Purification of organic compounds


Not all the compounds can be directly used for practical proposes soon after their extraction.  Many compounds require a lot of expenditure for their purification than mining but it is important to purify and remove the contaminating components of given substances. Only then, the substances come to a neutral and pure state which can be used based on the normal principles of chemistry. Some of the most commonly found impurities in the natural compounds are raw materials, decomposition compounds, compounds formed as products of side reactions, isomeric colourants, subsidiary colourants, and chance contaminants. There are many methods to purify them, but it depends upon the type and quantity of impurities in a compound. Let us discuss the most common methods of purification in practice.



Method of purification of organic compounds

Organic compounds can be purified by the following methods:

  1. Distillation method (Fractional Distillation, Vacuum Distillation and Steam Distillation)

  2. Sublimation method

  3. Crystallization method

  4. Chromatography method

  5. Differential Extraction




Distillation refers to the technique employed in separating volatile substances from non-volatile ones. It is applicable when the two substances have close but different boiling points. Distillation of chemical substances can further be group into three principal categories as follows:


1. Fractional distillation

It refers to the method of separating two or more liquids with different but close boiling points. For example, pure water with a boiling point of 100 and ethanol with a boiling point of 78 can be separated using this method as there are no huge differences in their boiling points. As the vapours of the participating liquids might condense at the same time, a fractionating column is fixed to the mouth of the RB flask. Usually, the component parts have boiling points that at least differ by less than 25 °C from each other under one atmospheric but if the difference in boiling points is greater than 25 °C, simple distillation is typically used rather than the fractional distillation. Fractional distillation typically requires a good laboratory setup that supplements heat source, such as a hot plate with a bath,  round bottom distillation flask, fractionating column, distillation head,  thermometer and adapter, condensers, such as a Liebig condenser or Allihn condenser, vacuum adapter and standard laboratory glassware with ground glass joints.



2. Distillation under reduced pressure

It is also known as vacuum distillation used to purify liquids that have high boiling points. Vacuum distillation separates compounds based on differences in boiling points and it is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. An instant reduction of pressure lowers the boiling point of compounds which can be calculated with the help of a temperature-pressure nomograph sing the Clausius–Clapeyron relation. Because the boiling point depends upon the atmospheric pressure, the liquids are boiled at a temperature lesser than their actual boiling points in case the substances were distilled in an atmosphere with lower pressure. This is accomplished by using a vacuum pump because the atmospheric pressure is lowered, the liquids also boil faster and hence the whole process of distillation is made fast.





3. Steam distillation

In this method, the steam is passed through the flask containing the liquids to be separated. Since the liquids will boil faster than the others, they create aqueous tension (vapour pressure of water) which helps in equalizing the atmospheric pressure. Therefore, total pressure = Aqueous tension + vapour pressure of liquid components.





Sublimation method

Sublimation refers to the technique used to separate substances that easily sublime on heating. Sublimation is the process of transition of a substance from the solid to the gaseous state without passing through the liquid state, in other words, a sublimation is said to happen in those solid compounds that enter the gaseous form without turning into liquids, for example, separation of iodine from the mixture of sand and iodine. The chemical is heated in a china dish containing an inverted funnel at its top where the top collects the sublimable compounds. As the funnel is kept cool, the vapours of the substance solidify on the funnel. To separate mixtures that contain a sublimate with the volatile component from a non-sublimate impurity, the sublimation process is used



Sublimation  for purification



Crystallization method

Crystallization refers to the method of separation of substances which mainly based on the difference between the solubility of substance and impurities in an appropriate solvent. In the crystallization method, the impure substance is first dissolved in a suitable solvent and then heated at regulated temperatures. The pure substance present in the mixture crystallizes out while others are not. Frequently used solvents for crystallization are water, chloroform, carbon- tetrachloride, acetone, benzene alcohol, ether and petroleum etc.



Chromatography method

Chromatography is the technique used to separate mixtures into their respective components, followed by this,  purify and test its purity. Chromatography is divided into two types; partial chromatography and adsorption chromatography. Partial chromatography or Partition Chromatography is the process of separating mixtures of chemical compounds by passing them through a column that contains a solid stationary phase.  Adsorption chromatography involves the analytical separation of a chemical mixture depending upon the interaction of the adsorbate with a specific adsorbent. The mixture of gas or liquid gets separated when it is passed over the adsorbent bed that adsorbs different compounds at different rates.  A good example of adsorbent material(s) is silica gel and alumina.



Chromatography method-image from Wikipedia



Differential Extraction

It suits the immiscible liquids as these liquids do not mix together. For example, oil and water are immiscible. The 2 or more immiscible liquids are taken in a separating funnel and left to be stabilized by themselves. After a while, they get segregated based on the respective specific gravities, with the heavier liquid at the bottom. Then they are later collected. 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.






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