Lab 2-Thin-Layer Chromatography Lab

Give the three basic steps in TLC in the order in which they are done.

1. spotting 2. development 3. visualization

4.) In a TLC analysis of analgesics, what would be the result if a solvent of too low polarity is used to develop the plate?

The center of spot will not move far along the plate, since the solvent can not compete well will the silica gel to develop the components along. As a result, the Rf values will decrease and separation of components may not occur, since they will be clumped together.

3.) What will be the result of adding too much sample to the TLC plate?

If too much sample is added, the spot will be overloaded, and you'll see tailing of the spot. The location of the center will be very inaccurate so we cannot calculate Rf values accurately.

In a TLC analysis, if the solvent was observed to travel 10.0 cm and the spot was observed to travel 8.40 cm, what is the Rf for that spot?

Rf = x/y = (distance traveled/solvent front) = 8.40 cm / 10.0 cm = 0.84 The Rf value is simply the ratio of the distance that the spot travels to the distance that the solvent travels from the starting spot. The value is always between 0 and 1. It is usually best to choose a development solvent that moves the spots into mid range - that is that results in Rf's of about 0.3 - 0.7.

5.) The Rf of ibuprofen was found to be 0.32 when neat t-butyl methyl ether was used as the development solvent. What effect would there be on the Rf of ibuprofen if acetone had been used to develop the TLC plate?

So t-butyl methyl ether is a less polar developing solvent than acetone, as a result acetone is able to better compete against the silica gel to carry ibuprofen further along the TLC plate. Therefore, the Rf value of ibuprofen will increase since it will the component will move further along the TLC plate increasing the height of spot. (Rf = center of spot/solvent front)

1.) Arrange the following compounds in order of increasing Rf in a TLC analysis: benzoic acid, benzaldehyde, 3-heptanone, nonane, and cyclohexanol.

The order of increasing Rf follows the trend of non-polarity, compounds that are the least polar will have the highest Rf . Therefore, the in order of increases Rf is benzoic acid, cyclohexanol, benzaldehyde, 3-heptaonone, nonane.

2.) Why must the spot be applied to the TLC plate above the level of development solvent?

The spot must be applied above the level of the development solvent or the spotted material will dissolve in the development solvent, and the plate will need to be redone

After development and visualization of a TLC plate, five spots were observed. What can be concluded about the number of components in the original mixture? There are at least five components in the mixture. No conclusion can be made. There are five components in the mixture. There are more than five components in the mixture

There are at least five components in the mixture.

A TLC analysis was run on an unknown mixture. After visualization by two methods, three spots were observed. What can be concluded? There are at least three components in the mixture. There are three components in the mixture. Six components are present in the mixture. No conclusion may be drawn.

There are at least three components in the mixture. All that can be concluded is that the number of components in a mixture is greater than or equal to the number of spots observed in a TLC analysis. Some compounds in the mixture may have very similar polarities, which would cause them to move as one spot. If several TLC analyses are run on the same mixture, using different conditions each time (such as changing the development solvent), and three spots are still observed under all the different conditions it can be more safely concluded that the mixture contains just three components.

In a TLC analysis if the Rf's of the spots are too small, a development solvent of higher polarity should be used. a different method of visualization should be used. a development solvent of lower polarity should be used. a spotting solvent of lower polarity should be used. a spotting solvent of higher polarity should be used.

a development solvent of higher polarity should be used. In general, to increase the Rf of a spot, use a development solvent of higher polarity. The spotting solvent has no effect on the separation. It is used simply to help transfer the sample to the TLC plate.

In a TLC analysis, if the Rf of a compound was found to be too large, a __________ (spotting or development?) solvent having a ______________ (lower or higher?) polarity should be chosen.

development, lower

Thin layer chromatography can be used to help determine (select all correct answers) the identity of a compound. the boiling point of a compound. the purity of a compound. the number of components in a mixture. the density of a compound.the melting point of a compound.

the identity of a compound, the purity of a compound, the number of components in a mixture.