Thin Layer Chromatography- Grade: 94 PDF

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Course was instructed by Dr. Rajani Srinivasan at Tarleton State University for a 6 week summer course.
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Thin Layer Chromatography by Isabella Manlapaz [email protected] CHEM 2423 Section 520 Experiment 5 June 13, 2018 Executive Summary The purpose of this experiment is to perform a thin layer chromatography (TLC) analysis on three different chemicals along with an unknown mixture. The three chemicals used in this lab were fluorene, fluorenone, and fluorenol. Once the spots are marked and later visualized in the UV chamber, one must determine the distanced traveled by the spot and solvent to calculate the Rf (ratio to the front) value. After writing down the information in the observation table given, I could determine that the unknown mixture was very consistent with fluorene based on the measurements taken from the experiment.

Thin-layer chromatography (TLC) is an analytical technique used to determine the number of compounds in a mixture as well as determining the identification of the components. In addition to determining whether two compounds are identical, TLC is also used in determining solvent polarity. To be able to further identify the composition of the unknown mixture, you will need to calculate the Rf value which determines the distance traveled by the spots and solvent. Overall, the purpose of this experiment was to study the effect of change in the concentration of the solvents on the separation of the mixture. To carry out a thin-layer chromatography analysis, you must first obtain a TLC plate with a thin layer of adsorbent coating the surface of the place providing the stationary phase. In this lab, silica gel was used as the stationary phase. Then, mark the TLC plate one centimeter from the bottom of the plate and one and a half centimeters from the top of the plate with a pencil. Once you’ve drawn the marks with your pencil, spot the appropriate solutions on marks according to the instructions. The solutions used in this lab were fluorene, fluorenone, fluorenol, and an unknown mixture. Prepare a developing chamber with a beaker and add just enough solvent to cover the bottom of the flask because the TLC spots must be above the liquid level or else your data could become skewed. The solvent used in this procedure consisted of dichloromethane, which was used as the mobile phase. Adding a filter-paper liner to the beaker will help keep the plate upright and not slanted. Place the TLC plate in the chamber, cover the chamber with the watch glass, and allow the solvent to move up the plate via capillary action. As soon as the solvent front has moved up the plate to the mark near the top, remove the TLC plate and mark the front with a pencil. Before placing the TLC plate in the ultraviolet (UV) chamber, you must allow the solvent to evaporate. Once the plate has completely evaporated, note the presence of spots in the

TLC by outlining them with a pencil. Using a ruler, determine the distance traveled by the spots and solvent by calculating the Rf (ratio to the front) to determine the composition of the unknown mixture. Vol. of hexane Vol. of Rf Fluorene CH2Cl2 10 0 0.33 9 1 0.38 8 2 0.50 7 3 0.57 6 4 0.58 5 5 0.56 4 6 0.60 3 7 0.56 2 8 0.69 1 9 0.69 0 10 0.77 Vol. of Vol. of Distance hexane CH2Cl2 travelled by the solvent 10 0 5.4 cm 9 1 4.0 cm 8 2 5.4 cm 7 3 3.7 cm 6 4 3.8 cm 5 5 4.1 cm 4 6 4.0 cm 3 7 4.1 cm 2 8 3.5 cm 1 9 3.5 cm 0 10 4.4 cm

Rf Fluorenol 0.056 0.15 0.17 0.24 0.29 0.32 0.35 0.34 0.49 0.49 0.61 Distance travelled by fluorene 1.8 cm 1.5 cm 2.7 cm 2.1 cm 2.2 cm 2.3 cm 2.4 cm 2.3 cm 2.4 cm 2.4 cm 3.4 cm

Rf Fluorenone

Rf Unknown

0.056 0.33 0.13 0.38 0.17 0.50 0.22 0.51 0.26 0.58 0.32 0.61 0.38 0.58 0.37 0.59 0.43 0.71 0.46 0.69 0.61 0.77 Fluorenol Fluorenone Unknown

0.3 cm 0.6 cm 0.9 cm 0.9 cm 1.1 cm 1.3 cm 1.4 cm 1.4 cm 1.7 cm 1.7 cm 2.7 cm

0.3 cm 0.5 cm 0.9 cm 0.8 cm 1.0 cm 1.3 cm 1.5 cm 1.5 cm 1.5 cm 1.6 cm 2.7 cm

1.8 cm 1.5 cm 2.7 cm 1.9 cm 2.2 cm 2.5 cm 2.3 cm 2.4 cm 2.5 cm 2.4 cm 3.4 cm

The Rf value for the solvents was calculated by taking the distance from the starting point to the top of the spot and dividing it by the distance from the starting point to the solvent front.

In conclusion, thin layer chromatography is a technique used to determine the identity and polarity of unknown compounds in a mixture. Based on the results of the experiment, the unknown mixture was most likely consistent with fluorene because the distance traveled by the

unknown mixture and the Rf value were very similar.

inquiry-driven

Post-lab Questions 1.

When 2-propanol was used as the developing solvent, two substances moved with the

solvent front (Rf = 1) during TLC analysis on a silica gel plate. Can you conclude that they are identical? If not, what additional experiments would you perform? No, you cannot conclusively conclude they are identical because by using only TLC analysis, all that you can conclude is that the two substances are similar in size and polarity. They may be different molecules that have similar substituents or enantiomers or just happen to have the same Rf. 2.

The Rf value of compound A is 0.34 when a TLC plate is developed in pentane and 0.44

when the plate is developed in diethyl ether. Compound B has a Rf value of 0.42 in pentane and 0.60 in diethyl ether. Which solvent would be better for separating a mixture of A and B by TLC? In TLC, you are trying to get the greatest separation possible, which would be the largest spread in Rf values between compounds A and B. Based on my calculations, the mix of diethyl ether would be the better solvent for separating a mixture of A and B by thin layer chromatography. A vs B in pentane= 0.34 vs 0.42 = 0.08 A vs B in diethyl ether = 0.44 vs 0.60 = 0.16

3.

A student needs to analyze a mixture containing an alcohol and ketone by silica gel TLC.

After consulting Table 18.1, suggest a likely developing solvent. Ethyl acetate should be a good solvent to be used for this mixture. Alcohol is more polar than the ketone portion and will react more with the plate, therefore you want to choose a solvent that will react with the ketone portion of the mixture and Rf value can be obtained. Ethyl acetate is used for ketones and aldehydes and will be a good solvent to use....