Science Homework Help

Thin layer chromatography (TLC) is a simple and relatively fast analytical tool that is used to measure the extent of a reaction or measure the relative polarity of a molecule. A TLC plate is a sheet of glass that is coated with a thin layer of adsorbent such as silica (or occasionally alumina). A small amount of the mixture to be analyzed is placed or spotted near the bottom of this plate. When monitoring reactions, it is common to place the starting materials on one side of the plate (or in the left lane) and the reaction mixture in the right lane. The TLC plate is then placed in a shallow pool of a solvent (20% ethyl acetate/hexanes) so that only the very bottom of the plate is in the liquid. This solvent, or the eluent, is the mobile phase, and it slowly rises up the TLC plate by capillary action.

As the solvent moves past the spot that was applied, it carries the spotted mixture up the plate where a competition occurs between the solvent carrying the mixture and the silica on the plate. Since the silica adsorbent is polar, polar molecules in the mixture will rise very little up the plate, but nonpolar molecules will have little attraction to the silica and will consequently rise to the top. Molecules of intermediate polarity will stop somewhere in between these two extremes. When the solvent has reached the top of the plate, the plate is removed from the solvent, dried, and then the separated components of the mixture are visualized by exposure to a UV lamp. The positions of each spot on the TLC plate are identified and recorded by assigning R_f values from 0.0 to 1.0, where a 0.0 indicates the spot is at the bottom of the plate and a 1.0 indicates the spot is at the top.

In this assignment, you will be guided through the steps of a simple esterification reaction as a demonstration of how to use thin layer chromatography as a tool to monitor a reaction until it reaches completion. This assignment will also serve as a tutorial to teach you how to utilize the various parts of the organic simulation that will be used in later assignments.

1. To start this activity, click this link for Using Thin Layer Chromatography. The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below. You will see a lab bench containing reagents on the back of the bench, aqueous reagents on the right, an equipment rack containing necessary laboratory equipment, a red disposal bucket for cleaning up the lab, and the organic stockroom in the back. Other pieces of laboratory equipment will be used in other assignments.

1. You will find a round bottom flask located on the stockroom counter. Select the starting materials for the reaction by first clicking on 2-phenylacetic acid (PhAcOH) in the Esterification menu of the Stockroom tab. Also add 3-methyl-1-butanol (PentOH). Now double click on the flask to move it to the stir plate on the lab bench.

1. The two starting materials are listed in the Live Data Flask Contents display, and hovering over the listed starting materials will display their structures on the chalkboard.

1. In order to perform an esterification reaction, sulfuric acid (H₂SO₄) must be added to the starting materials. This is done by double clicking on the H₂SO₄ bottle on the reagent shelf on the lab bench. The Live Data display will now show that the acid has been added to the reaction mixture.

1. Before the reaction can be started, we must be able to heat the reaction mixture so the reaction can proceed at a suitable rate. This is done by adding a heating mantle to heat the reaction mixture, adding a condenser so the mixture can be refluxed, and then adding nitrogen gas to maintain an inert atmosphere and to prevent pressure buildup. Double click on the Heater mantle on the bench in front of all of the reagent bottles. Now double click on the Condenser from the equipment rack. Finally, double click on the nitrogen gas hose to the right of the stir plate. Now click on the dials on the front of the stir plate to start the reaction. You should see the reaction mixture stirring in the round bottom flask.
2. Before waiting too long, perform a TLC measurement by clicking on the TLC jar located in front of the analytical equipment and dragging and dropping the TLC plate on the round bottom flask. A window should now open showing the TLC results. Click Save to save the TLC to your Lab Book. In the starting material lane (the left lane) you should see two large spots near the bottom and in the reaction lane (the right lane) you should see the same two spots.

Why are the spots the same for both the right and left lanes?

1. Now advance the reaction forward 15 minutes (or you can just wait 15 minutes) by advancing the time on the laboratory clock. This is done by clicking on the appropriate button under the minutes, tens of minutes, or hours digits on the clock. Now perform a new TLC measurement on the reaction mixture.

What is the new spot?

What has happened to the size of the starting material spots? Why?

8. Now advance the laboratory time forward until all of the starting materials have been consumed. You will need to monitor the reaction with TLC measurements until you observe that the starting materials have been consumed. You will also notice that some time passes each time you perform TLC by dragging over a plate.

How much time (approximately) did it take to run each TLC?

minutes

Why is running a TLC not instantaneous?

How much time did it take to complete the reaction?

minutes

What are the R_f values for the starting materials and product?

What can you say about the relative polarities of the starting materials and the product?

1. When a reaction is complete, the reaction mixture is rinsed or “worked up” by adding an aqueous reagent of an appropriate pH to the reaction mixture in a separatory funnel. After you shake the funnel to thoroughly rinse the reaction mixture, the water-soluble products will be in the aqueous phase and all others will be in the organic phase. Double click on the separatory funnel on the equipment rack to move it to the round bottom flask. The reaction mixture should now be in the funnel. Now double click the NaOH aqueous reagent (one of the big white bottles on the front of the lab bench) to add it to the funnel. There will now be two layers, the top being the organic phase and the bottom the aqueous phase. Click on the organic phase to extract the product and drag it to the cork ring on the lab bench. Perform a TLC measurement on the solution in the flask to confirm that it is the same product produced in the reaction.

An important note:

In Beyond Labz Organic Synthesis, the solvent system is 20% ethyl acetate in hexanes. Reactant and product molecules that are highly volatile (low boiling point and high vapor pressure) or very nonpolar (with no affinity for the silica molecules coating the TLC plate) will not be seen on a TLC plate. These molecules either evaporate too quickly or are pulled off the plate and into the solvent system when spotted. Also, to aid the user in focusing directly on the reactants and products of a reaction, reaction solvents, reagents added from the bench, and reagent byproducts will not be shown on TLC plates. All spots belong to either a reactant or a product molecule. Please keep this in mind as you work through activities in Beyond Labz.