Answer to problem 1.
a) 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 R f for compound A is 0.80/2.75 = 0.29.
Similarly, compound B travels 2.20 cm, and therefore the R f for compound B is 2.20/2.75 = 0.80. Compound C travels 1.70 cm, and therefore the R f for compound C is 1.70/2.75 = 0.62.
b) 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 R f .
c) 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 R f value if acetone were used to elute than when hexanes is used to elute the TLC plate.
d) Alumina is more polar than is silica (see the first paragraph under "The Adsorbent" in the TLC section). Therefore, each of the compounds would travel slower on an alumina TLC plate than on a silica TLC plate, the R f values for each of the compounds would be smaller.
Answer to problem 2.
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.
Answer to problem 3.
A couple 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.
Answer to problem 4.
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.
Answer to problem 5.
Biphenyl is an unsaturated hydrocarbon, and therefore is the least polar and will have the largest R f value of the three compounds. The benzoic acid - an 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".)
Answer to problem 5.
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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