Question 1

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?

Accepted Answer

(a) very polar: paper
(b) very polar: paper
(c) less polar: TLC

Question 2

Consider the TLC plate illustrated in Problem 8. Would the R_fof 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?

Accepted Answer

Yes, the R_fof Compound A be different. 50% Hexanes-50% ethyl acetate is a more polar sol- vent system than is 90% hexanes-10% ethyl acetate. Therefore, the R_fof Compound A would be greater because the compound will travel further up the plate.

Question 3

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?

Accepted Answer

Either too much compound was spotted or the sample is very impure. The student should dilute the sample and re-run the TLC. If a streak is again obtained, the student's product is impure.

Question 4

A student spots an unknown sample on a TLC plate. A single spot with an R_fof 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?

Accepted Answer

The answer of A student spots an unknown sample on...

Question 5

If two compounds have R_fvalues 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?

Accepted Answer

The answer of If two compounds have R_fvalues of...

Question 6

Calculate the R_fvalues 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

Accepted Answer

The answer of Calculate the R_fvalues for the following...

Question 7

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?

Accepted Answer

The answer of A wick of filter paper is placed...

Question 8

Again consider the TLC plate illustrated in Problem 12.8. Would the R_fof 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?

Accepted Answer

The answer of Again consider the TLC plate illustrated in...

Question 9

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_fvalues for the three compounds in the sample.

Accepted Answer

The answer of Consider a sample that is a mixture...

Question 10

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?

Accepted Answer

The answer of A student is following a reaction (the...

Exam 13: Thin Layer Chromatography

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?

Consider the TLC plate illustrated in Problem 8. Would the Rf 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?

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?

A student spots an unknown sample on a TLC plate. A single spot with an Rf 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?

If two compounds have Rf 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?

Calculate the Rf 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