Tech 707 and 708 Chromatography

Tech 707 and 708 Organic Chemistry

Chromatography

Abstract:

        Thin layer chromatography was used in this experiment to choose a good eluent solvent, identify an unknown compound, and for the separation of two compounds. The choice of a good eluent solvent was made by testing solvents of varying polarities, and seeing which produced the best results. With the best eluent chosen, we were then able to compare three standards to an unknown compound. Spotting a TLC plate, placing it in the eluent, and comparing the results allowed for the identification of the unknown. One last technique used was the separation of two compounds by use of a chromatography column. The two compounds were then compared using a TLC plate. The results from this experiment included a low percent recovery for ferrocene and a high one for acetylferrocene. These can be attributed to errors in separation and procedure.

Discussion:

        Chromatography is used is used in chemistry for a few different techniques. It can be used for separating, analyzing, and identifying compounds. Chromatography has two main parts, or phases, in which it undergoes in order to achieve the desired results. First there is the mobile phase. The mobile phase is the liquid or gas in which the sample is placed into. This moves the samples through the solid support. This solid support is called the stationary phase, which can contain an adsorbant or another liquid. Each sample will move through the stationary phase at different rates. This is due to the different intermolecular forces of attraction between the sample and each phase. Adsorption column chromatography utilizes a glass column, which houses the adsorbant and allows the eluent to move through it slowly. This technique allows for a compound mixture to be added and separated by use of different eluents. The different eluents will differ in their polarity, allowing the compounds to be collected separately. This is due to the differences in attraction between the eluent, stationary phase, and the compound. The less polar solvent will dissolve the less polar compound, therefore allowing it to separate out first. A more polar eluent solvent is then used to draw out the more polar compound for collection.

        Two different techniques can be used to achieve separation. There is the Dry Pack Method and the Slurry Pack Method. The Dry Pack Method, used in this experiment, is performed by using a dry adsorbant above and below the sample mixture. The eluent is added to the top of the column and it makes it way down through the adsorbant and elutes out through the bottom. In comparison, the Slurry Pack Method uses a mixture of adsorbant and eluent. When the slurry is added, the stopcock is open allowing the eluent to drain out. This allows for the packing of the adsorbant.

Rƒ calculation:

Standards:

Biphenyl: Rƒ= = 0.838[pic 1]

Benzophenone: Rƒ= = 0.467[pic 2]

Benzhydrol: Rƒ= = 0.191[pic 3]

Unknown:

Unknown: Rƒ= = 0.838[pic 4]

        Our unknown Rƒ was the same as the Rƒ for biphenyl. Therefore, we can conclude that the unknown compound was that of biphenyl. The recorded Rƒ for each compound make sense due to the polarity of each compound. Since biphenyl is the least polar of the three, it will have the highest Rƒ. This is due to the lack of attraction to the adsorbant and how fast it will move with the eluent solvent. Since benzophenone has an intermediate polarity, it will fall somewhere in the middle. Finally, since Benzhydrol is the most polar of the group, it is more attracted to the adsorbant. This will cause it to move more slowly with the eluent and attach more strongly to the adsorbant.

Elution Solvent Sequence:

        In Tech 708, the separation of two compounds in a mixture was achieved by column chromatography and the addition of two eluents. These two eluents, hexane and TBME, were added individually in order to separate one compound at a time. Hexane was added first in order to separate out the ferrocene. This is because hexane has a lower polarity than TBME, so it must be used first. Had the TBME been added before hexane, there would have been no separation. This would have resulted in both compounds dissolving and eluting out together. TBME was added after all of the yellow ferrocene had been eluted and collected. The TBME then dissolves the more polar acetylferrocene and allows it to be eluted and collected as well.

Discussion of Elution Solvents:

        In tech 707 five different elution solvents were tested in order to decide which one was best to use. The elution solvents were, methanol, ethyl acetate, toluene, hexane, and dichloromethane.  It was decided that toluene was the best elution solvent to use. This is because it has a middle polarity, three spots appeared, and it is a more environmentally friendly compound. The environmental friendliness was used a determining factor because dichloromethane also produced three spots and has a middle polarity.  The other three solvents were eliminated right away due to how many spots appeared. Methanol only produced one spot and it is the most polar solvent in consideration. This makes it a bad elution solvent, since it would cause the compounds to move too rapidly. Ethyl acetate is also a polar solvent, and it produced one spots as well. Finally, hexane was not chosen as the best solvent since it is not polar enough. Although it produced two spots, which is more than the other two, there needed to be three spots in total.

Data Analysis (TECH 708)

        The quality of the separations can be indicated by the color of the appearance, the percent recovery, and the results of the TLC.

        The appearance of the mixture of ferrocene and acetylferrocene was a brown color. When separated out in the column, the ferrocene moved down the alumina as a bright yellow color. The solution draining out of the column was a clear bright yellow. This evaporated out of the beaker and produced a bright yellow crystal that did contain some separate orange crystals. This result indicates that the ferrocene separation did not lead to an entirely pure compound. This may mean that some acetylferrocene filtered out along with the ferrocene. Upon the addition of the TBME, the acetylferrocene separated through the alumina as a bright orange. This filtered out into the beaker as a bright light orange. The acetylferrocene was a much darker orange while in the column, but came out lighter. Once the liquid was evaporated out, using the air system in the fume hood, the crystals produced were a bright orange. This orange was much darker than the yellow-orange of the ferrocene. This color indicates a more pure sample of the acetylferrocene was recovered.