Question

1. When 2-propanol was used as the developing 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. 3. A student needs to analyze a mixture containing an alcohol and a ketone by silica gel TLC. After consulting Table 17.1, suggest a likely developing solvent.

Answer 1

1. They may be identical or may not be identical. If they are identical then R_f will be same but if they are not identical, then R_f will be identical only if their polarity difference is less or close to same. Since the mobile phase(2-propanol) used here is very polar, so their R_f value is not differ due to their nonpolarity . But according to the question they are not identical.

To see the different R_f value, it is necessary to run the TLC plate in a mixture of polar(less amount) solvent(ethyl acetate and nonpolar(large amount) solvent(hexane) around 1:15 ratio.

2. Since the separation of R_f value of A and B in ether medium is large(0.60 - 0.44) = 0.16 so for separation purpose ether solvent is better than hexane(as R_f difference is only 0.42 -0.34 = 0.08).

3. A mixture of ethyl acetate (polar solvent) and pet-ether(nonpolar solvent) in 1:10 ratio will be effective for developing a solvent for the mixture of alcohol and a ketone.