Part 2: Preparing and purifying an organic solid

Explain the purpose of reflux, recrystallisation, filtration and melting-point measurement, including how practical errors change yield and purity.

Match the apparatus to the physical task

A preparation may use Quickfit apparatus to heat a reaction under reflux or to distil a volatile component. In reflux the condenser is vertical above the reaction vessel, cooling vapour so liquid returns. In distillation the vapour passes through a condenser towards a separate receiver.

Cooling water enters a condenser at the lower inlet and leaves at the upper outlet so the jacket fills effectively. The apparatus must not become a sealed heated system. Use suitable clamping and a heating method compatible with the flammability and hazards of the mixture.

Anti-bumping granules provide nucleation sites for smoother boiling when appropriate. Add them before heating; adding them to a hot superheated liquid can cause sudden boiling. These explanations connect apparatus features to purpose rather than merely naming glassware.

Exploit a temperature-dependent solubility difference

Recrystallisation works when the desired solid is much more soluble in the chosen solvent when hot than when cold. Dissolve the crude solid in the minimum practical volume of hot solvent. Too much solvent leaves more product dissolved after cooling and lowers recovery.

If insoluble impurities remain, filter the hot solution while keeping it hot enough to avoid premature product crystallisation in the funnel. This removes insoluble material; it is not the same operation as later collecting the product crystals.

Allow the solution to cool so crystals form. Slow initial cooling can favour better crystal growth; further cooling can improve recovery. Soluble impurities ideally remain in the mother liquor, while product molecules enter the crystal lattice. A solvent must also be chemically suitable, not react with the product, and be manageable to remove.

Wash impurities away without redissolving the product

Collect crystals by filtration under reduced pressure using suitable apparatus such as a Büchner funnel and side-arm flask. Wet the filter paper with the chosen solvent so it seats properly, and use apparatus appropriate to reduced pressure.

Wash with a small amount of cold solvent to remove adhering mother liquor and its soluble impurities. Hot solvent or a large washing volume can dissolve product and reduce yield. Continue suction as appropriate, then dry the crystals by a suitable method before weighing or testing them.

Wet crystals can give an apparently high yield because retained solvent contributes mass. Drying is therefore part of obtaining a defensible yield, not just presentation. Transfer losses and product remaining in solution can lower recovery even when the reaction itself proceeded well.

Interpret a range, not a single reassuring number

A pure crystalline substance usually melts over a narrow range close to its reference melting point under comparable conditions. Impurities often depress and broaden the range. Heat slowly near the expected melting region and record onset and completion rather than a single guessed temperature.

Agreement supports identity and purity, but melting point alone is not absolute proof: different substances can have similar values and decomposition or measurement conditions can complicate interpretation. Combine it with the reaction route and other analytical evidence.

If the measured range is broad and low, consider impurity, residual solvent and technique. Recrystallising again may improve purity but usually reduces recovered mass. Explain the purity–recovery trade-off instead of assuming the highest mass is the best product.

Worked example: distinguish yield from purity

An original preparation has a theoretical product mass of 3.20 g. Dry purified crystals weigh 2.24 g, giving percentage yield = 2.24/3.20 × 100 = 70.0%. An earlier wet mass of 3.36 g would suggest 105%, signalling that the weighed material cannot all be dry pure product under the stated assumptions.

A low yield alone does not identify the cause. The reaction may be incomplete, side reactions may occur, some solid may remain dissolved, or product may be lost during transfer and filtration. Match a proposed improvement to evidence about the actual loss.

Practical questions can assess why a method works, how uncertainty enters and how changes affect results. Written knowledge of this procedure is distinct from demonstrating the practical competencies required for the practical endorsement; completing these notes does not itself satisfy the endorsement.

Quick checks

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

Sources