Organic Chemistry I - Thin-Layer Chromatography Lab PDF

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Organic Chemistry I - Thin-Layer Chromatography Lab...

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Lab 4 – Thin-Layer Chromatography Part I – Purpose and Procedure The purpose of the experiment is to understand the process of thin layer chromatography in seperating multiple compounds

Materials           

Mortar and pestle Small beaker Pencil Ruler Silica coated TLC plate Scissors Spotting capillaries Chamber (beaker/bottle with lid) Filter paper Tweezers UV light set to wavelength 254 nm

Substances

Aspirin

Acetaminophen

Ibuprofen

Caffeine

Procedure Preparing the Sample  Prepare a sample of only a few milligrams each of Aspirin, Acetaminophen, Ibuprofen, Caffeine, and an Unknown by dissolving each separately in diethyl ether Spotting the TLC Plate  With a pencil, lightly draw a line one centimeter from the end of a silica gel coated TLC plate  Place 5 evenly separated marks on the one centimeter line that will indicate the placement of each sample

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Using a spotting capillary, place a small amount of each sample on its designated mark on the TLC plate

Picking a Solvent  Create a 4-mL mixture containing 95% ethyl acetate and 5% acetic acid Developing the Plate  Place the solvent mixture into a 4-oz wide-mouth bottle or beaker  Line the beaker with filter paper saturated with the solvent  Using tweezers, place the TLC plate into the chamber ensuring that the sample spots are not submerged  Cover the container and allow the solvent to travel within a centimeter of the top of the plate  Remove the plate then mark with a pencil a line indicating where the solvent had travelled up to  Allow the plate to dry Visualizing the Results  Observe the TLC plate using a UV light of wavelength 254 nm, marking with a pencil the spots Calculate the RF Values  Calculate the RF value of each spot for each sample by dividing the distance travelled by the spot by the distance travelled by the solvent

Part I – Data Observations Had to conduct procedure a second time with Acetaminophen due to a spot not appearing for the sample on the original TLC plate

Data Sample Ibuprofen Aspirin Acetametophine Caffeine Unknown Solvent

Calculations Used Used to dermine Rf:

Distancethe spot travelled Distance the solvent travelled

Distance Travelled 6.30 cm 4.75 cm 4.45 cm 2.05 cm 2.10 cm | 4.45 cm | 5.5 cm 8.10 cm

Rf Value 0.78 0.59 0.55 0.25 0.26 | 0.55 | 0.68 NA

Part III – Discussion and Conclusion Discussion Thin-Layer Chromatography is used to analyze mixtures by separating the components that make up the mixtures caused by their molecular interactions with two phases – one moving and one stationary. The stationary phase is normally a polar solid absorbent while the mobile phase can be a single solvent or a combination of solvents. The sample is dissolved in an easily evaporated solvent. The solvent must be volatile for it to be mostly evaporated from the sample once on the TLC plate. The chamber that includes the TLC and a piece of filter paper must also include a solvent (used Acetic acid and Ethyl acetate) that will travel up the TLC plate. The solvent is important, for it will interact with the silica gel on the plate: the silica gel is polar so it will hold onto the sample; however, the solvent will try to carry the sample while it travels up the plate. The polarities of the different substances that make up the sample will cause for the substances to travel different distances and show up in separate locations. The location of the samples is observed using a UV light. After completion of the lab, the TLC plate was observed under the UV light. The larger dots may indicate several different substances of close polarity. The distances between the marked ending point of the solutions and the starting points is determined. The distance travelled for each sample is then divided by the distance travelled by the solvent in order to determine its R f value.

Conclusion It could be understood that the ingredients that make up the unknown are similar to that of Caffeine, Acetaminophen, and Aspirin, for the calculated R f values of the Unknown are most similar to that contained in those three samples. There are several sources of error for thin-layer chromatography: (1) not using a clean TLC sheet, (2) using too much solvent at the base of the chamber, (3) a solvent of an inappropriate polarity 1. After completion of the lab, the TLC plate is observed under a UV light. The UV light will allow for the samples to be viewed, shown as dark dots. If the TLC plate wasn’t clean before carrying out the experiment, then different contaminants could be mistaken as the samples when observing the distance traveled by the samples. With contaminates on the plate, there is no sure way to determine what spots indicate the traveled substances, and what spots are contaminants. 2. The solvent at the base of the chamber is used to separate the different components of the samples as it travels up the TLC plate; however, if too much solvent is used, it can completely submerge the samples initially rather than travel through them. If this situation occurs, the solvent may wash the samples from the plate into the bottom of the chamber. 3. The polarity of the solvent is very important for chromatography, for a solvent with an appropriate solvent can optimize the separation of components of the compounds that can have a variety of polarities. If the polarity of the solvent is too low, the separation of components will be very minimal, creating for a low Rf. If the polarity of the solvent is too high, the components may travel as far as the solvent front with no separation, causing for a high R f value.

Questions 6. Arrange the following in order of increasing Rf with TLC: acetic acid, acetaldehyde, 2-octanone, decane, and 1-butanol Acetic acid, 1-butanol, acetaldehyde, 2-octanone, decane 11. One of the analgesics has a chiral center. Which compound is it? One of the two enantiomers is far more effective at reducing pain than the other. Advil has the chiral center. 12. Using a ruler to measure distance, calculate the R f value for substance B in figure 8.12 0.73...