Organic Chemistry Lab: Crystallization and Recrystallization
Kristine--obviously my comments here are made knowing your report is incomplete. I'm going to refrain from saying "write more here!" etc. but will look for problems with what you have already put down. Let me know if you have questions--Carol
Introduction
Crystallization is commonly used in organic chemistry to purify organic substances and/or separate out components of a mixture. The process of crystallization includes dissolving an impure solid organic substance in an appropriate and easy to use solvent. Then the solvent is cooled, allowing the solubility of the sold to decrease to the point where the solid comes completely out of solution. The newly formed pure and large crystal are formed and then filtered out from the remaining liquid. In order to achieve crystallization, there are seven specific steps that are to be followed in detail, otherwise crystals will not form from a variety of reasons. The following lab experiments demonstrate the steps of solvent selection, decolorization and examples of crystallization and analysis of successful and unsuccessful crystallization.

Procedures

The experiments conducted as part of the Crystallization and Recrystallization labs are found in the following citation:
Williamson, Kenneth, L. Macroscale and Microscale Organic Experiments. 4th ed. Boston: Houghton Mifflin Company. 2003. Print.

The following experiments were completed in the first lab;
Experiment #1: Solubility Tests, pages 56-57.
Experiment #2: Crystallization of Pure Phthalic Acid (Naphthalene and Anthracene was not done as part of this lab), pages 57-59.

The following experiments will be completed in the second lab:
Experiment #4: Decolorization of Brown Sugar (Raw sugar will be used instead of Brown Sugar), pages 59-60.
Experiment #7: Purification of an Unknown, pages 61-62.
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Data/General Observations
Experiment #1: Solubility Tests
Three test compounds, approximately 10 mg of Anthracene, Benzoic Acid, and Resorcinol were mixed individually with approximately 0.25 mL of three solvents, Water, Toluene, and Ligroin to test the solubility of the each solid relative to each solvent. The best solute and solvent reactions are summarized in the discussion/conclusion section below.
Compounds
Water (0.25ml) Cold
Water (0.25 ml) Hot
Anthracene
not soluble
not soluble, no crystals formed
Benzoic Acid
not soluble
soluble with heat, formed cloudy flat crystals
Resorcinol
soluble
did not heat, due to initial solubility

Compounds
Toluene (0.25ml) Cold
Toluene (0.25 ml) Hot
Anthracene
not soluble
not soluble
Benzoic Acid
soluble
did not heat, due to initial solubility
Resorcinol
not soluble
soluble, formed pointed crystals

Compounds
Ligroin (0.25ml) Cold
Ligroin (0.25 ml) Hot
Anthracene
not soluble
not soluble
Benzoic Acid
not soluble
not soluble
Resorcinol
not soluble
soluble, formed prism like crystals
your tables are fine.
Experiment #2: Crystallization of Pure Phthalic Acid
The crystallization of pure phthalic acid was accomplished by starting with 0.060 g of phthalic acid. This experiment was conducted twice, because during the first experiment, the quantity of phthalic acid was incorrect, so it was redone with the exact quantity needed. Water was used as the solvent, adding approximately 7 drops slowly with the Pasteur pipette. The crystallization steps were followed and clear, large crystals formed as expected. The weight of the pure phthalic acid crystals were 0.031 g at the completion of the lab after drying with a hirsch funnel and drying in the oven for 15 minutes. The crystals were set aside in the lab to reweigh the following week, however they were not kept, so they could not be reweighed after they had dried completely or melting points recorded. Thus, the weight of 0.031 g is assumed to be the dried weight and there is no melting point data.

The percent recovery of the pure phthalic acid is 0.031g/0.060g x 100 = 52%.

Experiment #4: Decolorization of Raw Sugar
In order to remove colored impurities when conducting crystallization, activated charcoal can be used to decolorize a solution. Raw sugar was used in this experiment instead of brown sugar. There was 15.030 g of raw sugar weighed and dissolved in 30mL of dioinized water. The solution was split into 2 flasks, and then added 0.255 g of charcoal into sample 1 and added 0.051 g of charcoal into sample 2. Sample 1 was a clear amber color and sample 2 was also a clear amber color, but with a slightly darker tint.

Experiment #7: Purification of an Unknown
This experiment was conducted to identify an unknown compound by following the seven steps of crystallization and then recording the mixed melting point determination. No crystals formed during this experiment; therefore there is no data for percent recovery and melting point determination.

The solvent used in this experiment was 18mL of Ethanol and 2 mL of dioinized water. The unknown was weighed at 0.289 g and had not fully dried, therefore may have been a slight overestimate from the dried weight. This would be useful information if crystals had formed and the percent recovery was calculated.

During the step of dissolving the solute, approximately 0.75 mL of solvent was added. The solution turned to a cloudy, yellowish substance. As the substance was heating, it began exploding, therefore, the tube was pulled up slightly to keep the explosions (superheating) from happening. Then approximately 2.0 mL of solvent was added and continued to heat the substance. The total volume of the solvent and the time it was removed from the heat was approximately 1.5 mL. The solute had not fully dissolved and therefore heating was discontinued, assuming that the solvent was unable to dissolve 100% of the solute with heat. The substance was settled and the impurities had fallen to the bottom of the reaction tube. The next step was to conduct a hot filtration to remove most of the impurities from the solution. Mistakenly, a vacuum filtration was conducted instead of a hot filtration. The associated problems with this step are discussed in the conclusion/discussion section below. The remaining filtered liquid was clear and yellow and was cooled in the ice bath. After approximately 20 minutes, only very small and barely visible crystals had formed. They were too small to filter and continue the experiment with weighing and melting point measurements.

Discussion/Conclusion

Experiment #1: Solubility Tests
In crystallization experiments, solvents are usually chosen by experimentation. Polarization of a molecule is difficult to determine by the structure of the molecule and experimentation will demonstrate whether or not the solute will dissolve in the solvent. The ideal crystallization solvent is one that will fully dissolve the solute when it is hot, but not cold. Williamson (2003) on page 40 and 41 provides a list of common solvents and their properties. The following table summarizes the best solute and solvent reactions from the experiment that meet the criteria of a good solvent for crystallization experiments. There does not appear to be any source of error in this experiment.
Compounds (Solute)
Solvents
Benzoic Acid
Water
Resorcinol
Hot Toluene
Resorcinol
Hot Ligroin

Experiment #2: Crystallization of Pure Phthalic Acid
The crystallization of pure phthalic acid was accomplished by following the seven steps outlined in Williamson (2003). The crystals formed as expected and the percent recovery was 52%. With 52% recovery, either a maximum of 48% of the original compound was lost in the process and/or water was a good solvent and removed impurities in the original compound. There also may have been an error in the dried weight and the 52% recovery is actually higher than what it may have been if a dried weight was measured a week later. Since there is no melting point data, a comparison between the original pure phthalic acid and the crystallized phthalic data melting point ranges cannot be compared. However, if the original substance was pure, then the melting point ranges should be similar before and after crystallization. Without the melting point data, it is not feasible to adequatley assess the possible errors.

Experiment #4: Decolorization of Raw Sugar
The decolorization of raw sugar was tested using two samples, sample 1 with 0.255 g of charcoal and sample 2 with 0.051 g of charcoal. The amount of charcoal used in decolorization is difficult to determine and takes some refinement. This experiment resulted in sample 2 being slightly darker in tint, suggesting that only 0.051 g of charcoal was not enough. Sample 1 was a clear amber color, suggesting that 0.255 g of charcoal was enough to remove the color impurities from the raw sugar. If more charcoal was added to sample 2, and the color resulted in the same color of sample 1, then it can be concluded that 0.255 g of charcoal was sufficient for this step of the crystallization experiment. There does not appear to be any source of error in this experiment.

Experiment #7: Purification of an Unknown
The purification of the unknown experiment did not result in crystallization or identification of the unknown. The steps of crystallization were taken as followed:
1. Choose the solvent
2. Dissolve the solute
3. Decolorize the solution (not necessary)
4. Filter suspended solids
5. Crystallize the solute
No crystals formed therefore the last two steps were not conducted (collect and wash the crystals and dry the product).

Error Analysis
There are several reasons that can account for the lack of formation of crystals. One is that during the step of dissolving of the solute, approximately 0.75mL of solvent was added and then additional 2.0 mL of solvent was added. There may have been too much solvent used when it was added the second time, which would impact the cooling and crystallizing step. There were also undissolved impurities in the bottom of the reaction tube, so they were filtered out. Instead of conducting a hot filtration, a vacuum filtration was done mistakenly, which may have cooled down to a temperature where crystals may have collected in the hirsch funnel along with the impurities. Due to each of these changes from recommended steps, crystallization did not occur. If more time was available, the experiment would have been repeated making the necessary adjustments.

Post Lab Question
Ty Trate has been feverishly working in the Organic lab to recrystallize a substance he synthesized using the Nobel-award winning Heck reaction. Ty started out with 1.106 grams of product with a melting point of 131.6-144.3 degrees C. After recrystallizing from an ethanol/water mix, he is left with 0.884 grams of product that melts from 143.7-144.9 degrees C.

1. Calculate the percent recovery for Ty, reporting your answer with an appropriate number of significant figures.
Percent recovery = 0.884 g / 1.106 g * 100 = 79.9%.

2. Then tell me whether Ty appears to have succeeded in purifying his product during the recrystallization, and explain to me how you know.
Ty was successful in purifying his product during the recrystallization due the small range in melting points compared to the initial ranges in melting points. This suggests that the substance prior to recrystallization was more impure than the substance after recrystallization. Also, the resulting percent recovery of 79.9% demonstrates that 20% of the original substance was impure; therefore there should be a smaller range in the melting points of the purified product.