Researcher

Matthew McBride

Objective

To recrystallize caffeine using the high boiling point solvent 1-octanol in Erlenmeyer flasks and then washing with the low boiling point solvent pentane from a 2.2M, 0.77M and a 0.26M solution.

Procedure

Created a 2.2M, 0.77M and 0.26M solutions by adding 1-octanol to 1 gram of caffeine in 25 mL Erlenmeyer flasks. The solutions were stirred and heated on a hot plate until all of the crystals had entered the solution. The flasks were removed from the hot plate and allowed to cool to room temperature. The recovered crystals were filtered and washed in the flask with pentane five times. The presence of 1-octanol was monitored by TLC plate. The crystals were filtered on a Buchner funnel and analyzed by HNMR and melting point to determine purity.

Results

Recrystallization Sheet - Flask A
The crystals of the 2.2M solution (flask A) were not recovered due the crystals being caked on the walls of the flask. There was so little solvent present that the crystals just remained stuck to the walls and never entered the heated solution.

Recrystallization Sheet - Flask B
The crystals of the 0.77M solution (flask B) were not recovered due to there being no separation of the crystals from the 1-octanol.

Recrystallization Sheet - Flask C
The crystals of the 0.26M solution (flask C) were recovered: Sample C
Amount: 0.7868 g (from 1.0261 g - 77% yield)
Appearance: White platelet crystals
Melting Point: 255-256°C (stock sample melting point: 255-256°C)
HNMR Spectrum of ONSEXP342_SampleC
Purity: A very pure sample of caffeine with a less than 1% of 1-octanol still remaining in the sample

Picture #1: A picture of the three flasks right after they were placed on the hot plate
exp342realpic1.png
Picture #2: A picture of the crystals on the walls of Flask A that could not be dissolved into the solution due to the small amount of solvent
exp342pic1.png
Picture #3: A picture of Flask C with the solid gel of the crystals in the 1-octanol that had to be scraped out of the flask and onto the filter to remove the 1-octanol
exp342pic3.png

Discussion

The 2.2M solution was heated in too large of an Erlenmeyer flask so the crystals could not properly enter the solution. Future experiments will use a smaller flask or a screw cap test tube to carry out the recrystallization.

The temperature dependent solubility curve for caffeine in 1-octanol predicted that the crystals of the 0.77M solution would enter the solution at 127°C and this was an accurate prediction. The crystals were observed to enter the solution at 135°C. After cooling to room temperature, the 1-octanol did not separate from the crystals and so could not be pipetted off. This means that any impurity contained in the 1-octanol could not be removed. The addition of pentane without the removal of any 1-octanol would just cause any impurity to come crashing back out into the sample. A future experiment will perform the recrystallization in a test tube so the sample can be centrifuged to separate the crystals and the 1-octanol.

The temperature dependent solubility curve for caffeine in 1-octanol predicted that the crystals of the 0.26M solution would enter the solution at 91°C and this was an accurate prediction. The crystals were observed to enter the solution at 95°C. Some sample was lost due to transferring the sample from the flask to the Buchner funnel and then into a beaker to wash with pentane. The recovered 0.7868 g of caffeine was a 77% yield and the expected yield was 90%, since solubility of caffeine in 1-octanol at room temperature is 0.019M and the solution was not cooled down on ice. To limit the lose of crystals, the recrystallization will be done in a test tube and a centrifuge will be used to improve the removal of the 1-octanol. A TLC plate was used to monitor the presence of 1-octanol, because if any 1-octanol was present, a shadow was left on the TLC plate. Following the final wash with pentane, no shadow was present. The melting point (identical to the stock) and the NMR confirmed that the recovered crystals were pure. The NMR determined that the caffeine was pure and was compared to the NMR of pure caffeine found here.

Conclusion

The recrystallization should be conducted in a test tube to limit the lose of sample and a centrifuge should be used to improve the removal of 1-octanol by washing with pentane. A 0.26M solution had a 77% recovery and demonstrated that this method can successfully perform a recrystallization.

Log

2013-01-07

13:33 Added 1.0135 g of anhydrous caffeine into a 25 mL Erlenmeyer flask. This is flask A (2.2 M solution).
13:37 Added 1.0251 g of anhydrous caffeine into a 25 mL Erlenmeyer flask. This is flask B (0.77 M solution).
13:39 Added 1.0261 g of anhydrous caffeine into a 25 mL Erlenmeyer flask. This is flask C (0.26 M solution).
13:50 Added 1.4104 g of 1-octanol into flask A.
13:53 Added 5.0395 g of 1-octanol into flask B.
13:57 Added 16.4073 g of 1-octanol into flask C.
13:58 Took Picture #1 of all three flasks.
14:02 Added a stir bar to each of the flasks and placed on the stir plate. Turned on the heat to warm the solutions.
14:14 Crystals were visible in each of the flasks.
14:18 Flask C reached 95C and all of the crystals entered the solution except for a few that were stuck on the sides. The crystals on the sides were scrapped into the solution, but some crystals remained on the sides of the flask.
14:33 Flask B reached 135C and all of the crystals entered the solution except for a few that were stuck on the sides. The crystals on the sides were scrapped into the solution, but some crystals remained on the sides of the flask.
14:40 The crystals were caked on the walls of flask A due to the small quantity of solvent. Each of the flasks were removed from the hot plate and placed on the lab bench at room temperature to cool. Took Picture #2.
14:41 Flask B formed crystals.
14:45 Flask A formed crystals.
14:50 The crystals in flask A and in flask B formed a thick gel. None of the 1-octanol could be poured off. The flasked were covered and stored at room temperature.

2013-01-09

10:15 Took a Picture #3 of flask C with the white crystals in the solution. No liquid could be poured off without losing the crystals. The thick solution was poured onto the Butcher funnel and filtered by suction filtration. Pentane was used to transfer the solution from the flask to the funnel.
10:23 The crystals were rinsed with n-pentane.
10:47 The crystals were clumping (indicating the presence of octanol). The crystals were transferred into a beaker. The beaker was filled with a few mL of pentane and mixed on the stir plate.
10:51 The beaker was filled with 20 mL of pentane and allowed to mix and then removed from the stir plate. The crystals settled to the bottom. The pentane that formed the top layer was spotted on a TLC plate to determine the presence of octanol (the octanol was completely removed when no shadow remained from the spot). The top layer of pentane was removed and replaced with fresh pentane.
10:58 The crystals were washed in the same manner and the solution spotted on the TLC plate. A slight shadow still remained on the TLC plate from the spot.
11:04 The crystals were washed in the same manner and the solution spotted on the TLC plate. A slight shadow still remained on the TLC plate from the spot.
11:15 The crystals were washed in the same manner and the solution spotted on the TLC plate. A slight shadow still remained on the TLC plate from the spot.
11:33 The crystals were washed in the same manner and the solution spotted on the TLC plate. No shadow remained from the spot on the TLC plate.
11:47 The crystals were filtered on the Butcher funnel using suction filtration and allowed to dry.
11:50 The weight of an empty (no cap) snap cap 5-dram vial was 11.9941 g. This was labeled Sample C.
11:56 The dried caffeine crystals were transferred to the vial and the vial weighed 12.7809 g. This means there were 0.7868 g of recovered caffeine from flask C.
12:02 The melting point of Sample C was measured to be 255-256C. The melting point of the stock caffeine was measured to be 255-256C.
12:57 0.0497 g of sample C was placed in a 5-dram vial and dissolved in 0.5 mL of CDCl3. The solution was placed in NMR Tube 1.
13:55 NMR Tube 1 was analyzed by 500MHz HNMR and the file was saved as ONSEXP342_SampleC.