Methods of Purification of Organic Compounds

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The organic compounds whether isolated from a natural source or prepared in the laboratory are mostly impure. These are generally contaminated with some other substances. A number of methods are available for the purification.
The choice of a particular technique or method depends upon the nature of the compound whether solid or liquid and also upon the nature of the impurities associated with it.
Common techniques used for purifications are as follows:-
(i) Sublimation
(ii) Crystallisation
(iii) Distillation
(iv) Distillation
(v) Differential extraction
Once the compounds have been properly purified and the purity checked by finding out the m.p. (melting point) or b.p. (boiling point) depending upon their nature. Most of the pure compounds have sharp melting and boiling points. 
Nowadays, the purity of an organic compound is determined by new methods based on different types of chromatographic and sepectroscopic techniques. Lets us briefly learn about different methods for purifying the organic compounds.
(i) Sublimation:– Certain organic solids on heating directly change from solid to vapour state without passing through a liquid state. Such substances are called sublimable. This process is called sublimation. The vapours on cooling change back to the solid form.
                                Solid – Heat—cool – Vapours
The sublimation process is used for the separation of sublimable volatile  compounds (such as camphor, naphthalene, anthracene, benzoic acid etc.) from non-sublimable impurities.
(ii) Crystallisation:- This is most common method for purifying organic solids. This method is based on the differences in the solubility of the organic compound and its impurities in a suitable solvent. This methods involves the following steps:-
(a) Preparation of the solution:- The organic compound is dissolved in a suitable solvent by heating. The amount of solvent should be just sufficient to dissolve the whole of the solid on heating and the solvent should not dissolved the impurities and chemically unreactive with the compound.
(b) Filtration of the compound:- The hot solution obtained above is then filtered immediately.
(c) Crystallisation:- The hot filtrate is allowed to cool slowly and undisturbed in the beaker. After some time crystals of the pure compound are formed. When the crystallization is complete, the crystals are separated from the mother liquor by filtration. The crystal left in the funnel are washed once or twice with little of the cold solvent to remove adhering impurities, if any.
(d) Drying of crystals:- The crystals are dried by pressing between the folds of filter paper and then placed in a steam or air oven for some time. The crystals are finally dried over sulphuric acid or calcium chloride in a desicator
(e) Removal of colour:- Sometimes, the crystals obtained are slightly coloured due to presence of certain coloured impurities. In such cases, crystals are redissolved in the same solvent and a small amount of activated charcoal is added to it.
The mixture is boiled for 15-20 minutes. During this treatment, charcoal adsorbs all the coloured impurities. The charcoal is then filtered out and the filtrate is allowed to cool when colourless crystals of the pure substance are obtained.
(iii) Distillation:– “Distillation is the process of converting a liquid into vapours upon then cooling the vapours back to the liquid state.”
Liquid ——A —-Vapour —–Cool —- Liquid.
The process of simple distillation is used to purify those organic liquids which are quite stable at their boiling points and the impurities present are non-volatile. Liquids such as benzene, toluene, ethanol, acetone, chloroform, carbon tetrachloride can be purified by simple distillation.
(a) Fractional Distillation:- This method is used to separate two liquids whose boiling points are very close to each other (say differ by 10 K to 30 K) and the separation cannot be achieved by simple distillation. This is achieved by the use of fractionating column.
Mixtures whose compounds can be separated by fractional process are:
(i) Fractional distillation of crude oil (Petroleum)
(ii) Methyl alcohol (CH3OH) and acetone (CH3COCH3)  
(iii) A mixture of (C6H6) benzene and (CH3C6H5) Toluene

(b) Distilation under Reduced Pressure or vacuum Distillation:-

This method is used for the purification of the high boiling points liquids and which decompose at or below their boiling points.
A liquid boils when its vapour pressure becomes equal to the external pressure. Oviously, the same liquid would boil at lower temperature if the pressure acting on it is reduced. Since the liquid now boils at a lower temperature, its decomposition does not occur.
Pressure can be reduced up to 10-20 mm of Hg with the help of water section pumps in the laboratory. However withvacuuum pump, pressure of the order of 0.1 mm of Hg can be easily obtained. Some of the examples where vacuum distillation has been used to purify liquids.
(i) Steam distillation:- The process employed for the distillation of those substances (solid or liquid) which
(i) are insoluble in water
(ii) have high molecular mass
(iii) have fairly high vapour pressure at about 373 K and
(iv) are volatile in steam but the impurities present in them are non-volatile.
In this process, a mixture of two immiscible liquids i.e. water and an organic liquid is heated. Each would exert its own vapour pressure independently of the other and the mixture will begin to boil at a temperature when the sum of the vapour pressure of the organic liquid (p1) and that of the water (p2) becomes equal to the atmospheric pressure (P).
                       P = p1 + p2 
Evidently, the temperature at which the mixture boils must be lower than the normal boiling poit of the both the compoments. In other words, the organic liquid boils at a temperature lower than its normal boiling point and hence, the decomposition is avoided.
The process od steam distillation can be applied for the separation of a mixture of o-nitrophenol and p-nitrophenol.
In this process, water vapours carry along with them vapours of o-nitrophenol which is more volatile and they get condensed in the receiver. P-nitrophenol with higher boiling point remains in the distillation flask.This method can be used for the purification of impure sample of aniline.
(iv) Differential Extraction:- This technique is normally used to separate certain organic solids dissolved in water by shaking with a suitable organic solvent. The process called extraction is done in a separating funnel. The organic solvent selected should be such that
(a) The given solid must be more soluble in the organic solvent than a water.
(b) Water and organic solvent should not be miscible with each other.
The most common organic solvent is ether. Benzene and acetone can be also used. However alcohol is not the proper solvent because water and alcohol are highly miscible with each other.
The common methods for purifying organic compounds include distillation, crystallization, sublimation, chromatography, and extraction. Each method is chosen based on the nature of the compound and the impurities present.
Distillation separates components of a mixture based on differences in boiling points. Simple distillation is used for compounds with a significant difference in boiling points, while fractional distillation is used for separating compounds with closer boiling points.
Chromatography is a technique where components of a mixture are separated based on their different affinities to a stationary phase and a mobile phase. It is particularly useful for purifying small amounts of compounds and is widely used in analytical chemistry.
Recrystallization is used to purify solid organic compounds by dissolving them in a suitable solvent and allowing them to crystallize slowly. Impurities are left behind in the solvent, while pure crystals form.
Sublimation is the process of converting a solid directly into vapor without passing through a liquid state. It is used to purify substances that can sublime, such as camphor and naphthalene, from their non-volatile impurities.
Extraction involves transferring a solute from one solvent to another based on its different solubilities in the two solvents. This technique is commonly used in liquid-liquid extractions to separate organic compounds from aqueous mixtures.

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