Part 2: Reflux, distillation and organic-liquid purification

Explain the purpose and order of each operation, including how to identify and dry the correct layer.

Assemble the apparatus for the job

Use joined ground-glass apparatus with appropriate support. Reflux has a vertical condenser above the flask: vapour condenses and returns, so reactants can be heated for longer. Distillation directs vapour into a condenser and receiver to separate a volatile product. Put the thermometer bulb near the side-arm entrance so it measures the vapour entering the condenser.

Cooling water enters the lower condenser port and exits the upper one. Add anti-bumping granules before heating. Use appropriate electric heating for flammable liquids and keep the apparatus open to a safe outlet: never heat a sealed system.

Separate immiscible liquid layers

Cool the crude mixture before using a separating funnel. Allow layers to settle. Identify the organic layer using density information or a small water-drop test; it is not always the upper layer. Remove the stopper when draining and collect labelled fractions rather than discarding a layer before identification.

If a wash is specified, mix carefully and vent with the outlet directed away from people. A carbonate wash of an acidic organic product mixture can release CO₂, so pressure must be released frequently. Use a wash compatible with the desired product; the core requirement is understanding separation, not adding an unnecessary wash to every procedure.

Dry the organic layer, then redistil

Add a suitable anhydrous drying agent such as MgSO₄ or Na₂SO₄ to the isolated organic layer. It removes traces of water; it does not separate large aqueous and organic layers. Choose a drying agent that does not react with or dissolve significantly in the product. Swirl, allow contact, then decant or filter away the solid.

Redistil and collect the fraction over the product’s expected boiling range. Lower- and higher-boiling impurities can be separated where their boiling points differ sufficiently. A narrow matching boiling range supports purity but is not an absolute identity proof. These operations develop PAG 5 preparation of an organic liquid.

Worked yield and extended-response plan

Suppose 6.00 g propan-1-ol (M = 60.0) is converted 1:1 into 1-bromopropane (M = 123). Theoretical n = 0.100 mol and product mass = 12.3 g. An isolated dry mass of 8.61 g gives 8.61/12.3 × 100 = 70.0% yield. Use dry product mass after purification, not the crude wet mixture.

For a method answer, explain reaction conditions, separation of the correctly identified layer, drying, removal of drying agent and final distillation in order. Link each step to the impurity or loss it addresses.

Match each operation to the impurity it removes

A separating funnel removes a bulk immiscible liquid phase. A compatible aqueous wash can transfer a soluble impurity or neutralise an acid, when the procedure calls for it. A drying agent removes traces of water remaining in the organic phase. Distillation separates suitable volatile substances by boiling behaviour. These operations solve different problems.

For a stated organic product of density 1.20 g cm⁻³ with an aqueous layer of density 1.00 g cm⁻³, the organic phase is the lower layer. Collect it accordingly. For a product less dense than the aqueous layer, it would be upper. If density data are absent, a water-drop test can identify the aqueous phase; do not discard either layer before identification.

Drying-agent particles often clump while taking up water; follow the specified procedure and allow contact before separating the solid. Excessive drying agent can retain product and reduce recovery. Do not heat the wet crude mixture with drying salts left in it as a substitute for the proper separation sequence.

A large collected mass is not proof of a successful synthesis

Suppose the theoretical mass is 5.00 g but the crude liquid weighs 5.40 g. The apparent 108% yield indicates the measured mass cannot be entirely the intended pure dry product under the stated assumptions. Water, solvent or unreacted starting material may inflate it; an incorrect theoretical calculation is another possibility.

After drying and redistillation, 3.85 g is collected, giving 77.0% isolated yield. A lower mass can represent a better-characterised product. A narrow boiling range close to the expected value supports purity, but identity should be checked against other evidence where available.

Low yield may result from incomplete conversion, competing reactions, transfer losses, evaporation or leaving product in discarded fractions. Distinguish an improvement in reaction conversion from one in recovery: better transfer technique cannot change the equilibrium constant or remove a competing reaction pathway.

Repair a practical method by explaining its consequences

“Heat a sealed flask, add granules once boiling, pour the hot mixture into a separating funnel and collect the top layer” contains several independent faults. A heated sealed system can build pressure; granules belong in the flask before heating; the mixture should cool before separation; the layer must be identified rather than assumed.

In a distillation drawing, the thermometer bulb should measure the vapour entering the condenser, not sit deep in the boiling liquid or above the vapour path. Condenser cooling water travels through the jacket, separate from the organic vapour. The receiver collects condensed product; reflux instead returns it to the reacting flask.

For an extended response, put the steps in physical order and link each to a purpose. In the 2025 H032/02 Q5(c) assessment context, purification, reasoning and the yield calculation were considered together through level descriptors. This is why an ordered explanation is stronger than an unordered list of equipment names.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources