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Prelab 4 - lab 4 thin layer
Mod Organic Chem I (CHEM 2211)
University of Georgia
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TLC for Unknown Mixtures
Introduction:
In this lab we will identify active ingredients of six unknown analgesic mixtures using TLC. Chromatography is a separation technique that is based on a compound’s distribution between, and relative affinity for, a stationary phase and a mobile phase. There is gas-liquid and solidliquid chromatography. We will use solid-liquid chromatography to identify our unknowns. Solid-liquid chromatography is used for the separation of solutes by the adsorption onto the surface of an inorganic plate. If a compound favors a mobile phase it will move along the plate faster allowing it to be distinguished from compounds who do not favor a mobile phase as much. Organic compounds stick to through molecule interactions between the absorbent of silica which is used on the plate. More polar molecules will stick to the adsorbent stronger causing them to move slower. The compounds are carried up the plate using a solvent often polar, and in this case, Acetone will be used. Using the Rf values determined from distance traveled on the plates we can identify the components of our unknown mixtures. In this lab an eluent of 1:1 ratio of methylene chloride to ethanol will be used. The Rf values is the ratio of the distance traveled by the compound divided by the distance moved by the solvent. Using known Rf values of known compounds we can compare it to the determined Rf values of the unknown compounds so that the unknown compounds can be identified. The makeup of these mixtures can then be compared to the chart to determine the name for each mixture. Mixture Composition Table Patrick’s Potent Powder Caffeine Ibuprofen ---- Anastasia’s Analgesic Elixir Acetaminophen Acetylsalicylic Acid ----
Excedrin Acetaminophen Caffeine Acetylsalicylic Acid Excedrin AF Acetaminophen Caffeine ---- Tylenol Acetaminophen ---- ---- Motrin Ibuprofen ---- ----
Table of Reagents:
Compound Name Structure MW (g/mole) MP (°C) Density (g/ml)
Acetaminophen 151 169 1.
Acetylsalicylic acid 180 135 1.
This lab involves the use of materials that will go up to high temperatures, thus proper precautions and equipment are necessary to stay safe. When heating be sure you do not heat a closed vessel, and while heating a liquid make sure a boiling chip was added to the liquid prior to heating it. Throughout the lab safety goggles and gloves must be worn. Methylene chloride and ethyl acetate are highly flammable and toxic. Care must be taken to keep toxins under snorkel, avoid breathing in the fumes, and to avoid sparks and hot surfaces. Before dismantling an apparatus make sure it is fully cooled. Acetic acid causes burn if it comes in contact with skin or eyes. Notify your ta of any toxic exposure and make sure to dispose of liquids in the correct waste.
Procedures:
Place small amount of each mixture into separate test tubes, and label each tube with its number
Add 1 ml of a 1:1 methylene chloride/ethanol solution.
Using a dull pencil draw a line 1cm from the bottom of each plate that the TA gives you. (do not press down too hard)
For the first plate label 4 lanes to indicate where the known analgesic compound will be spotted.
Put the corresponding samples on plate 1.
Using pipets put each of known compounds on origin line, make sure they are at least 5 mm from edges of TLC plate and the first and fourth spot.
Use a 100 ml beaker and watch glass to develop a chamber.
Add developing solvent to cover the bottom of the beaker, and place TLC plate into chamber with labeled side facing the interior.
Once the solvent travels within 1 cm of top, remove it and draw a line across solvent front. 10 this process with the remaining two plates and the six unknown mixtures. 11 all three plates are developed and marked use an ultraviolet light on them. 12 all the dark spots seen on your plates and measure the distance from the center to calculate Rf values.
Data:
Unknown 1: 415623C Unknown 2: 324561E Unknown 3: 536124D Unknown 4: 642312A Unknown 5: 253319K Unknown 6: 761428B
Un #2 .9 / 3 =. 2 / 3 =.
Caffeine Acetaminophen
Excedrin AF Un #3 2 / 3 = .570 Acetaminophen Tylenol Plate 3 Unknown Rf Value Closest Match Identified Substance Un #4 3 / 3 = .860 Ibuprofen Motrin Un #5 .9 / 3 =. 3 / 3 = .886 Caffeine Ibuprofen
Patrick’s Potent Powder Un #6 2 / 3 = .571 2. / 3 =.
Acetaminophen Aspirin
Anastasia’s Analgesic Elixir
Results:
To find the identities of the six unknown compounds, the obtained Rf values were matched to the known Rf values of the four compounds. Then using the chart provided the ingredients (compounds) of the unknown compounds were matched with the ingredients of known mixtures. Unknown #1 had Rf values of .229, .570, and .800. which values correspond to caffeine, acetaminophen, and aspirin. From the provided chart, Excedrin contained these three compounds, meaning that Unknown #1 is Excedrin. Unknown #2 had Rf values of .257 and .570, these values match with caffeine and acetaminophen which are found in Excedrin AF. Unknown # 3 has one Rf value of .570 that corresponds with acetaminophen which is the one ingredient of Tylenol according to the chart provided. Unknown # 4 had a Rf value of .860 that matches with ibuprofen ‘s Rf value, meaning it must be Motrin, because ibuprofen is its only ingredient.
Unknown # 5 had Rf values of .257 and .886, matching with caffeine and ibuprofen, which makes up Patrick’s Potent Powder. Finally, unknown #6 had Rf values of .571 and .743, matching those of acetaminophen and aspirin, which makes up Anastasia’s Analgesic Elixir.
Discussion:
Using Thin-layer chromatography, the identities of the six unknown compounds could be found by comparing the patterns and Rf values found from the TLC plates to the Rf values of the known ingredients. To see the spots of the unknown compounds on the TLC plate, they were dissolved in methylene chloride, whereas acetone was used to drive them up the plates. Polar molecules such as aspirin and ibuprofen travelled further up the plate due to the solvent of acetone also being polar. After finding the Rf values and obtaining the known ingredients in each unknown, the chart provided was used to compare the now known ingredients of the unknowns to those of the given mixtures in order to identify the unknowns through the mixtures with the same composition of ingredients. Human error caused very minor differences in rf values between unknown 2 and 3 when compared to the pure substances of plate 1. This could be due too adding to much solvent on the plate making the solutes travel further than it should. Another human error could arise from how carefully the marking on the plates were drawn, causing scrapes or inaccurate marking could affect the calculated Rf values.
Post Lab:
- If the origin line was positioned in the chamber low enough to be below the level of the eluent, the spots from the compound’s sample would dissolve away in the solvent making any interpretations of the spots impossible.
- From calculating the Rf values by dividing the distance traveled by the solvent front measurement on a TLC plate, we find that the Rf value of Compound Q is 3.4cm/5 equaling .65 and the Rf value of aspirin is 2.70cm/4 which also equals .65. Both compound Q and Aspirin obtained the same Rf values from the TLC plate. Since the same solvent of dichloromethane was used to develop the compounds and acetone was
used to move up the compounds, it can be concluded that Compound Q is Aspirin. And since Compound Q is not a mixture, there are no other compounds on the plate with Compound Q. 6. It is possible to compare TLC results to of an unknown compound to refence standard on TLC plates with different lengths because Rf values equal the distance traveled by spot divided by distance traveled by the solvent front, creating a ratio. Although other factors affect Rf values those factors are at a constant on both TLC plates, meaning that the only factors affecting the RF are the distance traveled and the solvent front. Therefore, only looking at the Rf values produced from the different TLC plates would allow for comparison and identification of compounds. Although their distance traveled would not be significant to compare using their produce Rf values would be experimentally significant to identify a compound.
Prelab 4 - lab 4 thin layer
Course: Mod Organic Chem I (CHEM 2211)
University: University of Georgia
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