Purification of Organic Compounds

The purification of organic compounds is necessary, though a complex step after its extraction from a natural source or synthesis in the laboratory.  The method of purification of the organic compound depends mainly on the nature of the compound and the impurities present.

Table of Contents

• Sublimation
• Crystallisation
• Distillation
• Fractional Distillation
• Vacuum Distillation
• Steam Distillation
• Differential Extraction
• Chromatography

One easy method to check the purity of an organic compound is to either melt or boil it, as organic compounds tend to have sharp melting and boiling points.

Purification means the removal of unwanted impurities present in an organic compound. The general methods of purification are listed below:

• Sublimation
• Crystallisation
• Distillation
• Differential Extraction
• Chromatography

Methods of Purification

Sublimation

Some solids can directly pass to the vapour state without going through the liquid phase. The purification technique which exploits this property is called sublimation. It is helpful in separating sublimable compounds from non-sublimable ones.

Methods of Purification – Sublimation

The substance is heated in a China dish above, and an inverted funnel is kept to collect the sublimable compounds. The funnel is kept cool so as to hasten the process. Vapours of the substance solidify on the funnel.

Check: Sublimation

Crystallisation

The principle here is that the compound and the impurities have different solubilities in a solvent. A solvent is chosen, where the compound to be purified is sparingly soluble, that is, it is sparingly soluble at a lower temperature and soluble at a higher temperature. The solution is heated to get a saturated solution, and on cooling, the crystals of the compounds are removed via filtration.

For example, crystals of benzoic acid can be crystallised with water. Benzoic acid is sparingly soluble in cold water and soluble in hot water.

If the mixture contains impurities that have the same solubility as that of the compound to be purified, repeated crystallisation is performed.

Check: Crystallisation

Purification of Liquids

Distillation

The underlying principle behind distillation is that the mixture of liquids can be separated by the difference in their boiling points. The boiling point is defined as the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure. This method separates volatile liquids from non-volatile liquids. The setup is given below.

Purification of Liquids – Distillation

The mixture is taken in the RB flask and boiled. The more volatile, i.e., component with a lower boiling point, evaporates faster and is collected in a separate container. A condenser is used to hasten the process of condensation.

For example, a mixture of chloroform and aniline can be separated by distillation. The boiling point of chloroform is 60°C, and that of aniline is 189°C. Therefore, distillation can be used to separate a mixture of chloroform and aniline.

Check: Distillation

Fractional Distillation

This method is employed when the difference between the boiling points of the liquids isn’t much. Since the vapours of such liquids might condense together, a fractionating column is fixed to the mouth of the RB.

Check: Fractional Distillation

Purification of Liquids – Fractional Distillation

Vacuum Distillation

Since the boiling point is dependent on the atmospheric pressure, the liquids will boil at a temperature lesser than their boiling points if they were distilled in an atmosphere having lower pressure. This is achieved by using a vacuum pump. Since the atmospheric pressure is reduced, the liquids also boil faster, and hence the whole process of distillation is made fast.

Process of Vacuum Distillation

Steam Distillation

In this variant, steam is passed into the flask containing the liquids to be separated. The principle here is that the liquids will boil faster because aqueous tension (vapour pressure of water) helps in equalising the atmospheric pressure.

Total pressure = Aqueous tension + vapour pressure of liquid components

Purification of Liquids by Steam Distillation

In the absence of aqueous tension, the process of boiling would have been continued until it equalises the atmospheric pressure. Now, with the addition of steam, this process is expedited.

Check: Steam Distillation

Types of Distillation – Video Lesson

Differential Extraction

This method is used for immiscible liquids, that is, liquids that do not mix together. For example, oil and water are immiscible.

The immiscible liquids are taken in a separating funnel and left undisturbed. After a while, they separate out according to their specific gravities, with the heavier liquid at the bottom. Then, they are collected later.

Substances can also be separated according to their preferential solubilities in the liquid. For example, if phenol is to be extracted, it can be preferentially extracted using NaOH solution as one of the liquids used.

Chromatography

Chromatography is an important separation technique used to separate constituent particles of a mixture of substances to purify the compounds and check the purity of organic compounds. In this technique, on a stationary phase (solid or liquid), a mixture of substances is applied. The mixture of gas or the pure solvent is allowed to move slowly on the stationary phase. Due to this, the components of the mixture start separating from one another.

Chromatography is of two types:

• Adsorption Chromatography
• Partition Chromatography

Adsorption Chromatography

It is based on the principle that the constituents are adsorbed on an adsorbent in varying degrees. The adsorbents used are generally silica gel or alumina. When a mobile phase moves over the fixed phase, different constituents of the mixture get adsorbed at various distances over the fixed phase.

Check: Adsorption Chromatography

Adsorption Chromatography is further classified into:

• Column Chromatography
• Thin Layer Chromatography

Column Chromatography

Here, a mixture is separated over a column of either silica gel or alumina, packed in a glass column. The constituent with the most affinity with the fixed phase is adsorbed at the top, and so on. It is then retrieved by using an eluant. The solvent is then evaporated to get the constituent.

Check: Column Chromatography

Thin Layer Chromatography

Here, a sheet of alumina is taken (0.2 mm thick), over which a small spot of the mixture is placed, and it is kept in a suitable solvent. The solvent rises due to capillary action, and the constituents also rise with the solvent depending on their differential adsorption, and thereby, they are separated.

Check: Thin Layer Chromatography

Also Read: Crystallization of Impure Sample