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Organic Chemistry - Separation and Distillation

You need to know how to separate and purify organic compounds from mixtures. So, you need to be familiar with the different methods of purifying and isolating substances, which are thin layer chromatography, extraction, distillation and recrystallization. 

Thin Layer Chromatography (TLC) 

This is a quick and easy procedure to separate mixtures of organic compounds based on phase distribution - a mobile phase and a stationary phase. TLC indicates the number of compounds in the mixture. 

This is how it works. Suppose we were given a mixture X and we wanted to identify how many compounds it contains. Here is what we do; take an adsorbent paper and place it on a glass plate. This is the stationary phase. Then dissolve the compound in an organic solvent like benzene. This is the mobile phase. We then take a sample of the unknown solution and add a drop of the sample near the edge of the plate. Place the plate in a container with a small amount of an eluting solvent. The solvent will then migrate up the plate by capillary action carrying with it the components of the mixture at different rates. In the end, we will find several spots on the plate each representing different compounds. So, if for mixture X we found three different spots, it means that it contains three unknown compounds. 


An extraction is a procedure used to isolate individual components of a mixture by repeatedly adding an aqueous solvent to extract different compounds. An extraction involves the distribution of a solute between two immiscible solvents. Each of the solvents has different acid-base properties that isolate specific types of compounds. Let us look back at mixture X. The TLC chromatography indicated that the mixture contained three compounds. Now, we want to know what types of compounds are in the mixture. If one of the compounds is moderately basic like an amine, it will be extracted from the solution by HCl. So, if we add HCl to the solution and shake vigorously, we will see two layers in the solution. We drain off the phase containing the base. If we now take the remaining solution and mix it with weak base like sodium carbonate, it will extract compounds such as carboxylic acids. 

We once again removed the layer we are interested in and proceed with the extraction. If we take the solution and mix it with a strong base like sodium hydroxide, it will separate out phenols. So, for mixture X we have identified three substances, an amine, a phenol, and a carboxylic acid. 


Distillation is used to purify a volatile liquid product. It is based on the ability of substances to turn into gasses. Here is how distillation works. Take the solution and put it in a flask and heat the liquid until it boils. The vapors in the flask are collected in a receiver flask and allowed to recondense. The nonvolatile impurities remain in the original flask. We can also separate two or more liquids in a flask if they have vastly different boiling points. The lower boiling mixture will separate first followed by the higher boiling mixture. 


Recrystallization is used to purify solid compounds. To do this we take a solid and dissolve it in a hot solvent to disrupt its crystalline structure. We will need an appropriate solvent that will dissolve the solid at its boiling point. The best solvents are generally those that dissolve the impurities. Remember like dissolves like. Filter the solutions so that the impurities are removed with the hot solvent. Then, the pure crystals are allowed to reform.