Organic Chemistry Lab - Thin layer Chromatography

What is Chromatography?

The separation of two or more compounds or ions caused by their molecular interactions with the two phases mobile and stationary.

What are the uses for TLC?

1. To Determine the number of components in a mixture.

2. To determine the identity of two substances.

3. To monitor the progress of a reaction

4. To determine the effectiveness of purification.

5. To determine the appropriate conditions for a column chromatographic separation.

6. To monitor column chromatography

what are the two states of chromatographic separation?

Free - dissolved in the liquid or gaseous mobile phase.

absorbed - sticking to the surface of the solid stationary phase.

What do the strength of attractive forces depend on?

Size, polarity, and hydrogen bonding ability of molecules A and B.

Polarity and hydrogen bonding ability of the stationary phase.

Polarity and hydrogen bonding ability of the mobile phase solvent.

Define elution

Process of solvent moving up the paper

Which type of compound will elute first and the farthest?

the Non-polar substance.

What kind of compounds can only be seen using an Iodine chamber?

alkanes alcohols and ethers.

How do you calculate RF values?

Rf= spot travel / solvent travels

What is the stationary phase?

A fixed material on a surface that can absorb compounds

What is the mobile phase?

A moving liquid or gas that dissolves compounds and carries them along a surface.

what is absorption?

the strength of attraction between the compounds and the stationary phase.

what is separation?

a measure of the elution or migration rate of compounds.

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

The separation of cis- and trans-2-butene would be very difficult to visualize by thin layer chromatography due to the fact that 2-butene evaporates very easily and neither cis- nor trans- would show up on the TLC plate.Also the seperation between these two compounds is very difficult to visualize because the difference in polarity is negligible.

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

If the top is left off the developing chamber, the solvent will evaporate as it rises up the plate and the solvent will rise slowly up the plate but to a shorter distance on the TLC plate. Rf value obtained will be lower or lesser than the Rf value obtained when the chamber is closed.

What problem will ensue if the level of the developing liquid is higher than the applied spot in a TLC analysis.

If the level of the developing liquid is higher than the applied spot in TLC then the sample may subsequently submerge, get dissolved and not drawn up the plate.

In carrying out an analysis of a mixture, what do you expect to see when the TLC plate has been allowed to remain in the developing chamber too long, so that the solvent front has reached the top of the plate?

When the solvent front reaches the top of the plate it begins to evaporate and the spots continue to move upward on the TLC plate, but the solvent front appears to stop. This will lead to incorrect Rf values. If the TLC plate has been allowed to remain in the developing chamber too long the components of the mixture being evaluated will eventually become diluted into the solvent and will spread over the silica gel causing their marks to disappear.

Arrange the following in order of increasing Rf on thin layer chromatography: acetic acid, acetaldehyde, 2-octanone, decane, and 1-butanol

The Rf value of any component in Thin Layer Chromatography drops down with increase in the polarity of the component.

Acetic Acid < 1-Butanol < Acetaldehyde < 2-Octanone < Decane.

What will be the result of applying too much compound to a TLC plate?

Too much compound should not be applied to the TLC plate. After the solvent has traveled up the TLC plate by capillary action, the UV light allows for visualization of the results. If too much compound is applied to the TLC plate the results will be large spots that mesh together and distinguishing them becomes a lot harder.

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

If the container in which the TLC is run is not closed and if the interior vapor is not saturated with the solvent nothing will show up on the TLC plate. The substances being tested as well as the solvent being used will both evaporate and the experiment will go to waste.

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

The solvent of a TLC plate must only be slightly polar. If the polarity is too low, the components of the spot will not travel very far and will not separate very well (will be very close to each other). If the polarity is very high, the components of the spot will travel very far and may run off the plate.

A TLC plate showed two spots with Rf values of 0.25 and 0.26. The plate was removed from the developing chamber, the residual solvent was allowed to evaporate from the plate, and then the plate was returned to the developing chamber. What would you expect to see after the second development was complete?

Rf values after 2 runs can be given by the following expression:

(remaining distance)(Rf value) + (Distance originally travelled)=total distance traveled

(1-0.25)(0.25)+(0.25)+0.4375

What is the rule regarding functional groups and polarity?

The more functional groups the more polar a compound tends to be.