Question

Suppose you are attempting the separation of three compounds (A, B, and C) by

column chromatography on silica gel. You start by running a number of TLC’s in

order to find the best solvent system for the separation. The results are

summarized in the table below.

Solvent Rf for A Rf for B Rf for C

Hexane 0.00 0.20 0.00

Toluene 0.00 0.30 0.05

Diethyl ether 0.05 0.75 0.29

Ethyl acetate 0.35 0.80 0.80

a) How polar are the three compounds: low, medium, or highly polar (please

estimate)?

b) Would the column chromatographic separation of all three compounds be

possible if either toluene or diethyl ether were to be used as the mobile phase?

Please explain.

Answer 1

A) Compound A is the least polar, as it has the lowest retention factor even with the most polar solvents which are Diethyl ether and Ethyl Acetate. Similarly, compound B is the most polar because it has a higher retention factor with the polar solvent Diethyl ether. Hence, the increasing order of polarity is A<C<B.

B) the separation of all three compounds could be possible if we were to use Diethyl ether as the solvent because the difference in the retention factor is considerable. Whereas, for Toluene, the retention factor for compound A and C are very close (i.e. 0.00 and 0.05 respectively).