3.6.2Organic Chemistry
Testing for Organic Functional Groups — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 25 of 55
Part 1 of 6Planning the Five Tests
The practical uses a known compound for each group so that the expected results can be checked.
Alcohol
- Add a small piece of sodium to dry ethanol;
- fizzing shows hydrogen is released: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.
Aldehyde
- Warm ethanal with Fehling’s solution in a water bath; the blue solution gives a brick-red precipitate.
Alkene
- Add a few drops of cyclohexene to bromine water and shake; the orange-brown colour is lost.
Carboxylic acid
- Add ethanoic acid to solid sodium hydrogencarbonate; CO₂ is released with fizzing.
Halogenoalkane
- Warm 1-bromobutane with sodium hydroxide, acidify with nitric acid and add silver nitrate to get a cream precipitate.
Part 2 of 6Working Safely and Accurately
Good practical technique protects the experimenter and makes the observations reliable.
1
Wear eye protection and gloves because sodium, bromine water and acidified solutions are hazardous.
2
Keep sodium under oil until needed, use only a small piece, and dispose of leftovers in ethanol and never in water or the bin.
3
Use a water bath rather than a naked flame when warming flammable liquids such as ethanal and bromobutane.
4
Use clean tubes and add reagents dropwise so the colour changes can be seen clearly.
5
Record all observations in a results table with columns for the compound, the reagent and what was seen.
Part 3 of 6Interpreting Unknowns
Results from several tubes are combined to identify an unknown compound.
Interpreting Unknowns
Fizzing with sodium
- Suggests an –OH group, but a carboxylic acid also reacts, so check for fizzing with NaHCO₃ as well.
Fizzing with NaHCO₃
- Only a carboxylic acid is acidic enough to give CO₂ with hydrogencarbonate; alcohols do not.
Red precipitate with Fehling’s
- Shows an aldehyde; a ketone leaves the solution blue.
Coloured precipitate with silver nitrate
- After hydrolysis and acidification, the colour (white, cream, yellow) shows whether Cl, Br or I is present.
Part 4 of 6Evaluating the Method
Evaluating the Method
Qualitative nature
- The volumes need not be exact because only a clear change is looked for, not a measurement.
False results
- Skipping the nitric acid step can give a false precipitate, and impure samples can give misleading colours.
Control comparison
- Running a test on a known compound alongside the unknown makes the colour change easier to judge.
Limitations
- Tests show only the presence of a group, so identifying a whole molecule needs further techniques such as spectroscopy.
Part 5 of 6Worked Example
Question
- 0.46 g of ethanol (Mr = 46.0) reacts completely with excess sodium.
- Calculate the volume of hydrogen released at room temperature and pressure (molar volume 24.0 dm³ mol⁻¹).
- Moles of ethanol = 0.46 ÷ 46.0 = 0.0100 mol.
- From 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂, 2 mol of ethanol give 1 mol of H₂.
- Moles of H₂ = 0.0100 ÷ 2 = 0.00500 mol.
- Volume = 0.00500 × 24.0 dm³ = 0.120 dm³.
Answer The volume of hydrogen is 0.120 dm³, which is 120 cm³.
Part 6 of 6Exam Checklist & Tips
3.6.2 – Key Points for the Exam
- Qualitative test A test that shows whether a substance is present, without measuring how much.
- Hydrolysis A reaction with water or hydroxide that breaks a bond, here releasing a halide ion.
- Observation What is seen, heard or smelt during a test, such as colour change or bubbles.
- Water bath A container of hot water used to heat flammable liquids gently and safely.
Common Mistakes
- Writing conclusions such as ‘it is an aldehyde’ in the observation column instead of what was seen.
- Putting sodium into water or the sink for disposal instead of reacting it with ethanol.
- Assuming fizzing with sodium alone proves an alcohol, when a carboxylic acid would also react.
- Heating bromobutane over a naked flame instead of a water bath.
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