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.
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 (\(\text{C}=\text{C}\) double bonds)
Alkenes undergo electrophilic addition reactions with halogens across the double bond. Adding bromine water (\(\text{Br}_2\)) 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 \(\text{CO}_2\). 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 \([\text{Ag(NH}_3)_2]^+\). Aldehydes oxidise to carboxylates while reducing silver(I) ions to metallic silver (\(\text{Ag}^0\)):
\[ \text{RCHO} + 2[\text{Ag(NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag}(\text{s}) + 4\text{NH}_3 + 2\text{H}_2\text{O} \]- 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 \(\text{Cu}^{2+}\) ions to copper(I) oxide (\(\text{Cu}_2\text{O}\)):
\[ \text{RCHO} + 2\text{Cu}^{2+}(\text{complex}) + 5\text{OH}^- \rightarrow \text{RCOO}^- + \text{Cu}_2\text{O}(\text{s}) + 3\text{H}_2\text{O} \]- Mix equal parts of Fehling's A (blue aqueous \(\text{CuSO}_4\)) 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) (\(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\)) 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 (\(\text{Cr}_2\text{O}_7^{2-}\)) turns green as it is reduced to chromium(III) ions (\(\text{Cr}^{3+}\)). Tertiary alcohols show no colour change (solution remains 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.
Common Exam Questions
1. State why Tollens' reagent must be freshly prepared rather than stored in a reagent bottle.
Tollens' reagent decomposes upon standing to form silver nitride, \(\text{Ag}_3\text{N}\), which is a highly sensitive and dangerous explosive when dry.
2. Explain why cyclohexene decolourises bromine water, but cyclohexane does not.
Cyclohexene is unsaturated, containing a reactive \(\text{C}=\text{C}\) double bond. This double bond has a high electron density that polarises the bromine molecule, leading to electrophilic addition. Cyclohexane is saturated, containing only single bonds, and does not react with bromine unless UV light is present to initiate radical substitution.
3. A student oxidises an alcohol and obtains a product that does not react with Fehling's solution. What can you deduce about the starting alcohol?
The product does not react with Fehling's solution, so it is not an aldehyde; it is a ketone. A ketone is formed by the oxidation of a secondary alcohol. Therefore, the starting alcohol must have been secondary.
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.