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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
Typology: Summaries
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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.
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