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Chemistry > Organic Chemistry > Purification of Organic Compounds
Organic Chemistry

Purification of Organic Compounds

Almost everything that we see these days is impure, isn’t it? The water we drink and the food we eat also need to go through levels of purification processes. Similar is the case with organic compounds. There are several methods of purification of organic compounds. Why are these important and how do we do it? Let us learn all that in this chapter!

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Types of Purification

A large number of methods are available for the purification of substances. The choice of method, however, depends upon the nature of substance (whether solid or liquid). It also depends on the type of impurities present in it. We commonly use these methods for purification of substances:

  • Simple crystallisation
  • Fractional crystallisation
  • Sublimation
  • Simple distillation
  • Fractional distillation
  • Distillation under reduced pressure
  • Steam distillation
  • Azeotropic distillation
  • Chromatography

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Let us now study about these methods in brief for better understanding.

Simple Crystallisation

This is the most common method that we use to purify organic solids. For crystallisation, a suitable solvent is one

  • which dissolves more of the substance at a higher temperature than at room temperature
  • in which impurities are either insoluble or dissolve to an extent that they remain in solution (in the mother liquor) upon crystallisation
  • which is not highly inflammable and
  • which does not react chemically with the compound to be crystallized. The most commonly-used solvents for crystallisation are water, alcohol, ether, chloroform, carbon- tetrachloride, acetone, benzene, petroleum ether etc.

Fractional Crystallisation

It is the process of separation of different components of a mixture by repeated crystallisations. In the first step, we dissolve the mixture in a solvent in which the two components have different solubilities. When we cool a hot saturated solution of this mixture, the less soluble component crystallises out first while the more soluble substance remains in solution.

The mother liquor left after crystallisation of the less soluble component is again concentrated and then we allow it to cool. Hence, we obtain the crystals of the more soluble component.

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Purification of Organic Compounds

Sublimation

Certain organic solids on heating directly change from solid to vapour state without passing through a liquid state. These substances are sublimable. This process is sublimation.

We use this process for the separation of sublimable volatile compounds from non-sublimable impurities. We use this for the purposes of purification of camphor, naphthalene, anthracene, benzoic acid, Iodine and salicylic acid etc containing non-volatile impurities.

Simple Distillation

Distillation is the joint process of vapourisation and condensation. We use this method for the purification of liquids which boil without decomposition and contain non-volatile impurities. We can also use this method for separating liquids having sufficient difference in their boiling points.

Purification of Organic Compounds

Fractional Distillation

We can use this process to separate a mixture of two or more miscible liquids which have boiling points close to each other. We carry out this process by using fractionating columns. The fractionating column is a special type of long glass tube that has obstructions to the passage of the vapour upwards and that of liquid downwards. This method can separate a mixture of acetone (b. p. 330 K) and methyl alcohol (b. p. 338 K) or a mixture of benzene and toluene.

Distillation under Reduced Pressure

We use this method for the purification of high boiling liquids and liquids which decompose at or below their boiling points. Practical examples include the crude oil industry, sugarcane industry etc.

Steam Distillation

This method is applicable for the separation and purification of those organic compounds (solids or liquids) which:

  • are insoluble in water
  • are volatile in steam
  • possess a high vapour pressure (10-15 mm Hg) at 373 K and
  • contain non-volatile impurities.

Azeotropic Distillation

An azeotropic mixture is a mixture having a constant boiling point. The most familiar example is a mixture of ethanol and water in the ratio of 95.87: 4.13 (a ratio present in rectified spirit). It boils at 78.13oC. We can’t separate the constituents of an azeotropic mixture by fractional distillation. Hence, we have to use a special type of distillation (azeotropic distillation) for separating the constituents of an azeotropic mixture.

In this method, we use the third compound in distillation. The process uses the fact that dehydrating agents like diethyl ether etc. depress the partial pressure of one of the original components. As a result, the boiling point of that component raises sufficiently and thus, the other component will distil over.

Chromatography

This is a modern method that we can use for the separation of mixtures into its components, purification of compounds and also test the purity of compounds. The name chromatography comes from the Greek word ‘chroma’ meaning colour and ‘graphy’ for writing because the method was first used for the separation of coloured substances found in plants. This method was described by Tswett in 1906.

Principle of Chromatography

The technique of chromatography uses the difference in the rates at which the components of a mixture move through a porous medium (stationary phase) under the influence of some solvent or gas (moving phase).

Thus, this technique consists of two phases- one is a stationary phase of the large surface area while the second is a moving phase which is allowed to move slowly over the stationary phase. The stationary phase is either a solid or a liquid while the moving phase may be a liquid or a gas. There are also some other methods of purification like differential extraction and other chemical methods.

Solved Examples for You

Question: Give two practical applications of simple crystallisation.

Answer: Practical applications of simple crystallisation include:

  • Sugar having an impurity of common salt can be crystallized from hot ethanol since sugar dissolves in hot ethanol but common salt does not.
  • A mixture of benzoic acid and naphthalene can be separated from hot water in which benzoic acid dissolves but naphthalene does not.
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