Question

Referring to your experiment; if your first component that eluted with hexane was completely off, but you still had hexane at the top of the column, is it permissible to add the hexane-methylene chloride mixture? If so, why?

**I know it is not permissible because it would not be diserable, just looking for an explanation why.

Details: It was a Column Chromotography experiment. Three solvents are used: 1) hexane; 2) 1/1 hexane/methylene chloride; 3) 9/1 methylene chloride/methanol. Began eluting with hexane, then changed to solvent 2 and then 3.

Answer 1

Ans:

Column chromatography is based on principle of adsorption-desorpion.

Column chromatography comprises solid stationary phhase and liquid mobile phase and mixture of organic compounds to be separated or impure organic compound to be purified.

Steps: 1: If your organic compound is liquid then adsorb it on sufficient quantity of Silica or if it is Solid then dissolve it in appropriate solvent (generally ethyl acetate) and then use this solution to adsorb your mixture on dry silica.

2. make a slurry of pure silica in non-polar solvent Hexane or cyclohexane and pour it in a column. (lower outlet of column must be plugged with cotton)

3. Let off excess solvent and put mixture adsorbed on silica above the slurry and then put cotton (spread) on it and again pour some silical above it.

4.start elution using appropriate solvent mixture or pure hexane depending on nature of compound oor impurity.

Now, your mixture contains three compounds having different polarities.

First use hexane which dissolves less polar component i.e. desorption of first component is more in hexane than adsorption. (adsorption rate is less than desorption rate) and get eluted first.

while doing this 2nd and 3rd component has less desorption rate because these components are more polar than 1st component and has less solubility in hexane.

As Polarity increases desorption rate increases in more polar solvent and elution rate also increases. Hence component runs faster in gravity column.

Polarity order of solvents:

Hexane (Non-polar) < Hexane+ MDC = 1:1 (Less polar)< MDC + MeOH = 9:1 (Highly polar)

Polarity order of components based on solvents used:

1st component < 2nd component < 3rd component.

Order of Solubility in Hexane : 1st component > 2nd component > 3rd component.

Order of solubility in Hexane + Methylene dichloride = 2nd component > 3rd Component.

this is the reason why 1st component comes out completely when eluent is Hexane.

[Note: MDC+Methanol = 9:1 is highly polar and it can dissolve all three components of mixture. Its dissolving power is high. In fact after completion of elution columns are washed by Ethyl acetate or methanol]

when solvent system is Hexane+ MDC = 1:1 component 3rd which is more polar has less solubility than comonent 2nd and hence runs with slower rate.

NOW, question is when component eluted completely and HEXANE remained in excess, is it permissible to add Hexane+ MDC = 1:1 ?

You can add Hexane+ MDC = 1:1 solvent solution but as hexane is already present in the column above the solid / stationary phase your solvent system ratio increases increases.

Hexane+ MDC will not be equal to 1:1 but it is greater than 1. And polarity of your solvent system decreases as % of hexane increases. But it has no problem as mixure contains only component 2 and 3.

And component 3 has less solubility in Hexane+ MDC = 1:1 and hence it will be even lesser soluble in solvent in which Hexane : MDC ration is > 1 (which is less polar than Hexane+ MDC = 1:1) .

So by adding second solvent system i.e. Hexane+ MDC = 1:1 ELUTION time of 2nd component decreases slightly but it will not affect the separation of 2nd from 3rd component.

effects of adding Hexane+ MDC = 1:1 without removing excess hexane:

1) Solvent required will be slightly higher

2) Time required to separate component 2 frm 3 is higher

So it is not permissible to add Hexane+ MDC = 1:1 without removing excess hexane.