Researcher

Matthew McBride

Objective

To recrystallize caffeine using the high boiling point solvent 1-octanol and then washing with the low boiling point solvent pentane from a 1.5M, 0.75M and a 0.25M solution using the centrifuge method to wash the crystals.

Procedure

Created three solutions of caffeine in 1-octanol in a 10mL screw cap test tube that were 1.53M (Tube 1, 0.2458 g), 0.77M (Tube 2, 0.2546M) and 0.25M (Tube 3, 0.2522 g). The caffeine was dissolved in the 1-octanol by heating using an ethylene glycol bath for each solution. The solutions were cooled in an ice bath to form crystals. The solutions were centrifuged for 5 minutes to separate the 1-octanol and caffeine crystals and then the 1-octanol was pipetted off. Each solution was washed with pentane (1.5 g) 12x by adding the pentane to each tube, vortexing the sample, centrifuging the sample, and then removing the pentane.

Results

Recrystallization Sheet for Tube 1
Concentration: 1.53M
All of the crystals did not enter the solution

Recrystallization Sheet for Tube 2
Concentration: 0.77M
The sample was contaminated with water and so no product was recovered

Recrystallization Sheet for Tube 3
Concentration: 0.25M
Amount of caffeine recovered: 0.2237 g
HNMR Spectrum of ONSEXP343M3_sample1
Purity: Trace amount of 1-octanol at 0.9ppm, but pure caffeine crystals
Predicted yield: 92% with a solubility of 0.019M at 25°C

Discussion

According the temperature curve for the solubility of caffeine in 1-octanol, all the crystals of the 1.5M solution should have entered the solution at 155°C, but this was not observed.

According to the same temperature curve, all the crystals for the 0.77M solution should have entered the solution at 128°C and the contents of this solution were observed to enter the solution between 130-160°C. This solution (Tube 2) could not be analyzed because at some point in the experiment, the sample was contaminated with water.

According the same temperature curve, all the crystals for the 0.25M solution should have entered the solution at 90°C. Upon being placed in the 130°C bath, the crystals were observed to enter the solution. There was some difficulty in removing the 1-octanol from the caffeine, because after being recrystallized, the caffeine crystals formed a thick gel with the 1-octanol. A spatula was used to loosen the gel and then the 1-octanol was successfully separated from the caffeine crystals. However, only 2.2036 g of the original 4.0867 g of 1-octanol (54%) that was originally added to the solution was removed before washing with pentane.

The measured yield of 89% strongly agrees with the predicted yield of 92% using the room temperature solubility value of 0.019M for caffeine in 1-octanol.

Conclusion

Using the centrifuge to wash the crystals with pentane 12x, the 0.25M solution successfully recrystallized 0.2237 g of pure caffeine (89% yield).

Log

2012-01-16

10:11 Filled a 250 mL beaker to the 175 mL mark with ethylene glycol. Turned on heat of the plate to 300°C.
10:15 Weighed three empty 10 mL screw cap test tubes. Tube 1 weighed 11.4616 g. Tube 2 weighed 11.5028 g. Tube 3 weighed 11.3810 g.
10:27 The following amounts of anhydrous caffeine was added to each tube: Tube 1 - 0.2458 g. Tube 2 - 0.2546 g. Tube 3 - 0.2522 g.
10:28 The temperature of the ethylene glycol bath was 87°C.
10:44 The following amounts of 1-octanol were added to each tube: Tube 1 (1.5M) - 0.5416 g. Tube 2 (0.75M) - 1.2512 g. Tube 3 (0.25M) - 4.0867 g.
10:54 Hot plate was set to 400°C. The temperature of the bath was 160°C.
10:55 The screw cap tubes were capped and placed in the hot bath at 160°C.
11:20 The solutions were observed to be bubbling slightly, so the caps were loosed to avoid any pressure build up.
11:27 All the crystals of tube 3 went into the solution. The temperature of the bath was at 130°C. Tubes 1 and 2 were vortexed to mix the solutions. Tube 3 was placed in the ice bath.
11:57 All of the crystals of tube 2 went into solution. The temperature of the bath was 160°C. Tube 2 was placed in the ice bath.
12:06 A small clump of crystals at the bottom of tube 1 would not enter the solution, so tube 1 was removed from the hot bath.
12:12 Tube 2 and 3 were removed from the ice bath and centrifuged (6-holed) for 5 minutes. Separation of liquid and crystals was visible in tube 2, but not in tube 3. Tube 3 had formed a thick gel.
12:23 A spatula was used to loosen the crystals in tube 3 and both tube 2 and 3 were centrifuged for 5 minutes (6-hole).
12:31 There was liquid separated from the crystals in both tube 2 and 3. The tubes were centrifuged for an additional 5 minutes.
12:50 2.2036 g of 1-octanol was removed from tube 3.
12:57 0.7265 g of 1-octanol was removed from tube 2.
12:58 1.5350 g of pentane was added to tube 2. (Wash #1)
13:00 1.5268 g of pentane was added to tube 3. (Wash #1)
13:05 The tubes were vortexed and centrifuged (6-hole) for five minutes.
13:13 The pentane was removed from both tubes. 1.4433 g of pentane was added to tube 2. 1.3850 g of pentane was added to tube 3. The tubes were capped and stored in a drawer. (Wash #2)

2012-01-28

11:24 Centrifuged tube 2 and 3 for 5 minutes in 6-holed centrifuge.
11:30 Removed pentane and added approximately 1.5 g (half of way up the tube) of pentane in each tube. Tubes were vortexed and centrifuged for 5 minutes. (Wash #3)
12:00 Water contamination was observed in tube 2, so it was not used any more.
12:24 Pentane was removed from tube 3 and fresh pentane added (half the tube - 1.5 g). The solution was vortexed and then centrifuged for 1 minute. (Wash #4)
12:35 Dotted solution on the TLC plate (shadow was observed showing presence of 1-octanol) and then refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #5).
12:39 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #6).
12:45 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #7). Shadow observed on the spotted TLC plate.
12:49 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash # 8).
13:07 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #9). No shadow observed on the spotted TLC plate.
13:14 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #10)
13:29 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #11)
13:32 Refreshed pentane. Vortexed and centrifuged for 1 minute (Wash #12).
13:47 Removed pentane from tube 3 using a pipette. Then placed the tube with a loose fit cap in the temperature bath at 110°C. The excess pentane evaporated off. The weight of tube 3 with the cap and the recovered product was 11.6047 g (0.2237 g of caffeine). This is Sample 1.
13:53 0.0255 g of Sample 1 was placed in a 5-dram vial and 0.5 mL of CDCl3 was added to the vial. The sample dissolved right into the CDCl3. The sample was placed in an NMR tube and analyzed by 500MHz HNMR. The file is saved as ONSEXP343M3_sample1.