Three Unlabelled Bottles: Identifying C₄ Compounds from Physical and Chemical Evidence
A laboratory receives three unlabelled bottles of colourless liquid. Bottle A (labelled C₄H₈O₂, Mr = 88) boils at 164 °C; its 0.10 mol/L aqueous solution has pH 2.9, and adding solid Na₂CO₃ produces vigorous bubbling. Bottle B (C₄H₈O₂, Mr = 88) boils at 77 °C; its aqueous solution is neutral, it does not react with Na₂CO₃ or with sodium metal, and it has a fruity odour. Bottle C (C₄H₁₀O, Mr = 74) boils at 118 °C; its aqueous solution is neutral, and it reacts with sodium metal to release a gas that burns with a pop. All three dissolve appreciably in water. The teacher asks the class to assign a structure to each bottle and to justify every assignment from the data rather than from the labels.
1. Assign a structural formula and a name to each bottle, and justify each assignment using the specific observations given.
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Bottle A - butanoic acid, CH₃CH₂CH₂COOH
- pH 2.9 for a 0.10 mol/L solution: a weak acid (a strong acid at 0.10 mol/L would be pH 1.0).
- Bubbling with Na₂CO₃ confirms a carboxylic acid: 2CH₃CH₂CH₂COOH + Na₂CO₃ → 2CH₃CH₂CH₂COONa + H₂O + CO₂.
- Formula C₄H₈O₂ matches (one COOH, one C=O plus C–O).
Bottle B - ethyl ethanoate, CH₃COOCH₂CH₃
- Neutral solution and no reaction with Na₂CO₃ → no acidic O–H.
- No reaction with sodium metal → no alcohol O–H either, so the oxygens must be an ester.
- Fruity odour and the lowest boiling point of the three fit a small ester.
Bottle C - butan-1-ol, CH₃CH₂CH₂CH₂OH
- Neutral in water, so not a carboxylic acid (and the formula has only one oxygen).
- Reacts with Na metal to release H₂: 2CH₃CH₂CH₂CH₂OH + 2Na → 2CH₃CH₂CH₂CH₂ONa + H₂. Only an alcohol O–H reacts this way.
- Mr = 74 matches C₄H₁₀O.
2. Bottle C has the lowest molar mass of the three, yet it boils 41 °C higher than bottle B. Explain the boiling point order using intermolecular forces.
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Order: A (164 °C) > C (118 °C) > B (77 °C)
A - butanoic acid: each molecule has both a C=O and an O–H, so it forms strong hydrogen bonds and pairs up as dimers through two complementary hydrogen bonds. Breaking these dimers into the gas phase requires the most energy, so the boiling point is highest despite the same Mr as B.
C - butan-1-ol: hydrogen bonding through the O–H group, but no dimer formation and a smaller hydrogen-bond donor/acceptor set than the acid. Boiling point is high but below A. Its higher boiling point than B despite its lower molar mass shows that hydrogen bonding outweighs the dispersion-force advantage of the heavier molecule.
B - ethyl ethanoate: no O–H, so no hydrogen bonding between its own molecules. It relies on dipole–dipole forces plus London dispersion forces across 8 carbon/oxygen atoms, which are weaker than hydrogen bonds. Hence the lowest boiling point.
General point: hydrogen bonding dominates the trend; the size/mass difference between B and C is real but secondary.
3. Bottle C is treated with acidified potassium dichromate solution. Predict the organic product under (i) distillation as the product forms and (ii) prolonged heating under reflux, and describe a simple chemical test that distinguishes the two products.
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Reaction with acidified K₂Cr₂O₇ (orange → green as Cr₂O₇²⁻ is reduced to Cr³⁺)
Butan-1-ol is a primary alcohol, so it can be oxidized in two stages:
- CH₃CH₂CH₂CH₂OH → CH₃CH₂CH₂CHO (butanal)
- CH₃CH₂CH₂CHO → CH₃CH₂CH₂COOH (butanoic acid)
(i) Distillation as formed - butanal, CH₃CH₂CH₂CHO. The aldehyde has a much lower boiling point (≈75 °C) than the acid (164 °C) and is removed from the oxidizing mixture before it can be oxidized further. Distillation therefore traps the aldehyde.
(ii) Prolonged reflux - butanoic acid, CH₃CH₂CH₂COOH. Reflux returns the aldehyde to the dichromate so oxidation goes to completion.
Distinguishing test
- Add a few drops of each to Na₂CO₃(aq): the acid gives steady CO₂ bubbling; butanal gives no reaction.
- Alternatively, Tollens' reagent or Fehling's solution: butanal gives a silver mirror / red precipitate, while butanoic acid does not.
A pH check also works: a 0.1 mol/L solution of the acid is around pH 3, the aldehyde is neutral.