Chromatography Thin Layer 1. Data Collection (3 points) Distance solvent Distanc
ID: 504574 • Letter: C
Question
Chromatography Thin Layer 1. Data Collection (3 points) Distance solvent Distance sample Color Dye moved in cm moved in cm 1 Bromothymol blue 40 cm S.O 2 Bromo resol green 25 3 Methyl orange 4 Phenol red Unknown No. B. (1 point) The unknowns are A. 2-Post lab questions 1. Thin layer chromatography is routinely used to monitor the progress of a reaction. The following TLC was performed for a certain reaction. Discuss the progress of the reaction (SM 1.1 ethyl acetatelhexane starting material, P products, RxN reaction mixture) (1 point SM RXN PExplanation / Answer
Data collection: The Rf is calculated as (distance moved by sample)/(distance moved by solvent).
So, Bromothymol blue, Rf = 5/40= 0.125
methyl orange, Rf = 3.5/35= 0.1
POST LAB questions:
1. The basic theory behind TLC is that when compounds having different polarity is spotted on the TLC plate, they rise along with the solvent. The surface of the silica gel is very polar and, because of the -OH groups, can form hydrogen bonds with suitable compounds around it as well as van der Waals dispersion forces and dipole-dipole attractions.The height to which the spots are raised will depend on the polarity of the compound. Polar compound's spot will appear near the base and the least polar compound will be the farthest from the base. The distance of the spots from the base is highly dependent on the polarity of the solvent. If we use a weakly polar solvent then the spots will not rise much but if you use a highly polar solvent then the spots will rise much higher from the base. The polarity of the solvent is varied by using different mixing ratios of ethyl acetate (which is a polar solvent) and hexane (which is a non-polar solvent). In the picture provided in the question, we observe that the TLC was run with ethyl acetate-hexane (1:1). The first spot is due to the starting material and we can see that it is near the base which suggests that the starting material is quite polar(with respect to the product). The reaction mixture gives two spot; this is due to the product and the starting material. We see two spots because, the TLC was taken for a solution containing a mixture of the starting material and the product. As the reaction progresses the starting material starts converting to the product so, we obtain a mixture. In the third spotting we can see that only one spot is visible. This is due to the product which appears to be less polar as compared to the starting material. In summary, we say that the three spottings indicate the progress of the reaction i.e starting material-mixture-product.
2. The TLC plates are not proper because of the following reasons:
The TLC plate A: The solvent is too polar with respect to the spotted material, so the spot is appearing at the base. The ideal location of the spot should be around the middle of the TLC plate i.e. Rf 0.5.
The TLC plate B: The spotting is smudged so, we observed a trailing spot which looks like two overlapping spots.
The TLC plate C: The solvent's polarity is quite less as compared to the polarity of the spotted material. So the spot is appearing near the final solvent front. The ideal location of the spot should be around the middle i.e. Rf 0.5.
3. The base line is drawn by a pencil and not by a pen because pen ink contains some dyes/chemicals which might give some spot of its own. But, a pencil is made of graphite which is inert and insoluble in the solvent.
4. TLC are used for the following purposes:
a) Checking the presence of any impurities in a sample and roughly determining the number of impurities.
b) Monitoring the progress of a reaction.
c) For purification of small quantities of a mixture i.e. Preparative TLC.
5. The first spot is due to C (Reason: C is the least polar molecule among the three given compounds)
The second spot is due to B ( Reason: The compound B is more polar than C due to the presence of C=O group. However, A is more polar than B due to the -COOH group and -OH group.
The third spot is due to A (Reason: The compound A is the most polar compound among the three due to reasons mentioned above)
The compound which is more polar will appear near the base of the TLC plate. This is because the compound is strongly adsorbed on the surface of the silica. Only solvents with high polarity can carry the compounds up along with the solvent front. So, less polar compounds will be easily carried up the TLC plate by polar solvents.
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