Question

Cis-3-hexene and trans-3-hexene can be separated using GC. Explain why these two compounds cab be separated by GC and not by fractional distillation

Answer 1

Gas chromatography works on the principle of retention time which is connected to the retention time of the molecule to the stationary phase. This results in different eluting time for different molecules.

In the case of fractional distillation, the important aspect is the considerable difference between the constituent molecules. The difference should be less than 25⁰ C. According to the boiling point of the molecules involved they will be separated corresponding to the change in temperature.

In the given mixture of Cis-3-hexene and trans-3-hexene, the difference between both the molecules is not so considerable. The boiling point of Cis-3-hexene is around 66-68 ⁰C whereas the boiling point of trans -3-hexene is 67⁰ C.

As you can see the temperature is almost similar so clearly the fractional distillation cannot be used here.

Which is logical since both the molecules are geometrical isomers of each other which results into similar properties.

In gas chromatography, there will be a significant difference in the strength of adsorption on the stationary phase of each molecule which will result into different retention time which makes the separation of them possible.

Hence in order to separate them out, we need to use Gas chromatography.