TLC Technique Quiz
1) Consider the following silica gel TLC plate of compounds A, B, and C developed in hexanes:
- Determine the Rf values of compounds A, B, and C run on a silica gel TLC plate using hexanes as the solvent.
- Which compound, A, B, or C, is the most polar?
- What would you expect to happen to the Rf values if you used acetone instead of hexanes as the eluting solvent?
- How would the Rf values change if eluted with hexanes using an alumina TLC plate?
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- Take a ruler and measure the distance between the origin and the solvent front: 2.75 cm. Then measure the distance between the origin and the center of spot A: 0.80 cm. Therefore the Rf for compound A is 0.80/2.75 = 0.29.
- Similarly, compound B travels 2.20 cm, and therefore the Rf for compound B is 2.20/2.75 = 0.80. Compound C travels 1.70 cm, and therefore the Rf for compound C is 1.70/2.75 = 0.62. Compound A is the most polar because it does not travel as far as the other two compounds. Remember, polar compounds stick to the adsorbent more readily, and thus do not travel as far and have a lower value for Rf.
- Acetone is a more polar solvent than is hexanes. If it were used to elute the same three compounds, each of the compounds would travel faster because the more polar eluting solvent is more proficient at eluting the compounds from the polar adsorbent. Since each compound travels faster, each compound would have a larger Rf value if acetone were used to elute than when hexanes is used to elute the TLC plate.
- Alumina is more polar than is silica. Therefore, each of the compounds would travel slower on an alumina TLC plate than on a silica TLC plate, the Rf values for each of the compounds would be smaller.
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?
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In order to get the three compounds X, Y, and Z to separate, you need to get them to move further up the plate. To do this, you need to increase the polarity of the eluting solvent. Since the eluting solvent tried was hexanes/ethyl acetate 95:5, the first thing to try is a higher percentage of ethyl acetate in the eluting solvent, such as hexanes/ethyl acetate 85:15. If the three compounds still do not separate, try hexanes/ethyl acetate 50:50; continue increasing the percentage of ethyl acetate until separation is achieved. You could also try a more polar solvent such as acetone or methanol to replace the ethyl acetate.
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?
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A couple of things can cause a TLC plate to streak as illustrated in the diagram of the plate shown. The plate might be overloaded, in other words, the solution used to spot the plate is too concentrated (to fix this, dilute the solution and try the TLC again). Or, there are simply too many components in the mixture to be separated by TLC.
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 Rf 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?
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The fact that you see only one spot on a TLC plate does not necessarily mean that the solution spotted contains only one component. This is because two compounds can have the same value of Rf in a particular eluting system. You must run the sample in a different eluting solvent and see if it again gives only one spot. This is a good indication that the sample is pure. However, you still should verify the purity of the sample by melting point, boiling point, and/or spectroscopic analysis.
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 Rf values for the three compounds in the sample.
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Biphenyl is an unsaturated hydrocarbon, and therefore is the least polar and will have the largest Rf value of the three compounds. The benzoic acid would be the most polar and therefore will have the smallest Rf value. Benzyl alcohol is between these two compounds in polarity and will have an Rf value between the two. (See the chart in the TLC section under "Interactions of the compound and the adsorbent".)
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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A compound will move with the same Rf value no matter what length the TLC plate. So, determine first the Rf value on plate A, and then see which of the longer plates gives the same Rf value for the compound. Take a ruler and measure the distance between the origin and the solvent front on plate A; you should measure about 2.5 cm. Then measure the distance to the center of the spot; you should measure about 1.75 cm. Determine the Rf by dividing 1.75 by 2.5; you should get 0.7 for this Rf value. Now look at plates B, C, and D. On plate C, if you measure how far the spot moves (2.3 cm) and divide this value by the solvent front (3.4 cm), you get about 0.7 for the Rf. Thus, the answer is plate C.
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