The caffeine molecule is a polar molecule. The carbonyl groups act as electron-w

Question

The caffeine molecule is a polar molecule. The carbonyl groups act as electron-withdrawing groups to draw electrons out of the rings. This polarity allows for caffeine to be soluble in water. This experiment will remove the caffeine from the aqueous soda solution by taking advantage of the preferential solvation of caffeine in isopropyl alcohol after enough sodium chloride is added to the soda in order to push the caffeine out.
The solubility of caffeine in water at room temperature is about 20 g/L. The solubility of caffeine in isopropyl alcohol is unknown. However, the solubility of caffeine in isopropyl alcohol is greater than the solubility of caffeine in salt water.
By adding salt to the Mt. Dew in this lab, we are overcoming the solute – solvent interactions between the caffeine and the water by pushing the caffeine out of the water. The sodium and chloride ions will dissolve preferentially over the caffeine. In comparison, the caffeine looks non-polar so it gets pushed out of the Mt. Dew and will separate into the isopropyl alcohol phase. Typically, isopropyl alcohol and water are miscible liquids. For the same reasons that the addition of salt pushes the caffeine out of the water, the isopropyl alcohol will form a layer on top of the soda. That isopropyl alcohol layer will contain the caffeine. **IF your solution does not separate, it is because you didn’t stir the salt into the soda. The salt needs a chance to dissolve before the organic solvent is added. Add more salt, stir, let it sit. Then, add more isopropyl alcohol. It will separate.
The technique used in this lab is called a liquid-liquid extraction. This technique utilizes the polarity differences of salt water and isopropyl alcohol and the preference for caffeine to be solvated in isopropyl alcohol over salt water. By removing the isopropyl alcohol layer from the water layer, the caffeine is removed. The solvent (isopropyl alcohol) will then be evaporated away and the extract purified by a second, warm isopropyl alcohol extraction. The extracted compounds will be verified using Thin Layer Chromatography (TLC) against a known standard.

Procedure Part 1: Extraction:
1. Pour 50mL of Mt. Dew into a 100 mL glass beaker. 2. Add 1 T of table salt to the Mt. Dew in the glass. 3. Stir gently with the wooden stir stick to be sure you have saturated the liquid
with salt. If there is no solid at the bottom of the glass, add additional salt. Allow this solution to sit for a few minutes. 4. Add about 25mL of isopropyl alcohol to the beaker and stir gently. 5. Allow the liquid to sit until there are two liquid layers visible. If there aren’t two liquid layers visible, add more salt, stir, then let it sit. Add more isopropyl alcohol and allow the layers to separate.

6. Using a pipet, remove the top layer of liquid into the clean, dry 100mL glass beaker.
7. Evaporate the isopropyl alcohol by leaving the beaker open for a day or two. This is flammable; do not heat. (Tip: Place the isopropyl alcohol / caffeine containing beaker on a heat or AC register or next to a sunny window and it will evaporate quickly.)
8. After the solvent has evaporated to dryness, move on to part 2.

Part 2: Purification:1. Warm approximately 10mL of isopropyl alcohol in a hot water bath. To heat this in a hot water
bath, place the isopropyl alcohol in a beaker and place that beaker in a bowl or mug of hot

water. 2. The boiling point of isopropyl alcohol is approximately 86 oF. The vapors are flammable. DO NOT have isopropyl alcohol near an open flame. Pour the warm isopropyl alcohol into the beaker containing the dried extract from Part 1.
3. Swirl the solution and the caffeine should dissolve. It might take a few minutes to allow the crystals to dissolve. There may be some residual crystals that don’t dissolve. Swirl and warm the mixture in a warm water bath for a few minutes to encourage the solvation of the caffeine. It is acceptable to have some residual, non-dissolvable substances; those are the impurities that are being separated from the caffeine.
4. If there are non-dissolvable substances in the liquid, use the filter paper to remove them. a. Fold the filter paper into a cone. b. Place the filter into the funnel and hold the funnel over the beaker used to warm the
isopropyl alcohol in step 1.
c. Pour the caffeine containing isopropyl alcohol into the filter and collect the liquid in the beaker from step 1.
d. When all of the liquid has drained, discard the filter paper in household trash.
5. Evaporate this beaker’s solvent to approximately equal to 1mL. In a 100 mL beaker, 1 mL is just enough liquid to cover the bottom. (approximately 20 drops = 1mL)

Part 3: TLC Characterization:
1. Prepare the developing chamber by taking a 100mL beaker and adding approximately 4 mL of acetone. Cover the beaker with plastic wrap or aluminum foil to increase the vapor pressure of the solvent in the beaker.

2. If the solvent has completely evaporated from the extracted caffeine and the caffeine crystals are apparent, place 1-10 drops of warm isopropyl alcohol into the crystals and swirl to allow some of the crystals to redissolve.
3. If the alcohol has evaporated to approximately 1mL left, then proceed to step 4. Otherwise, wait until the volume is further reduced. The purpose of this step is to get the solution as concentrated as possible.
4. **Handle the TLC plates with care and by the edges so as not to knock off the silica stationary phase. Using a pencil, draw a light pencil line about 0.5 cm from the short end of one of your TLC plates.
5. Make a light mark or draw a small circle on the line about 0.5 cm from the left edge. This will be where the standard solution will be placed.
6. Make a second light mark approximately 1cm from the standard mark. This is where the extract solution will be placed.
7. Gently touch the small end of a micropipette tip into your extract solution. Quickly ‘spot’ the extract onto the mark on the right side by holding the tip perpendicular to the TLC plate. Make a spot of analyte on the plate large enough to visualize but not so large as to leave a puddle on the plate; Approximately 1-2 mm in diameter.
8. Gently touch a clean micropipette tip into the standard solution. Using the same spotting technique, place a spot of standard solution on the mark on the left side.
9. Place the prepared TLC plate in the developing chamber (beaker from #1), pencil line side down.
**Be sure the solvent does NOT touch the analyte spots. You can visualize this by holding the plate on the outside of the beaker to judge the height of the eluent (solvent inside).  
10. Cover the beaker with plastic wrap or aluminum foil to keep the vapor pressure high. Do not jostle the beaker.

11. When the solvent appears to be within 1 cm from the top of the plate, remove the plate from the chamber. Lightly outline in pencil where the solvent stopped on the plate. This is called the solvent front.

12. Allow the plate to dry. 13. VISUALIZE - a. Plug the UV light apparatus into a standard US plug. DO NOT LOOK DIRECTLY AT THE
LIGHT. Shine the light on the dry TLC plate and gently mark with a pencil where the spots appear. Calculate the Rf of the spots that appear with the UV light.

13. VISUALIZE - a. Plug the UV light apparatus into a standard US plug. DO NOT LOOK DIRECTLY AT THE
LIGHT. Shine the light on the dry TLC plate and gently mark with a pencil where the spots appear. Calculate the Rf of the spots that appear with the UV light.

b. Lightly spray the dry TLC plate with bromocresol green. The plate should be colored and wet but not saturated. Expect to use 2-4 spray pumps to coat the plate. **It is best to wear gloves and spray the bromocresol green into the sink once or twice before spraying your plate. Use less, not more for this visualization. **This step is to show if there are any compounds extracted within the pH range of the bromocresol green. You may or may not see any additional spots with this method.

Top is procedure for the reference.

Question !!

1. This is an introduction to the idea of the nucleophile and the electrophile. Values for pKa’s are also discussed. Summarize how pKa’s are affected by the polarity of an atom. Tie that into the idea of a nucleophile and an electrophile. Include in that discussion a mention of the nucleophilic sites that would be expected in caffeine. (6pts)

2. Discuss this lab’s results. Describe the first extract prior to recrystallization and then after the purification step. Include an analysis of the TLC plate and conclude if you have extracted caffeine with this method. This is the majority of the lab points so be clear and thorough. (16pts)

Explanation / Answer

Caffeine separation and analysis

1. A more polar atom as in acetate with resonance stabilization in the ion, would give a lower pKa (higher Ka) value for the acetic acid. A less stable structue as in ethanol goes to ethanoate would have a higher pKa (lower Ka) value. So acetic acid dissociates to a greater extent when compared to ethanol as the substrate in the solution. Water is more polar and thus, NaCl dissoicates into ions more strongly in it, whereas, vcafeeine being non-polar dissociates less and goes in less polar i-propanol solvent layer.

2. The less polar caffeine goes in low polarity solvent i-propanol and is separated for isolation of caffeine. Once the caffeine is obtained after removal of solvent prior to recrystallization, it is dissolved in hot i-propanol for recrystallization. More polar impurities If any remained in undissolved state. On tlc, caffeine would rise higher in acetone with an Rf of approximately 0.4.

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