Question

 

1. When 2-propanol was used as the develop- ing solvent, two substances moved with the solvent front (Rf = 1) during TLC analysis on a silica gel plate. Can you conclude that they are identical? If not, what additional experiment(s) would you perform?

2.The Rf value of compound A is 0.34 when a TLC plate is developed in hexane and 0.44 when the plate is developed in diethyl ether. Compound B has an Rf value of 0.42 in hexane and 0.60 in diethyl ether. Which solvent would be better for separating a mixture of A and B by TLC? Explain.

Answer 1

1) If we get same Rf of two substances in one solvent, It doesn't mean the two are the same. Try a different solvent or a mixture of solvents.

As both the substances moved with Rf=1, the developing solvent should be less polar than 2-propanol. One should try nonpolar solvent or mixture of solvents like hexane/ethyl acetate, dichloromethane, diethyl ether etc.

If you still see single spot in different solvents, then apply a co-spot (apply both the substances at the same point of application on TLC) and develop the TLC in nonpolar solvent/mixture of solvents. If the two substances are diferent they produce two different spots on the TLC after development.

Ans.2) TLC should be developed to get the maximum possible separation, which would be the largest spread in Rf values between the compounds A and B.

Separation of compound A vs B in Hexane = 0.34 vs 0.42 = 0.08

Separation of compound A vs B in diethyl ether = 0.44 vs 0.6 = 0.16

So based on above observation, diethyl ether would be the better solvent for separating a mixture of A and B by TLC as it gives better separation than hexane.