Identifying organic functional groups is fundamental to organic chemistry and analysis. This practical guide covers tests to distinguish alkenes, alcohols, aldehydes, and carboxylic acids using simple test-tube reactions.
Core Specification Link
This practical links to topics 3.3.4 Alkenes, 3.3.5 Alcohols, 3.3.6 Organic Analysis, and 3.3.9 Carboxylic Acids and Derivatives.
Aim
To carry out simple test-tube reactions to identify the functional groups in unknown organic compounds (specifically alcohols, aldehydes, alkenes, and carboxylic acids).
Equipment List
- Test tubes and test tube rack
- Dropping pipettes
- Water bath (beaker of hot water or electronic water bath)
- Bromine water (Br₂(aq))
- Sodium hydrogencarbonate solution (NaHCO₃, 1.0 mol dm⁻³) or solid sodium carbonate (Na₂CO₃)
- Acidified potassium dichromate(VI) solution (K₂Cr₂O₇, 0.1 mol dm⁻³ acidified with dilute sulfuric acid)
- Tollens' reagent (prepared from silver nitrate, sodium hydroxide, and dilute ammonia solutions)
- Fehling's solution (equal parts of Fehling's A and Fehling's B)
- Unknown organic samples (labeled as Primary/Secondary/Tertiary alcohol, Alkene, Aldehyde, Carboxylic acid)
- Anti-bumping granules
Functional Group Identification Flowchart
This decision-tree outlines the logical pathway to classify an unknown organic substance into its correct functional class:
Detailed Procedures & Chemistry
1. Testing for Alkenes (C=C double bonds)
Alkenes undergo electrophilic addition reactions with halogens across the double bond. Adding bromine water (Br₂) converts the unsaturated alkene into a saturated halogenoalkane:
\[ \text{H}_2\text{C}=\text{CH}_2 + \text{Br}_2(\text{aq}) \rightarrow \text{CH}_2\text{Br}-\text{CH}_2\text{Br} \]- Add 1 cm³ of bromine water (orange-brown) to 1 cm³ of the sample.
- Shake the test tube gently.
- Observation: If an alkene is present, the orange-brown colour is decolourised immediately to yield a colourless solution.
2. Testing for Carboxylic Acids
Carboxylic acids are weak acids that react with metal carbonates in a standard neutralisation reaction to form a salt, water, and carbon dioxide gas:
\[ 2\text{RCOOH}(\text{aq}) + \text{Na}_2\text{CO}_3(\text{s}) \rightarrow 2\text{RCOONa}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g}) \]- Add a small spatula of solid sodium carbonate or 1 cm³ of sodium hydrogencarbonate solution to 1 cm³ of the sample.
- Observation: Rapid effervescence (fizzing). Bubble the evolved gas through limewater; it turns cloudy, confirming CO₂. Carboxylic acids will also turn blue litmus paper red.
3. Testing for Aldehydes (Tollens' vs Fehling's)
Aldehydes are easily oxidised to carboxylic acids, whereas ketones cannot be oxidised without breaking carbon-carbon bonds. Therefore, mild oxidising agents can distinguish aldehydes from ketones.
A. Tollens' Reagent (Ammoniacal Silver Nitrate)
Tollens' reagent contains the complex ion [Ag(NH₃)₂]⁺. Aldehydes oxidise to carboxylates while reducing silver(I) ions to metallic silver (Ag⁰):
\[ \text{RCHO} + 2[\text{Ag(NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag}(\text{s}) + 4\text{NH}_3 + 2\text{H}_2\text{O} \]- Add 1 cm³ of Tollens' reagent to 1 cm³ of the sample.
- Warm in a hot water bath (60 °C) for 5 to 10 minutes (never use a Bunsen burner).
- Observation: A reflective silver mirror coating forms on the inside wall of the test tube.
B. Fehling's Solution
Fehling's solution contains copper(II) ions complexed with tartrate ligands in alkaline solution (blue). Aldehydes reduce Cu²⁺ ions to copper(I) oxide (Cu₂O):
\[ \text{RCHO} + 2\text{Cu}^{2+}(\text{complex}) + 5\text{OH}^- \rightarrow \text{RCOO}^- + \text{Cu}_2\text{O}(\text{s}) + 3\text{H}_2\text{O} \]- Mix equal parts of Fehling's A (blue aqueous CuSO₄) and Fehling's B (alkaline sodium potassium tartrate).
- Add 1 cm³ of this blue reagent to 1 cm³ of the sample and heat in a water bath.
- Observation: The blue solution forms a brick-red precipitate of copper(I) oxide.
4. Testing for Alcohols (Primary & Secondary vs Tertiary)
Acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) oxidises primary alcohols to aldehydes (and then carboxylic acids) and secondary alcohols to ketones. Tertiary alcohols cannot be oxidised as they lack a hydrogen atom on the carbon atom holding the hydroxyl group.
- Add 1 cm³ of acidified potassium dichromate(VI) to 1 cm³ of the sample.
- Warm gently in a water bath.
- Observation: For primary and secondary alcohols, the orange dichromate(VI) solution (Cr₂O₇²⁻) turns green as it is reduced to chromium(III) ions (Cr³⁺). Tertiary alcohols show no colour change (solution remains orange).
Step 1: Identify ethanoic acid using sodium carbonate
Add a small spatula of solid sodium hydrogencarbonate (NaHCO₃) to a sample of each liquid in a test tube.
- The tube containing ethanoic acid will exhibit immediate effervescence (fizzing), releasing carbon dioxide gas: \[ \text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2(\text{g}) \]
- The other three liquids will show no fizzing.
Step 2: Identify cyclohexene using bromine water
Add bromine water dropwise to samples of the remaining three liquids and shake.
- The sample containing cyclohexene will rapidly decolourise the orange bromine water from orange to colourless: \[ \text{C}_6\text{H}_{10} + \text{Br}_2 \rightarrow \text{C}_6\text{H}_{10}\text{Br}_2 \quad \text{(colourless 1,2-dibromocyclohexane)} \]
- The other two liquids (cyclohexanol and cyclohexanone) will not decolourise the bromine water.
Step 3: Distinguish between cyclohexanol and cyclohexanone using acidified potassium dichromate(VI)
Add 1 cm³ of acidified potassium dichromate(VI) solution to the remaining two samples and warm in a water bath for 5 minutes.
- The sample containing cyclohexanol (a secondary alcohol) will be oxidised to cyclohexanone. The orange dichromate(VI) ions (Cr₂O₇²⁻) are reduced to green chromium(III) ions (Cr³⁺), causing a colour change from orange to green: \[ \text{C}_6\text{H}_{11}\text{OH} + [\text{O}] \rightarrow \text{C}_6\text{H}_{10}\text{O} + \text{H}_2\text{O} \]
- The sample containing cyclohexanone (a ketone) cannot be oxidised further under these conditions; the solution will remain orange.
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Tollens' reagent | Can form highly explosive silver nitride (silver fulminate) deposits if allowed to dry or stand. | Prepare Tollens' reagent freshly in minimal quantities immediately before use. Rinse test tubes with dilute nitric acid and flush down the drain with excess water immediately after the test. |
| Acidified potassium dichromate(VI) | Highly toxic, carcinogenic (causes cancer), and mutagenic. | Wear nitrile gloves and safety goggles. Avoid contact with skin. Dispose of waste in a designated toxic waste container. |
| Bromine water | Corrosive to skin and releases toxic bromine vapours. | Dispense and perform reactions using bromine water inside a fume cupboard. Wear gloves. |
| Organic liquids (ethanol, ethanal) | Highly flammable. | Ensure no Bunsen burner flames are active in the laboratory. Use a water bath for heating. |
Sources of Error & Improvements
- False positives with Tollens' reagent: If glassware is contaminated with organic residues, it can reduce the silver ions without aldehydes being present. Improvement: Ensure all test tubes are thoroughly washed with nitric acid and deionised water before use.
- Ethanal evaporating: Ethanal has a very low boiling point (21 °C) and evaporates rapidly at room temperature, which can reduce the concentration of aldehyde for testing. Improvement: Keep samples stoppered in ice baths before addition.
Exam questions, marked instantly
Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.
Exam-style question
3 marksExplain why cyclohexene decolourises bromine water but cyclohexane does not.
Show Mark Scheme
- Cyclohexene is unsaturated: it contains a C=C double bond [1]
- The high electron density of the C=C bond polarises bromine, so electrophilic addition takes place [1]
- Cyclohexane is saturated, with only C-C single bonds, so no addition reaction occurs (it only reacts with bromine in UV light) [1]
Exam-style question
3 marksAn 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.
Show Mark Scheme
- 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]
CPAC Skills Assessed
- CPAC 1: Correctly follows written procedures to carry out qualitative diagnostic tests.
- CPAC 3: Safely prepares and disposes of unstable Tollens' reagent and carcinogenic chromium waste.
- CPAC 4: Accurately records visual observations (colour changes, silver mirror formation, and gas evolution).
Do not use the word "clear" to describe bromine water turning colourless. Bromine water is clear (transparent) both before and after the reaction. The correct term is decolourised or turned from orange to colourless.