Purification of Organic Compounds

Purification of chemical substances or any matter works on the method of separating one substance from the other in a compound. A sample, in a mixed state, is transformed into new samples, each of which consists of a single substance.

After this, separation methods are used to separate these substances. These methods change the relative amounts of substances in a mixture.

The purification of organic chemicals and compounds depends on the nature of the compounds and the impurities in them. There are many methods of purification of organic compounds involved in the process of separating organic substances that we will be discussing in this guide.

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Methods of purification of organic compounds

Purification of organic chemicals and compounds could be done through many methods. The method involved only depends upon:

  • The nature of the substance
  • Type of impurities present

Some of the most important and commonly used purification methods are:

Filtration

  1. Exactly as it sounds, filtration involves passing a liquid containing the impurities through a filter to get a cleaner liquid. 
  2. The filter is usually made up of a funnel, preferably a Büchner funnel, which is lined with filter paper or some other screen to filter the impurities. 
  3. As the liquid passes through the filter, the impurities are left behind on the filter and a cleaner mixture is achieved.

Simple crystallization

  1. The purification of organic compounds by crystallization is a commonly used method to purify chemical compounds. Crystallization refers to dissolving a sample to be filtered in a solvent. 
  2. The solvent is chosen such that the desired chemical easily dissolves at higher temperatures and is sparingly soluble at room temperatures. The solute is heated well before the impure sample is completely dissolved in it. 
  3. After completely dissolving, the mixture is allowed to cool down to room temperature. As the temperature reduces, crystals of the desired pure substances begin to form, which can be removed by filtration. 
  4. The mother solute will now contain most of the impurities with some traces of the desired product.

Fractional crystallization

  • Similar to simple crystallization, fractional crystallization uses the solubilities of organic compounds to purify chemical substances 
  • When we have two solutes with different solubilities in a solvent, we dissolve the mixture in the said solvent at a high temperature. After we cool the solvent down, the less soluble substance will form crystals first that can be easily filtered out
  • The next step is to increase the concentration of the second solute and cool the mixture further. This cooling of the mixture will form crystals of the second desired solute, achieving fractional crystallization
  • Think of fractional crystallization as simple crystallization but done multiple times for different compounds
  • The more the concentration, the more effective and easier the purification of organic compounds will be
  • We increase the concentration of each subsequent solute for the same reason

Sublimation

  1. Certain organic compounds, when heated, directly change their state from solid to gas. 
  2. This property helps us separate mixtures with impurities that tend to sublimate.  Just by heating the mixture, the substances sublimate and leave us with pure products.
  3. A common example of sublimation is mothballs. When heated, mothballs directly turn into gas rather than a liquid.

Simple distillation

  1. Distillation involves evaporation and condensation of liquids for purifying organic compounds. 
  2. The mixture, which contains a liquid that boils without decomposition and non-volatile impurities, is heated to evaporate the liquid. The vapors then pass through a condenser where they are cooled and, thus, liquified. 
  3. The formed liquid, being pure, is collected and stored.

Fractional distillation

  • If two liquids in a mixture have boiling points close to each other, separating them using conventional distillation is very difficult. We achieve our objective of purifying organic compounds by introducing a fractionating column
  • The fractionating column provides hurdles to the rising vapours. As a result, the final vapours are richer in the more volatile substance while the less volatile liquid flows down back into the mixture
  • Fractional distillation is more effective than simple distillation
  • This is because the beads in the fractionating column act as many “Theoretical Plates”
  • These plates act as surfaces for the vapour to condense, re-evaporate, and re-condense again and again
  • This essentially works when the distillation of the mixture takes place repeatedly

Distillation under reduced pressure

  • This process is used for distilling liquids that decompose at or below their boiling points. Every liquid boils at a certain pressure and temperature
  • By decreasing the pressure at which the liquid is kept, the temperature required for boiling is also reduced. Hence, the liquid does not decompose upon boiling and can be readily distilled
  • Example: Glycerol decomposes at 563 K before it boils. This temperature is reduced to 453 K at a pressure of 0.0157895 atm, allowing it to be distilled

Azeotropic distillation

  • Particularly useful in separating components of a mixture whose boiling points are the same, Azeotropic Distillation is a conventional method of separating mixtures mixed with a third liquid
  • This decreases the partial pressure of one of the original solutes more than the other, making the complete solution prone to the other distillation methods
  • Azeotropes are mixtures that have a specific boiling point. The best-known example is a mixture of ethanol and water in the ratio of 95.87: 4.13. It boils at 78.13 ℃
  • Benzene is added to this mixture to cause a change in the properties of the constituents
  • This change in properties allows for the distillation of this mixture

Chromatography

  • The process of Chromatography employs the property of organic substances that dictates how well they move through porous substances under the influence of other moving mediums
  • The impure mixture separates into its constituting pure components as it moves through the porous medium, giving us the desired products
  • A simple example of chromatography can be seen in our homes
  • When water is poured on a paper with a black pen used on it, it is observed that the black ink gets dissociated into a different colour (usually blue or green)
  • This happens because the organic compounds in the black ink flow at different rates through the sheet of paper under the influence of water. The same concept is used for the purification of organic chemicals

    Conclusion

    The separation of the substances leads to the purification of organic compounds. And this separation is based on:

    • The quality of the material to be processed
    • Physical and chemical phenomena are used to affect the separation.
    • The nature of the substances