Four Unlabelled Liquids and a Badly Chosen First Test
Four unlabelled colourless liquids A, B, C and D are known to be ethanol, phenol, 2-methylpropan-2-ol and diethyl ether. The bench has sodium metal, anhydrous calcium chloride, neutral FeCl3 solution, bromine water, Lucas reagent (concentrated HCl with anhydrous ZnCl2) and a water bath. A student begins by adding a small piece of sodium to each tube, observes brisk effervescence in A, B and C but nothing in D, and concludes he has identified the ether but can go no further. He then runs the remaining tests: with Lucas reagent at room temperature A gives immediate cloudiness while B, C and D stay clear for ten minutes; B gives a violet colour with FeCl3 and a white precipitate with bromine water; C gives no colour with FeCl3 and no precipitate with bromine water. A teacher asks him to justify every assignment and to explain what his first test could and could not establish.
1. Assign A, B, C and D, and state precisely what the sodium test proves and what it fails to prove.
Read the worked answer
- D is diethyl ether. Only D fails to give hydrogen with sodium; ethers have no O–H hydrogen, so no alkoxide can form. D is also the only one of the four that is not fully miscible with water.
- A is 2-methylpropan-2-ol. Immediate cloudiness with Lucas reagent at room temperature is the signature of a tertiary alcohol: the 3° carbocation forms at once and the insoluble alkyl chloride separates as a cloudy second phase.
- B is phenol. Violet colour with neutral FeCl3 (the phenoxide–iron complex) and a white precipitate with bromine water (2,4,6-tribromophenol) both point to an activated aromatic ring bearing –OH.
- C is ethanol by elimination: no Lucas cloudiness at room temperature (primary alcohol), no FeCl3 colour, no bromine precipitate, but it does give hydrogen with sodium.
The sodium test proves only the presence of a relatively acidic hydrogen. It cannot separate A, B and C, because the tertiary alcohol, phenol and ethanol all liberate H2 (giving alkoxide or phenoxide). It is a good test for distinguishing an ether from an alcohol or phenol, and a poor test for identifying which alcohol. Test order matters: had the student begun with Lucas reagent and FeCl3, three liquids would have been assigned immediately and only one confirmatory sodium test would have been needed.
2. Write the equation for the Lucas test on A and explain why C needs warming while B gives no cloudiness at all.
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For A: (CH3)3COH + HCl → (CH3)3CCl + H2O, with anhydrous ZnCl2 acting as a Lewis acid that helps generate the carbocation. The mechanism is protonation of the –OH, loss of water to give the tertiary carbocation, then attack by Cl⁻. The 3° carbocation is strongly stabilised by three electron-releasing methyl groups, so it forms almost instantly and the insoluble chloride appears as turbidity within seconds.
For C (ethanol), the intermediate would be a primary carbocation, CH3CH2⁺, which is unstable; the SN1 route is therefore very slow at room temperature and the clear solution persists. On warming, an SN2 pathway with chloride becomes competitive, and cloudiness appears only after some minutes of heating - this delayed result is what distinguishes a primary alcohol from a tertiary one in the Lucas classification. The ZnCl2 concentration and the temperature must be controlled, otherwise 'slow' and 'very slow' cannot be told apart.
For B (phenol), no alkyl chloride separates. The C–O bond in phenol has partial double-bond character because the oxygen lone pair is delocalised into the aromatic ring by resonance; breaking that bond would destroy the resonance stabilisation, and the phenyl cation that would result is very high in energy. Phenol is therefore not dehydrated by HCl/ZnCl2 under these conditions, and the tube stays clear - a useful reminder that phenol is an acid, not an alcohol, in its reactions.
3. The four liquids must be stored after the practical. Give the storage precautions for B and D with chemical reasons, and comment on the safety handling of the sodium used.
Read the worked answer
Phenol (B) darkens from colourless to pink or brown on standing in air and light because it is slowly oxidised, partly to quinones and polymeric material. It should be kept in a tightly stoppered amber or dark-glass bottle, away from light and oxidising agents, and handled with gloves since it is corrosive and readily absorbed through skin, causing deep burns. Judgement point: a pink tint in an old bottle is an oxidation artefact, not a sign that the phenol has been converted to something else entirely, so a fresh sample should be used for the FeCl3 and bromine-water tests.
Diethyl ether (D) is the more serious storage problem. On exposure to air and light it forms peroxides, notably by autoxidation at the carbon adjacent to oxygen; these are involatile and explode on heating or on concentration. Ether must be stored in a dark bottle, over a drying agent such as anhydrous CaCl2 plus a little iron wire or KOH, and should be tested with acidified KI/starch or iron(II) sulphate before any distillation; never distil an old ether sample to dryness. Ether is also extremely flammable with a very low flash point, so no flame should be near the bench.
Sodium must be stored under kerosene, handled with dry tongs, and any residual pieces quenched by slow addition to excess ethanol, never to water, since the reaction with water is violent and can ignite the evolved hydrogen.