Purification and Analysis of Organic Compounds (Class 11)
Every technique here separates by exploiting one specific difference. Name the difference and the choice of method becomes obvious.
Choosing a purification method
| Method | Property exploited | Best when |
|---|---|---|
| Crystallisation | Difference in solubility with temperature | Solid, soluble hot and sparingly soluble cold |
| Simple distillation | Large boiling point difference | Liquids differing by a wide margin |
| Fractional distillation | Small boiling point difference | Liquids with close boiling points |
| Distillation under reduced pressure | Boiling point falls with pressure | Compound decomposes before its normal boiling point |
| Steam distillation | Volatility in steam, immiscibility with water | Compound is steam-volatile and water-insoluble |
| Sublimation | Direct solid-to-vapour transition | Compound sublimes, impurities do not |
| Chromatography | Differential adsorption or partition | Small quantities, or closely similar compounds |
Crystallisation in practice
The compound is dissolved in the minimum volume of hot solvent, hot-filtered to remove insoluble impurities, then cooled slowly so the compound crystallises while soluble impurities remain in the mother liquor. Slow cooling gives purer crystals, because rapid cooling traps impurities in the growing lattice.
Chromatography
All forms separate because components move at different rates between a stationary and a mobile phase. In adsorption chromatography, components adsorb to different extents on the stationary phase. In partition chromatography, they distribute differently between two liquid phases.
The retardation factor is defined as the distance moved by the component divided by the distance moved by the solvent front. It is characteristic under fixed conditions, which is why it is used for identification.
Qualitative analysis — detecting elements
Elements other than carbon and hydrogen are detected by the Lassaigne test. The organic compound is fused with sodium metal, converting covalently bound nitrogen, sulphur and halogen into ionic sodium salts that can be tested in solution.
The fusion step is the essential idea: covalent compounds do not give the ionic tests directly, so they must first be converted to ionic form. That reasoning is regularly asked.
- Nitrogen gives a Prussian blue colour on treatment with iron salts.
- Sulphur gives a violet colour with sodium nitroprusside, or a black precipitate with lead salts.
- Halogens give characteristic silver halide precipitates differing in colour and in solubility in ammonia.
- Nitrogen and sulphur together give a blood-red colour rather than the separate tests, because thiocyanate forms instead. This interference is a favourite question.
Quantitative analysis
Carbon and hydrogen are estimated by burning a known mass in oxygen and absorbing the carbon dioxide and water produced. Nitrogen is estimated either by measuring the volume of nitrogen gas liberated, or by converting it to ammonia and titrating.
Halogens and sulphur are estimated by oxidising the compound so the element ends up as a precipitable or titratable ion, then weighing or titrating it.
In every case the calculation is a proportion: the mass of element found, divided by the mass of compound taken, gives the percentage. These calculations are straightforward but require care with which product contains which element.
Frequently asked questions
Why is slow cooling better in crystallisation?
Because crystals grow more selectively, excluding impurity molecules. Rapid cooling traps impurities within the lattice as it forms quickly.
Why does a compound distil below 100 °C in steam distillation?
Because the compound and water are immiscible, so their vapour pressures add independently. The combined pressure reaches atmospheric at a temperature below the boiling point of either.
Why must sodium fusion be used before testing for nitrogen?
Because nitrogen is covalently bound and gives no ionic test. Fusion converts it to cyanide ion, which can then be detected.
Why does a compound containing both nitrogen and sulphur give a red colour?
Because thiocyanate forms during fusion rather than separate cyanide and sulphide, and thiocyanate gives a blood-red complex with iron instead of Prussian blue.
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