Question

In this experiment column chromatography, two different elution solvent systems were used. Why? If their order had been reversed (i.e., 20:80 EtOAc–Hex was used first), how would the separation of the fractions have changed?

Answer 1

In column chromatography, mixture of two solvents is used, out of which one is polar and another is non-polar solvent. As the stationary phase is also polar, so the polar solvent will bind with the stationary phase along with the polar compounds present in the products. And non-polar compounds will get separated and will come along with the non-polar solvent. As we go on increasing the content of polar solvent, the separation is more. If we only use non-polar solvent for column chromatography, then we will only get non-polar compounds and polar compounds will be left in the column. If we use only polar solvent, then also the case will be same. So, we want to recover both the polar and non-polar compounds, we should have a mixture of both the solvents.

Ethyl acetate is a polar compound and hexane is a non-polar compound. If more of non-polar compound is taken in the mixture and less is polar compound, then more of non-polar compounds will be eluted. The most non-polar compound will come first, then less non-polar will be eluted.