Organic Chemistry – Some Basic Principles and Techniques · Lesson 10 of 15
Purifying Compounds by Physical Properties
“Choose a purification method by connecting solubility, volatility, and boiling behaviour to the mixture.”
• Choose purification methods from the properties of the compound and its impurities. • Explain sublimation and crystallisation, including solvent selection. • Distinguish simple, fractional, reduced-pressure, and steam distillation. • Explain differential and continuous extraction and interpret apparatus diagrams.
Choose the property that separates the components
A compound isolated from a natural source or made by synthesis is rarely obtained alone. Before studying its properties or analysing its composition, impurities must be removed. No purification technique works equally well for every mixture. Ask which property differs: can one component sublime, dissolve more strongly, boil more readily, or distribute into a second solvent?
A sharp melting or boiling point can help assess purity, while impurities often broaden a melting range or change it. Such measurements are evidence rather than a complete structural identification. Chromatography and spectroscopic methods supply other ways to assess a sample; chromatography receives its own lesson because the separation principle and techniques deserve careful treatment.
Purifying solids by sublimation and crystallisation
Sublimation is the direct change from solid to vapour without an intervening liquid phase under the conditions used. A sublimable organic compound can be separated from a non-sublimable impurity by vaporising it and collecting it on a cooler surface. Camphor mixed with calcium sulphate is a suitable conceptual example: camphor sublimes, while calcium sulphate remains behind.
Crystallisation relies on different solubilities. A useful solvent dissolves much more of the desired solid when hot than when cold. The compound is dissolved in hot solvent, insoluble material is removed where necessary, and cooling lets the desired compound form crystals. The remaining solution, called the mother liquor, retains more soluble impurities and some dissolved product. A pure-looking crystal does not mean every trace of impurity has disappeared.
If the compound is very soluble in one solvent and barely soluble in another, an appropriate mixed-solvent system may help. Activated charcoal can adsorb coloured impurities before the crystals are recovered. Impurities with similar solubilities may require repeated crystallisation. Purity and yield are a trade-off: repeated treatment can improve purity while leaving some product in each mother liquor.
Problem
A solid is highly soluble in solvent A both hot and cold, but soluble in solvent B when hot and only sparingly soluble when cold. Which is more promising?
- 1.Solvent A will retain much of the desired compound after cooling, so crystal recovery may be poor.
- 2.Solvent B allows a hot solution to hold the compound and a cold solution to release it as crystals. It is the more promising first choice, provided it does not react with the compound.
- 3.The impurity behaviour must also be considered. An impurity that remains dissolved on cooling can stay in the mother liquor, whereas an insoluble impurity can be removed before crystallisation.
Distillation separates through vapour formation
A liquid boils when its vapour pressure equals the external pressure. During simple distillation, vapour leaves a heated flask and is cooled in a condenser, returning to liquid in a receiver. The technique separates a volatile liquid from non-volatile material or separates liquids whose boiling points are sufficiently far apart. Chloroform at about 334 K and aniline at about 457 K illustrate the latter case.
If boiling points are close, the two components vaporise over overlapping temperature ranges and simple distillation gives poor separation. A fractionating column creates repeated contact between rising vapour and descending condensate. The vapour becomes progressively richer in the more volatile, lower-boiling component. The remaining liquid becomes richer in the less volatile component. The column is doing many small separation steps before the vapour reaches the receiver.
Packing or bubble plates provide surfaces for heat exchange and repeated condensation–vaporisation. A theoretical plate represents one ideal equilibrium separation stage; it need not be a literal shelf in a packed column. More effective stages improve separation. Petroleum refining applies fractional distillation to separate boiling-range fractions of crude oil.
Problem
Two miscible liquids boil only a few degrees apart. Explain why collecting the first condensate from a plain flask may not give a pure liquid.
- 1.Both liquids contribute to the vapour because their volatility ranges overlap. Condensation alone does not remove that mixed composition.
- 2.Repeated partial condensation and vaporisation in a fractionating column enriches the rising vapour in the more volatile component.
- 3.Choose fractional rather than simple distillation for this close-boiling mixture, while recognising that separation depends on the actual mixture behaviour.
Lowering the temperature of distillation
Some liquids have very high boiling points or decompose when heated to their normal boiling points. Reduced-pressure distillation lowers the external pressure so the liquid reaches the boiling condition at a lower temperature. A vacuum or water pump supplies the reduced pressure. Glycerol can be recovered from spent lye in the soap industry using this approach.
Steam distillation is suitable for a steam-volatile compound that is largely immiscible with water. Water vapour and organic vapour both contribute to the total pressure. The mixture can therefore boil when their combined vapour pressures reach atmospheric pressure, even though neither component alone has reached that pressure. The condensed mixture is later separated into its water and organic layers.
Problem
Why can an immiscible aniline–water mixture distil below the normal boiling point of aniline?
- 1.Aniline vapour does not have to supply all the external pressure. Water supplies part of it.
- 2.Boiling begins when the two partial vapour pressures add to the external pressure. This can happen below either pure component’s normal boiling temperature.
- 3.After condensation, the water-insoluble organic component can be separated with a separating funnel. The method depends on steam volatility and immiscibility, not simply on adding water to any liquid.
Extracting into a second liquid
Differential extraction transfers a dissolved organic compound from water into an immiscible organic solvent in which it is more soluble. Shaking brings the phases into contact; allowing them to settle restores two layers. A separating funnel lets the layers be collected separately. The organic solvent can then be removed by evaporation or distillation to recover the solute.
Choose a solvent that forms a separate phase and has suitable affinity for the target compound. Do not assume that the organic layer must always be the top layer: its density relative to water determines its position. If the compound is poorly soluble in the extracting solvent, continuous extraction can repeatedly bring solvent into contact with the sample, improving transfer without using a fresh very large volume at each stage.
Do not confuse reduced-pressure distillation with steam distillation. One changes external pressure; the other uses the combined vapour pressures of immiscible liquids. Solvent extraction instead uses solubility differences between liquid phases.
Quiz
Which method is suitable for separating camphor from calcium sulphate?
A good crystallisation solvent generally dissolves the compound how?
What does a fractionating column mainly provide?
Why does reducing pressure lower boiling temperature?
Which pair of properties is central to differential extraction?
Practice Problems
- Choose and justify a method for camphor with a non-sublimable salt, a volatile liquid with dissolved non-volatile solids, and two close-boiling liquids.
- Explain the roles of hot solvent, cooling, mother liquor, and activated charcoal in crystallisation.
- Trace the vapour and cooling-water paths in a distillation diagram and explain why the condenser is needed.
- Compare simple, fractional, reduced-pressure, and steam distillation by separation principle and suitable sample.
- Explain how repeated vaporisation and condensation enriches a volatile component in a fractionating column.
- Describe differential extraction and explain why the organic phase cannot always be assumed to lie above water.
Key Takeaways
• Choose a method from a useful property difference between compound and impurities. • Crystallisation uses solubility; distillation uses vapour formation and condensation. • Reduced pressure and steam offer different ways to distil at lower temperatures. • Extraction transfers solute between immiscible liquid phases.