Post Lab Chromo - Lab experiment work for 322A PDF

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Lab experiment work for 322A...

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Chromatography Lab: Calculations: Sample: Rf value= Toluene: Rf ¿=

3.8 cm =0.93 cm 4.1 cm

Rf ¿=

2.9 cm =0.71 cm 4.1 cm

Rf ¿=

1.4 mm =0.34 mm 4.1 mm

Hexanes: Rf ¿=

2.4 cm =0.92 cm 2.6 cm

Rf ¿=

1.1 cm =0.42cm 2.6 cm

Rf ¿=

0.1 cm =0.04 cm 2.6 cm

DCM: Rf ¿

3.7 cm =0.97 cm 3.8 cm

Rf ¿=

3.4 cm =0.89 cm 3.8 cm

Ether: Rf =

3.4 cm =0.85 cm 4.0 cm

TCL Plates: Biphenyl: Rf =

3.9 cm =0.98 cm 4.0 cm

distance moved(spot) 3.8 cm =0.93 cm = distance moved (solvent) 4.1cm

Benzophenone: Rf =

3.4 cm =0.85 cm 4.0 cm

Diphenylmethanol: Rf =

2.2 cm =0.55 cm 4.0 cm

Mixture: Rf ¿=

2.1 cm =0.53 cm 4.0 cm

Rf ¿=

3.3 cm =0.83 cm 4.0 cm

Rf ¿=

3.8 cm =0.95 cm 4.0 cm

Critical Analysis and Reflection: Eluting at different rates: Due to the polarities of the molecules within the mixture, we could see that the substances did not elute up the TLC plate at the same rate. Molecules of the same polarities will travel up the plate faster with a solvent of similar polarity in comparison to molecules whose polarities differ from the solvent. It is also important to note that molecules with similar structure to the solvent will elute faster with a solvent of similar polarity. When we look at our molecules tested, we know that toluene and hexane are of similar polarities, as shown with our

Rf

green values.

When looking at molecules like biphenyl, we can assume it is a polar molecule due to the benzene rings that construct it. Polar molecules will move much slower at the plate when mixed with a non-polar eluent such as hexane. However, when mixed with a toluene, toluene made a better solvent since it’s slightly polar characteristics helped with the mixing of the benzophenone and diphenylmethanol. The cyclo-like structure of toluene was similar to benzophenone, biphenyl, and diphenylmethanol. It is clear to see that the structure and polarity of a molecule are very important in determining how a substance moves on a TLC plate.

Hexane and Toluene as solvents Both hexane and toluene worked better than diethyl ether and dichloromethane (DCM) because they were non-polar solvents that had similar properties to the molecules. Diethyl ether and DCM would not successfully separate the molecules because of the conflicting polarities; thus, they wouldn’t mix well. If DCM and diethyl ether couldn’t mix well then there would essentially be no separation of the mixture and the experiment would lead to poor results. Hexane and toluene were much more efficient at separating the compounds because of the polarities.

Column Chromatography: First, I would set up the column chromatography apparatus by taking a Paster pipette (5 inches) and placing a small amount of cotton into the tip of the pipette, which essentially would make a plug. It is also important to make sure not to pack the cotton too tight. Then, I would fill the pipette with two-thirds silica gel and then put the pipette into a rubber stopper with a hole in the middle that is attached to a clamp. Next, I would add some toluene solvent and put twentyfive milligrams of the biphenyl, benzophenone, and diphenylmethanol mixture added with about one milliliter of the toluene. Take this combined solution and add it to the five-inch Pasteur pipette. The compounds will then separate in the column. It is also important to keep the column wet with toluene so that it doesn’t dry up and destroy the separation.

Changing the ratio mixture If the hexane ethyl acetate ratio is changed from a three-to-one to a one-to-one ratio, then the

Rf

value would most likely become bigger. If there is a more equal ratio of polar to

nonpolar molecules, then the polar eluents will be able to move up the TLC plate much quicker than before. Yet, that does not mean it will move up the plate further. Therefore, the calculation will make the

Rf

value much larger than a three-to-one ratio.

We can take the information that we learned from the lab and apply it to the chromatography experiments of different solvents and molecules. In future experiment, we can test those different solvents and molecules to determine what is more effective with different molecules....