Part 7: Organic practical techniques and method selection

Select reflux, extraction, drying, distillation or recrystallisation by the job each technique performs, and explain how a specific error affects yield or purity.

Retain a reaction mixture or remove a volatile product

Reflux uses a vertical condenser above the heated flask. Vapour condenses and returns, allowing prolonged heating without losing volatile reactants or solvent. Cooling water enters the lower condenser connection and leaves the upper one, keeping the jacket full. The apparatus is open to the atmosphere through an appropriate outlet; never seal a heated reflux apparatus.

Distillation carries vapour into a sloping condenser and collects it in another vessel. It can remove a volatile product as it forms or separate liquids by volatility. Place the thermometer bulb at the level of the side-arm entrance so it measures vapour entering the condenser, not the heated liquid or cold receiver.

For primary-alcohol oxidation, distilling the aldehyde promptly can limit further oxidation; refluxing with excess oxidant favours the acid. Anti-bumping granules provide nucleation sites for even boiling and are added before heating, not dropped into a hot superheated liquid. Select electrical/water-bath heating when the vapour hazard makes a naked flame inappropriate.

Different solubilities transfer different components

Washing an organic layer removes impurities into a second immiscible phase. Solvent extraction transfers the desired substance into a solvent in which it is more soluble. In a separating funnel, stopper, mix gently as appropriate, vent safely, allow layers to settle, then separate them. Remove the stopper before draining to avoid irregular flow caused by pressure differences.

Identify layers using density data or a small water-drop test; organic does not automatically mean upper. Several smaller extractions with the same total solvent can recover more material than one large extraction because each fresh portion re-establishes the partition equilibrium.

For an acidic impurity, aqueous hydrogencarbonate converts it into a water-soluble salt. CO₂ production causes pressure, so vent frequently away from faces. Keep the desired product in mind: using alkali on an acid product would extract the product into the aqueous phase, and strong prolonged alkali can hydrolyse an ester.

Original interpretation: if a weak acid product disappears from the organic layer after a NaOH wash, it may still be in the aqueous layer as carboxylate. Acidify that layer and re-extract or crystallise as appropriate. This is a chemical form change, not necessarily destruction of the product.

Remove water without adding a new impurity

An anhydrous salt such as MgSO₄ or Na₂SO₄ can remove traces of water from an organic liquid. Choose a drying agent that does not react with the product and follow the amount/time appropriate to the method. After drying, decant or filter the liquid away from the solid before distilling. A clear liquid may still contain dissolved water; visual clarity alone is not a water assay.

A solid product is dried using a suitable warm oven, desiccator or airflow/absorbent support according to its stability. Mixing loose drying salt into product crystals contaminates them. A desiccator separates the drying agent from the sample and lowers water-vapour activity around it.

If wet product is weighed, residual liquid increases the apparent mass and percentage yield. This systematic upward bias can even give an apparent yield above 100%. Repeated mass measurements after further suitable drying can diagnose remaining solvent, although the requested exam procedure and compound stability determine the appropriate method.

Use the change in solubility with temperature

Choose a solvent that dissolves much more product hot than cold and does not react with it. Dissolve impure solid in the minimum suitable volume of hot solvent. If insoluble material remains, hot-filter using warmed apparatus to avoid crystallisation in the filter. Dissolved impurities need not all be insoluble; many are removed later because they remain in the mother liquor.

Allow the clear solution to cool so product crystals form, then cool further if appropriate. Collect by suction filtration, wash with a small volume of cold suitable solvent and dry. Slow initial cooling often produces cleaner crystals, while excess cold washing dissolves more desired product and lowers recovery.

Recrystallisation improves purity at the cost of some yield: some product stays in the mother liquor and some is lost on apparatus. Using too much solvent lowers recovery because more remains dissolved at the final temperature. More purification cycles are justified by actual impurity evidence, not automatically required.

Original mass balance: 3.20 g crude material contains 2.80 g desired product. Recrystallisation gives 2.45 g pure dry product. Recovery of desired product is 2.45/2.80 × 100 = 87.5%; 2.45/3.20 = 76.6% describes recovery relative to crude mass and answers a different question.

Choose the distillation that fits the sample

Simple distillation works well when a volatile liquid is separated from non-volatile impurities or a sufficiently different-boiling component. Fractional distillation provides repeated vaporisation/condensation in a fractionating column, improving separation of closer-boiling miscible liquids; an azeotrope can limit complete separation.

Steam distillation can isolate a steam-volatile, water-immiscible or sparingly water-soluble organic substance from non-volatile material. Water and organic vapours contribute to the total vapour pressure, so the mixture can boil below the organic compound’s normal boiling temperature, helping reduce thermal decomposition. It is not suitable for every high-boiling or fully water-miscible compound.

Condense the mixed vapour and separate/extract the organic component as appropriate. A diagram must show steam entry or in-situ steam generation, a vapour path, cooling-water flow and a receiver. Choose pressure-safe arrangements and prevent suck-back according to the taught procedure.

Measure a range and tie control measures to actual hazards

For a melting range, place a small dry powdered sample in a capillary and heat slowly near the expected transition. Record first liquid and complete melting. Impurities often lower and broaden the range; heating too fast creates thermal lag and can bias the observed temperature upwards. A sharp match supports purity and identity but is not conclusive alone.

For boiling temperature, measure saturated-vapour temperature during steady distillation and note pressure. Keep the thermometer correctly positioned and compare with a reference at the same pressure. Similar boiling points or an azeotrope can obscure contamination, so a narrow range is supporting evidence rather than proof of purity.

A useful risk explanation names a hazard, an exposure route and a control: volatile corrosive fumes → inhalation risk → suitable fume cupboard; flammable solvent → ignition risk → remove flames/use appropriate electrical heating; hot glass → burns → cooling and heat-resistant handling. Hazard depends on substance/procedure; risk also depends on amount, concentration and how it is used.

These written explanations support 9CH0 practical questions. Competence in assembling, handling and observing must also be demonstrated through supervised laboratory work for the separately assessed practical endorsement.

Quick checks

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

Sources