The separation of components of a mixture to get pure components is one of the most important practices in chemistry. The purification of organic compounds is a necessary, though complex, step after its extraction from a natural source or synthesis in laboratory. Organic compounds made in the laboratory are impure because several by-products are also formed along with the main product, during a chemical reaction. Thus, there is a need for a purification method.
There are many purification methods used for several reactions. The method of purification of an organic compound mainly depends on the nature of the compound and the impurities present in it. The factors that could be taken into account for selecting a particular method of purification of organic compounds are boiling point, melting point, solubility, etc.
The various methods of purification of organic compounds are:
Sublimation is one of the methods of purification of organic compounds. This method can be applied to only those compounds that change their state directly from solid to vapour on heating; hence, this method can be used for separating the sublimable compounds from the non-sublimable impurities. This process is known as sublimation. In this process, a solid is kept in a sublimation apparatus and heated in vacuum. Under low pressure, volatilization of solid takes place which condenses on the cool surface to form pure compounds, and the non-volatile impurities are left behind.
The difference in the solubility of an organic compound and that of the impurities present in it, in a suitable solvent is the basis for this method of separation. A concentrated solution is prepared by dissolving the compound in a suitable solvent; the pure compound crystallizes out on cooling. The dissolved impurities are removed through filtration. The filtrate contains some amount of pure compound too. Compounds containing impurities of comparable solubility are purified by repeated crystallization.
This method is mainly used for separating liquids with sufficient difference in their boiling points. It can also be used for separating volatile liquids from non-volatile impurities. The two kinds of distillation methods widely used in the industry for purifying organic compounds are:
- Fractional distillation: It is the process in which the components of a mixture are separated by heating the chemical mixture. Boiling point plays a vital role in the fractional distillation. The component with lower boiling point vaporizes first, followed by the components with higher boiling points. In this way, the components of the mixture are separated.
- Steam distillation: In this method, substances which are steam volatile and immiscible with water can be separated. It basically lowers the distillation temperature of the organic compounds which are immiscible with water, having high boiling point.
- Differential Extraction:
Differential Extraction is one of the methods of purification of organic compounds. It is used for the separation of an organic compound which is present in an aqueous solution. The compound is removed by dissolving it into a highly soluble solvent, forming a layer with the aqueous solution. The aqueous solution and the organic solvent should be immiscible and are separated using a separating funnel. Later on, the organic solvent and the compound are separated by evaporation or distillation.
Chromatography is an important separation technique used to separate constituent particles from a mixture of substances, to purify compounds and to check the purity of compounds. In this technique, on a stationary phase (solid or a liquid) a mixture of substance is applied. The mixture of gas or the pure solvent is allowed to move slowly to 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
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