Deck 13: Thin Layer Chromatography

Question 1

Calculate the R_f values for the following compounds.
(a) Spot, 5.0 cm; solvent front, 20.0 cm
(b) Spot, 3.0 cm; solvent front, 12.0 cm
(c) Spot, 9.8 cm; solvent front, 12.0 cm

Answer 1

(a) R_f = 5.0 cm / 20.0 cm = 0.25
(b) 0.25
(c) 0.82

Question 2

If two compounds have R_f values of 0.50 and 0.61, how far will they be separated from each other on a plate when the solvent front is developed to
(a) 5 cm?
(b) 15 cm?

Answer 2

(a) x cm / 5.0 cm = 0.50; x = 2.5 cm. Similarly, y = 3.05 cm. Separation = 3.05 - 2.5) = 0.55 cm.
(b) 1.65 cm

Question 3

A wick of filter paper is placed in a TLC developing jar, and the atmosphere in the jar is satu- rated with solvent before a plate is developed. What would happen if a plate were developed in a jar with an atmosphere not saturated with solvent vapor?

Answer 3

The solvent front would not move evenly up the plate because it would be evaporating from the plate. Also the R_f from such a plate would be unreliable because more solvent is moving up the plate than normal, moving the spot farther up the plate than it should be.

Question 4

A student spots an unknown sample on a TLC plate. A single spot with an R_f of 0.55 showed up on the plate after developing the sample in hexanes-ethyl acetate 50:50. Does this indicate that the unknown material is a pure compound? What can be done to verify the purity of the sample?

Question 5

5 As a separation and detection method, would TLC or paper chromatography yield better results in the analysis of each of the following pairs of compounds?

Question 6

Consider a sample that is a mixture composed of biphenyl, benzoic acid, and benzyl alcohol. The sample is spotted on a TLC plate and developed in a hexanes-ethyl acetate solvent mix- ture. Predict the relative R_f values for the three compounds in the sample.

Question 7

After a rather lengthy organic chemistry synthesis procedure, a student ran the product of the reaction on a TLC plate and obtained the result below. What might he / she have done wrong, if anything?

Question 8

A student is following a reaction (the conversion of compound A to Compound B) by TLC. Aliquots of the reaction mixture are taken and analyzed at time = 0, 10, 20, and 30 min. The TLC plate, developed in 90% hexanes-10% ethyl acetate, is shown below.
(a) Which is the more polar compound, A or B?
(b) Is the reaction complete at 10 min? 20 min? 30 min?
(c) How can the identity of Compound B be verified by TLC?

Question 9

Consider the TLC plate illustrated in Problem 8. Would the R_f of Compound A be different if the solvent system used to develop the plate was 50% hexanes-50% ethyl acetate? If so, would it be lower or higher?

Question 10

Again consider the TLC plate illustrated in Problem 12.8. Would the R_f of Compound B be dif- ferent if the solvent system used to develop the plate was 100% hexanes? If so, would it be lower or higher?