These questions are in the 5th edition of the Handbook for Organic Chemistry Lab. Once you've tried them on your own, click on the link at the bottom of the page to view the answers.
1) Consider the following silica gel TLC plate of compounds A, B, and C developed in hexanes:
a) Determine the R f values of compounds A, B, and C run on a silica gel TLC plate using hexanes as the solvent
b) Which compound, A, B, or C, is the most polar?
c) What would you expect to happen to the R f values if you used acetone instead of hexanes as the eluting solvent?
d) How would the R f values change if eluted with hexanes using an alumina TLC plate?
2) You are trying to determine a TLC solvent system which will separate the compounds X, Y, and Z. You ran the compounds on a TLC plate using hexanes/ethyl acetate 95:5 as the eluting solvent and obtained the chromatogram below. How could you change the solvent system to give better separation of these three compounds?
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?
4) A student spots an unknown sample on a TLC plate. After developing in hexanes/ethyl acetate 50:50, he/she saw a single spot with an R f of 0.55. Does this indicate that the unknown material is a pure compound? What can be done to verify the purity of the sample?
5) 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 mixture. Predict the relative R f values for the three compounds in the sample.
6) Plate A, below, represents the TLC chromatogram of a compound run in hexanes. The same compound was then spotted on a large TLC plate and again run in hexanes. Which TLC plate, B, C, or D, correctly represents how far the compound would run on the longer plate?
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