RP 5: Distillation: preparing an organic liquid

Hazards

Method

1. Load the flask (mass by difference)AQA's exemplar: dehydrate cyclohexanol to cyclohexene. Put ~20 cm³ cyclohexanol into a weighed 50 cm³ pear-shaped flask and re-weigh (mass by difference). Add ~8 cm³ concentrated phosphoric(V) acid dropwise with frequent shaking.
2. Distil below 100 °CAdd anti-bumping granules and assemble the distillation apparatus: still head with the thermometer bulb level with the side-arm opening, water condenser sloping down to a cooled receiver. Water enters the condenser at the bottom and leaves at the top. Heat gently and collect the distillate boiling below 100 °C.
3. Separate in the funnelTransfer the distillate to a separating funnel with ~50 cm³ saturated sodium chloride solution. Invert gently several times, opening the tap to release pressure. Let the layers settle and run off the lower aqueous layer — cyclohexene is less dense, so it floats.
4. Dry, weigh, % yieldDry the organic layer over anhydrous CaCl₂, MgSO₄ or Na₂SO₄ until it runs clear, then decant into a weighed container. Weigh and calculate % yield. Redistil if a purer sample is needed.
5. Confirm the alkeneA ~1 cm³ portion decolourises bromine water (or turns acidified KMnO₄ from purple to colourless/brown MnO₂).
6. Aldehyde vs acid: distil or refluxRelated spec point: when making an aldehyde from a primary alcohol (RP10 territory), distil it off as it forms — its boiling point is lower than the alcohol's, so it escapes before being oxidised on to the carboxylic acid. If the acid is what you want, reflux first (vertical condenser, never sealed), then distil.

Mistakes examiners see every year

What to expect

Cyclohexene boils at 82.9 °C (NIST WebBook) vs cyclohexanol at ~161 °C, so the <100 °C fraction is clean to collect. The dried product is clear, not cloudy, and decolourises bromine water. A typical student yield is 50–70% (indicative). RSC's alternative liquid prep: 2-methylpropan-2-ol + concentrated HCl → 2-chloro-2-methylpropane, washed with NaHCO₃ solution, dried over Na₂SO₄, redistilled.