Methods of Purification of Organic Compounds

Full Notes

Purification techniques are essential in organic chemistry to isolate the desired compound from a mixture or reaction product. The method chosen depends on the nature of the compound and impurities.

Sublimation

Sublimation is a process where a solid changes directly into a vapour on heating, without passing through the liquid state. It is used for compounds that sublime easily, like naphthalene or camphor. Impurities that do not sublime remain behind. The vapour is cooled and collected as pure solid.

Crystallisation

This is one of the most common techniques for purifying solid organic compounds. The impure substance is dissolved in an appropriate solvent at high temperature, and the solution is then cooled slowly.

The pure compound crystallises out, while impurities remain in the solution. The solvent should:

• Dissolve the compound at high temperature.
• Not dissolve the impurities.
• Be volatile and easily removable.

Distillation

Distillation is used for purifying liquids. It involves boiling the liquid and condensing its vapours to collect the pure compound.

Simple Distillation:

Suitable when the boiling point difference between components is more than 25 °C. The liquid with the lower boiling point distils first.

Fractional Distillation:

Used when the boiling points are close. A fractionating column provides repeated condensation and vaporisation cycles, allowing better separation (e.g. alcohol–water mixture).

Distillation Under Reduced Pressure:

Useful for liquids that decompose at high temperatures. Lowering the pressure reduces the boiling point (e.g., glycerol purification).

Steam Distillation:

Used for water-insoluble compounds that are steam-volatile (e.g., essential oils). The compound co-distils with steam at a temperature lower than its boiling point.

Differential Extraction

This technique separates compounds based on their differential solubilities in two immiscible liquids (usually water and an organic solvent like ether).

The aqueous and organic layers are separated using a separating funnel. It is commonly used for extracting organic acids or bases from aqueous mixtures.

Example Benzoic acid extraction

Benzoic acid in water can be extracted into diethyl ether, then recovered by evaporating the ether.

Chromatography

Chromatography is a powerful technique used for separating small quantities of mixtures into individual components.

Principle: Components move through a stationary phase at different rates due to their differing affinities for the mobile and stationary phases.

Types:

Adsorption Chromatography:

Column Chromatography:
A glass column is packed with an adsorbent which acts as the stationary phase (like alumina or silica gel). The mixture is loaded on top and eluted with a solvent. More strongly adsorbed components move slower.

Thin Layer Chromatography (TLC):
A thin layer of adsorbent is spread on a glass or plastic sheet. The mixture is spotted and developed in a solvent chamber.

Separated components are visualised under UV or iodine vapour.

Partition Chromatography:

The separation is based on the relative solubilities of components between a mobile phase (solvent, e.g. water) and a stationary phase.

Paper chromatography is a common example.

Summary

• Sublimation and crystallisation purify solids based on volatility and solubility.
• Simple and fractional distillation separate liquids by boiling point differences.
• Reduced pressure and steam distillation help when compounds decompose or are steam volatile.
• Differential extraction uses immiscible solvents to isolate target compounds.
• Chromatography separates small amounts based on affinities to stationary and mobile phases.