TLC questions?

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1. Why might it be very difficult to visualize the separation of cis- and trans-2-butene by thin-layer chromatography?

2.What error is introduced into the determination of an Rf value if the top is left off of the developing chamber?

3.Why is it necessary to run TLC in a closed container and to have the interior vapor saturated with the solvent?

4.What will be the appearance of a TLC plate if a solvent of too low polarity is used for the development? too high polarity?

5.A TLC plate showed two spots of Rf 0.25 and 0.26. The plate was removed from the developing chamber, dried carefully, and returned to the developing chamber. What would you expect to see after the second development was complete? why?

Please help me !!

4 Answers

  • 1. cis and trans 2-butene are very similar in structure. Compounds that are very similar in structure are very difficult to separate on TLC.

    2. The chamber needs to be saturated with the eluent vapor. If the top is left off some of the vapor evaporates. When this happens the Rf values will tend to be lower than expected due to the evaporation.

    3. The TLC needs to be in a closed container to ensure maximun resolution between components.

    4. If the solvent is of too low polarity the Rf of the compounds being eluted will be too low and no separation will be seen. If the solvent is of too high polarity the compounds being eluted will have Rf that is too high and the components will all run to the solvent front and no separation will be achieved.

    5. Many times when running a TLC the spots are too close in Rf. When this happens it is a good idea to dry the plate and run it again. The resolution between spots will be improved.

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  • 1. Cis and Trans butene have very similar polarity. They will both partition the same way in TLC.

    2. I’m not sure, I just don’t leave the top off.

    3. To avoid the errors introduced in 2.

    4. Assuming a polar stationary phase, a solvent with low polarity would lead to very low Rf values and poor separation. A solvent that was too polar would lead to rf values the were too large. The Rf values of the components would be high, but very similar, hence no separation.

    5. Rf = distance traveled by component/distance traveled by solvent

    So both these components have moved about a quarter of the way up the plate. If we run the plate again we would expect them to move about a quarter of the remaining distance. So they would be about 1/4 + 1/4(3/4) = 7/16 the way up the plate. The distance between the spots would roughly double.

  • how can i seperate high polar solvent and non polar solvent in TLC seperation ? please give me proper solvent ratios?

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