest (238 kJ mol⁻¹) and breaks most readily, despite being the least polar.

4. Distinguishing Primary, Secondary, and Tertiary Alcohols

Acidified potassium dichromate(VI) (K₂Cr₂O₇ / H₂SO₄) acts as a powerful diagnostic tool based on the structural class of the alcohol:

Primary & Secondary Alcohols

Both possess at least one hydrogen atom directly bonded to the carbinol carbon (C-OH):

  • Primary alcohols are oxidised to aldehydes, then carboxylic acids.
  • Secondary alcohols are oxidised to ketones.
  • Observation: Orange dichromate(VI) solution turns to a dark green chromium(III) solution.

Tertiary Alcohols

The carbinol carbon is bonded to three alkyl groups and has no hydrogen atoms attached:

  • Oxidation would require breaking a stable C-C bond, which does not occur under mild conditions.
  • Observation: The solution remains orange (no reaction).

5. Differentiating Aldehydes from Ketones (Tollens' vs Fehling's)

Both aldehydes and ketones contain the carbonyl group (C=O). However, aldehydes are easily oxidised because they possess a C-H bond on the carbonyl carbon, while ketones do not:

Tollens' Reagent (Ammoniacal Silver Nitrate)

Preparation: Add 1 drop of dilute NaOH to 2 cm³ of AgNO₃ to form brown Ag₂O precipitate. Add dilute NH₃ dropwise until the precipitate just dissolves, forming [Ag(NH₃)₂]⁺.

Test: Add 5 drops of organic sample. Warm in a beaker of hot water (~60 °C).

  • Aldehyde: Silver mirror forms on the test-tube wall: RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + 2H₂O.
  • Ketone: Remains colourless (no silver mirror).

Fehling's Test (Alkaline Copper(II) Tartrate)

Reagents: Equal volumes of Fehling's A (aqueous CuSO₄) and Fehling's B (alkaline sodium potassium tartrate) mixed to give a deep royal blue solution.

Test: Add 5 drops of sample and warm in a hot water bath.

  • Aldehyde: Blue solution turns to a brick-red precipitate of copper(I) oxide (Cu₂O(s)).
  • Ketone: Remains deep blue.

6. Testing for Carboxylic Acids with Carbonates

Carboxylic acids are weak organic acids, yet sufficiently acidic to displace carbon dioxide from carbonate or hydrogencarbonate salts:

Carbonate Test Protocol - Add a spatula measure of solid sodium hydrogencarbonate (NaHCO₃) or sodium carbonate (Na₂CO₃) to 2 cm³ of the organic liquid.
- Positive Observation: Immediate vigorous effervescence (fizzing).
- Confirmation: Bubble the evolved gas through limewater; limewater turns cloudy / milky due to CaCO₃ precipitate.
- Diagnostic Distinction: Neither alcohols, aldehydes, nor ketones react with carbonates.

7. Worked Unknown Deduction Problem

Worked Example: Identifying Organic Compounds A, B, and C

Problem: Three unlabelled bottles contain propanoic acid, propan-1-ol, and propanal. A student carries out three tests:

  • Test 1: Addition of sodium carbonate solid to sample A causes vigorous fizzing. The gas turns limewater cloudy. Samples B and C show no reaction.
  • Test 2: Warming sample B with Tollens' reagent produces a shiny silver mirror on the inside of the tube. Sample C shows no change.
  • Test 3: Warming sample C with acidified potassium dichromate turns the solution from orange to green.

Identify compounds A, B, and C, and write the structural formula of the organic product formed when sample B reacts in Test 2.

Deductions:

- Sample A fizzes with Na₂CO₃ producing CO₂ → Compound A is Propanoic acid (CH₃CH₂COOH).
- Sample B gives a silver mirror with Tollens' → Aldehyde → Compound B is Propanal (CH₃CH₂CHO).
- Sample C is oxidised by acidified dichromate but does not react with Tollens' → Primary alcohol → Compound C is Propan-1-ol (CH₃CH₂CH₂OH).

Organic product of propanal in Test 2:

In alkaline Tollens' reagent, propanal is oxidised to propanoate ion: CH₃CH₂COO⁻ (or propanoic acid CH₃CH₂COOH upon acidification).

Conclusion: A = Propanoic acid, B = Propanal, C = Propan-1-ol

8. Practice Exam Questions

Question 1: In testing for halogenoalkanes, why must the reaction mixture be acidified with dilute nitric acid AFTER heating with NaOH and BEFORE adding silver nitrate?

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Correct Answer: B

Hydroxide ions from excess NaOH react immediately with Ag⁺ ions: 2Ag⁺ + 2OH⁻ → Ag₂O(s) + H₂O. The resulting brown silver oxide precipitate obscures the delicate white, cream, or yellow silver halide precipitate.

Question 2: Which halogenoalkane hydrolyses fastest when heated under identical conditions with aqueous sodium hydroxide?

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Correct Answer: C

The rate of nucleophilic substitution is determined primarily by the carbon-halogen bond strength (bond enthalpy), not bond polarity. The C-I bond has the lowest bond enthalpy (238 kJ mol⁻¹) and therefore reacts fastest.

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An alcohol is warmed with acidified potassium dichromate(VI), which turns green. The organic product gives no change with Fehling's solution. Deduce the type of alcohol and explain your answer.

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  • The product is not an aldehyde, because Fehling's solution stays blue (no red precipitate) [1]
  • So the product is a ketone [1]
  • Ketones are formed by oxidising a secondary alcohol, so the alcohol is secondary [1]
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