Author Question: Consider the TLC plate illustrated in Problem 8. Would the Rf of Compound A be different if the ... (Read 32 times)

torybrooks · **on:** Aug 23, 2018

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 hexanes50 ethyl acetate? If so, would
it be lower or higher?

Question 2

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 hexanes10 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 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?

Question 4

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 hexanesethyl acetate solvent mix
ture. Predict the relative Rf values for the three compounds in the sample

Question 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?
(a)
and
CH3CH2SO3H CH3CH2SO2NH2

Question 6

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 hexanesethyl 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?

ghepp · **Reply #1 on:** Aug 23, 2018

Answer to Question 1

Yes, the Rf of Compound A be different. 50 Hexanes50 ethyl acetate is a more polar sol
vent system than is 90 hexanes10 ethyl acetate. Therefore, the Rf of Compound A would
be greater because the compound will travel further up the plate

Answer to Question 2

(a) Compound B is the more polar compound.
(b) At 10 and 20 min, the reaction is not complete. At 30 min, it is complete.
(c) The student should obtain a standard sample of Compound B and run it on the same TLC
plate as the reaction mixture.

Answer to Question 3

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.

Answer to Question 4

Table 11.1 (p. 122) gives the expected eltuion order of organic compounds in TLC. Biphenyl, an
aromatic hydrocarbon, is the least polar of the three compounds and will travel the farthest on
a TLC plate. Benzoic acid, a carboxylic acid, is the most polar and will travel the least far of the
three compounds. Benzyl alcohol, an alcohol, will travel in the middle.

Answer to Question 5

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

Answer to Question 6

A single spot on a TLC plate does not necessarily indicate that the mixture spotted contains
only one compound. Many compounds can have the same Rf value on a TLC plate. To verify
the purity of the sample, it should be run in more than one TLC system. Or, resort to other
methods of purity assessment, such as melting point determination or spectroscopy.