1. Master Diagnostic Matrix: Organic Functional Group Tests
In OxfordAQA Paper 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) | 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: Cr2O7^2- 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 + CO3^2- → 2RCOO- + H2O + CO2. |
2. Test for Alkenes (Carbon-Carbon Double Bond)
Alkenes contain an electron-rich π-bond that undergoes rapid electrophilic addition at room temperature:
- 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
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:
- Place 5 drops of halogenoalkane into a test tube. Add 1 cm3 of ethanol (co-solvent to dissolve both the halogenoalkane and water).
- Add 1 cm3 of dilute aqueous sodium hydroxide (NaOH).
- Place the test tube in a hot water bath at 50 deg C for 5 minutes (promotes alkaline hydrolysis).
- 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).
- 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 deg 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:
- 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
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?
Show Answer & Explanation
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?
Show Answer & Explanation
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.