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Thin Layer Chromatography Experiment: Separating Compounds using TLC, Summaries of Chemical Experimentation

An experiment for Thin Layer Chromatography (TLC), a technique used for separating and analyzing mixtures of compounds. the concept of Rf value, its significance, and how to perform the experiment. Students will spot different compounds on TLC plates and develop them using various solvent mixtures to determine their Rf values. The larger the Rf value, the less polar the compound is.

What you will learn

  • What are the structures of Acetophenone, Anisole, Benzoic Acid, Benzyl Alcohol, and Benzaldehyde?
  • What happens if too much sample is added to the TLC plate?
  • Calculate the Rf values for each compound in the experiment.

Typology: Summaries

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Experiment 2 Chem 276 Fall Semester 2010 Page 1 of 5
Experiment 2 - Thin Layer Chromatography
Introduction
Chromatography is an experimental technique by which a mixture of compounds can be
separated into its individual components. In thin layer chromatography, there is a
stationary phase as well as a mobile phase. For this experiment, the TLC plate consists of
an aluminum backing on which is placed a thin layer of silica gel. The silica gel is the
stationary phase. The eluding solvent is the mobile phase which carries with it the
components of the mixture. Thin layer chromatography is used mostly for analytical
purposes. Microgram to picogram quantities can be separated by this technique and then
characterized by their Rf value.
The retention factor, or Rf, is defined as the distance traveled by the compound divided
by the distance traveled by the solvent.
Rf=distance traveled by compound
distance traveled by solvent front
In the example shown below, a compound travels 16.2 cm while the solvent front travels
23.0 cm, the Rf is 0.704:
Rf=
solvent front
origin
new position
of compound
16.2 cm
23.0 cm
16.2
23.0 =0.704
The Rf for a compound is a constant from one experiment to the next only if the
chromatography conditions below are also constant:
solvent system
adsorbent
thickness of the adsorbent
amount of material spotted
temperature
pf3
pf4
pf5

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Experiment 2 - Thin Layer Chromatography

Introduction

Chromatography is an experimental technique by which a mixture of compounds can be separated into its individual components. In thin layer chromatography, there is a stationary phase as well as a mobile phase. For this experiment, the TLC plate consists of an aluminum backing on which is placed a thin layer of silica gel. The silica gel is the stationary phase. The eluding solvent is the mobile phase which carries with it the components of the mixture. Thin layer chromatography is used mostly for analytical purposes. Microgram to picogram quantities can be separated by this technique and then characterized by their Rf value. The retention factor, or Rf, is defined as the distance traveled by the compound divided by the distance traveled by the solvent.

Rf =

distance traveled by compound distance traveled by solvent front

In the example shown below, a compound travels 16.2 cm while the solvent front travels 23.0 cm, the Rf is 0.704:

Rf =

solvent front

origin

new position of compound

16.2 cm

23.0 cm

23.0 =^

The Rf for a compound is a constant from one experiment to the next only if the chromatography conditions below are also constant:

 solvent system  adsorbent  thickness of the adsorbent  amount of material spotted  temperature

Since these factors are difficult to keep constant from experiment to experiment, relative Rf values are generally considered. “Relative Rf” means that the values are reported relative to a standard, or it means that you compare the Rf values of compounds run on the same plate at the same time.

The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When comparing two different compounds run under identical chromatography conditions, the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a mixture, you can predict that a compound of low polarity will have a larger Rf value than a polar compound run on the same plate.

The Rf can provide corroborative evidence as to the identity of a compound. If the identity of a compound is suspected but not yet proven, an authentic sample of the compound, or standard, is spotted and run on a TLC plate side by side (or on top of each other) with the compound in question. If two substances have the same Rf value, they are likely (but not necessarily) the same compound. If they have different Rf values, they are definitely different compounds. Note that this identity check must be performed on a single plate, because it is difficult to duplicate all the factors which influence Rf exactly from experiment to experiment.

Questions

  1. Draw the structures of the following compounds.

Acetophenone

Anisole

Benzoic Acid

Benzyl Alcohol

Benzaldehyde

Compound R f in 1: EtOAc/Hex

R f in 1: EtOAc/Hex

Appearance in

___________ stain

Appearance in

__________ stain Acetophenone

Anisole

Benzoic Acid

Benzyl Alcohol

Benzaldehyde

  1. Calculate the R f values of each spot.
  1. What would occur if you had added too much sample to your TLC plate?
  2. From your results, which compound is the least polar? Do the structures of these compounds support your experimental date? Explain.
  3. From your results, which eluding mixture is the most desirable? Explain.
  4. From your R f values, are all the compounds pure? If not, identify what the impurities might be in the contaminated sample.
  5. From your R f values, what are the identities of the compounds in the unknown sample?