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Introductory Sequence: TLC of unknown compounds

Page history last edited by jbfriesen 9 years, 8 months ago

Summary of Procedure:

Thin Layer Chromatography is the first experiment of the 1st semester introductory sequence.

The students perform a series of TLC experiments on 6 known compounds.

They observe the effects of three solvents (hexane, dichloromethane, & ethyl acetate) on the Rf values of the compounds.

Students work in groups to combine their data by entering their Rf values into a laptop before they leave lab.

They then each receive a mixture of unknowns that is analyzed with TLC.

Students are encouraged to analyze their class data as well as previous years' data.

2010 version of TLC experiment

2008 version of TLC experiment

2007 version of TLC experiment

Positive aspects of this experiment:

+The beauty of TLC is that a complete experiment can be done in a matter of 15 minutes. This allows for them to “learn as they go” and experiment with different techniques.

+ In our labs we do many TLCs over the course of the year, so this is a good place to start.

+ TLC gives some clue as to the polarity and function groups of the unknowns, but much more information is necessary to make a definitive identification.

Neutral (may be positive or negative) aspects of this experiment:

± The theory of TLC is rather involved. Some aspects of solvent polarity are explained but the theory is not described in detail.

Negative aspects of this experiment:

- TLC Rf values are notoriously unreproducable, especially for novices. At the best, general trends can be observed.

- “Learn as you go” can use up a lot of relatively expensive TLC plates.

Inquiry & Extension Ideas:

● Students can experiment with various pure solvents and solvent mixtures to target separations and Rf values.

● This is a good experiment to use “class data” as part of the discussion.

● The previous years’ results can be used to prepare for the experiment as well as to analyze and discuss data.

A good article to consult is "Thin-Layer Chromatography: The 'Eyes' of the Organic Chemist" Dickson, Kittredge, & Sarquis J. Chem. Ed. (81) 2004 p. 1023

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