Introduction: The molecules present in biological system or in synthetic chemistry are produced through a series of reactions involving intermediates. As discussed in previous lecture, at any moment of time biological organism has major fraction as desired product but has other compounds in minute quantities. The minor species present in a product is always referred as “impurities” and these compounds need to separate from desired product for biotechnology applications. How two molecules can be separated from each other? To answer this question we can take the example of three molecules given in Figure 28.1. These 3 molecules (benzene, phenol, aniline) are similar to each other but have distinct physical and chemical properties which can be used as a criteria to separate them. The physical and chemical properties which can be use to separate molecules are-

Physical Properties

1. Molecular weight

2. Boiling point (in case both are liquid, as in this case)

3. Freezing point

4. Crystallization

5. Solubility

6. Density

Chemical Properties

1. Functional Group, for example, phenol has –OH where as aniline has NH2.

2. Reactivity towards other reagent to form complex

Now for example you have a mixture of compound 1 (benzene) and compound 3 (Aniline) and you would like to purify benzene rather than aniline. In this situation, you can take the physical and chemical properties of benzene into the account and isolate it from the mixture.