Separating an alkene–carboxylic acid mixture after simple tests
A student is given a single colourless liquid sample of 20.0 cm³ that is a homogeneous mixture of two organic compounds. The possible compounds are hexane, hex-1-ene, ethanol and ethanoic acid. The student shakes a small portion with bromine water: the orange colour disappears. A second portion is treated with sodium carbonate solution: brisk effervescence occurs. The student concludes the sample contains hex-1-ene and ethanoic acid, but the teacher points out that the tests identify functional groups, not necessarily those exact two compounds. The mixture boils over a range. The student must separate the two components in the laboratory and provide evidence for each.
1. Interpret the two test results and explain why the student cannot conclude the exact identities from these tests alone.
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Decolourisation of bromine water shows a carbon–carbon double bond, so an alkene is present; hexane and ethanoic acid would not give this reaction. Effervescence with sodium carbonate shows a carboxylic acid: 2RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O.
However, these tests identify functional groups, not the whole molecule. Another alkene could also decolourise bromine water, and another carboxylic acid could also release CO₂. The exact identities would need supporting evidence such as boiling point, titration equivalent mass, or infrared spectroscopy.
2. Describe a sequence of steps to separate and isolate the two components, including the reagents and expected observations.
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Add excess sodium carbonate solution to the mixture and stir until effervescence stops. The carboxylic acid is converted to its sodium salt and enters the aqueous layer:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O.
The alkene remains as the organic layer. Transfer the mixture to a separating funnel, run off the aqueous layer, and keep both layers.
Wash and dry the organic layer (e.g. with anhydrous sodium sulphate), then distil it to collect hex-1-ene at about 63°C. Confirm it with bromine water.
Acidify the aqueous layer carefully with dilute sulphuric acid: 2CH₃COONa + H₂SO₄ → 2CH₃COOH + Na₂SO₄. Distil the acidified solution to collect ethanoic acid at about 118°C. Confirm it with sodium carbonate solution.
3. Explain why simple distillation of the original mixture is less suitable than the chemical separation method.
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Simple distillation relies on the boiling-point difference between hex-1-ene (about 63°C) and ethanoic acid (about 118°C), which is large enough that some separation would occur. However, the acid is corrosive and can char or bump when heated, and some acid vapour may co-distil and contaminate the alkene fraction. The distillation also gives mixtures rather than clean components unless a fractionating column is used.
Chemical separation converts the acid into a salt, which is held in the aqueous layer, so the alkene can be distilled cleanly. Acidifying the aqueous layer then regenerates the carboxylic acid. This gives better purity and clearer evidence for each functional group.