Post Lab 1 chm 205 PDF

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Lucrezia Lucchi 9/17/18 CHM 205 - U6 Post Lab Write Up Experiment 1 (Recrystallization) Introduction: The objective of this experiment was to understand solubility, crystal formation of a solid and melting points through the purification of a solid compound. Recrystallization involves the dissolution of a solid in a hot solvent filtration of a heated solution, crystal formation, and the isolation and measurement of a crystalline compound. Because it relies on slow formation of crystals in order to obtain pure materials, slow and controlled cooling is more likely to lead to pure solids. Formation of crystals also depends on concentration of a solute, and on the type and temperature of the solvent – ideally, a solvent should be at a high temperature to dissolve a compound. Insoluble impurities are resolved through the rapid process of hot water filtration, whereas colored, soluble impurities may be removed through the additional process of adding charcoal chips. For a productive, pure crystal yield; the solid substance should never be added to a solution at/near boiling point, compounds should be dissolved with as little hot solvent as possible, and the final solution (post filtration) should be perfectly clear and colorless. The unknown compound we had selected originally (#8), did not necessarily require boiling chips for even heating; nor did it require the use of any charcoal chips, as no soluble impurities or colors were present in solution.

Materials and Methods: In Part A (Macroscale recrystallization), we dissolved .49 g of an unknown compound in approximately 150 mL of near boiling DI H2O. Boiling chips and charcoal did not need to be added fro the reaction to proceed appropriately. Hot gravity filtration was performed using a prewarmed filter funnel fitted with an 11cm Whatman #4 filter paper. The solution was cooled slowly over the course of the week to allow crystals to form. Crystals were then collected and dried by vacuum filtration (approx. 45 min) to be prepared for weighing and melting point determination. Total crystal yield was measured, and percent yield was calculated. A sample of recrystallized unknown was crushed, two pure melting points (slow ramp rate = 1 degrees C/min and fast ramp rate = 10 degrees C/min) and two mixed melting points (unknown + pAcetamidophenol, unknown + Salicylamide) were performed on the DigiMelt SRS. Unknown was identified. In Part B of the experiment (Microscale), .8 mg of unknown was dissolved in approximately 1.3 mL of near boiling DI H2O. A microspatula was used to mix the solution rapidly to accelerate dissolving, and charcoal pellets and boiling chips prior to gentle reheating. Hot gravity filtration was performed in the same manner as in Part A; hot filtrate was then transferred to a Craig tube and crystallization occurred rapidly as the solution cooled to room temperature. The Craig tube was then centrifuged for 2 minutes to recover dry crystals. Identity of unknown was confirmed through measurement of total crystal yield (and percent yield), and two pure melting points utilizing the same ramp rates as in Part A.

p-Acetamidophenol (MP = 169-172 Celsius)

Salicylamide (MP = 140-144 Celsius)

Results (table):

Mass of unknown added (#8) Mass after Recrystallization Percent Recovery

Macroscale (Part A) .49 g .246 g 50%

Microscale (Part B) .8 mg (.0008 g) .7 mg (.0007 g) 87.5%

(post-mass/pre-mass x 100 = % recovery) Experimental Melting Point Range Mixed MP of

167.2-169.0 degrees C 167.9-169.9 degrees C

167.0-168.8 degrees C ----

Unknown + p-Acetamidophenol Mixed MP of

142.6 – 144.4 degrees C

----

Unknown + Salicylamide Possible Unknown Compounds chart is found in lab notebook.

Discussion: Though crystals form through initial formation of a small seed crystal which grow in a reversible layer by layer manner to form a lattice structure, we did not directly observe the formation of the crystals, as the ‘unknown substance’ we had chosen had to remain in the laboratory overnight/week to cool and crystallize. For part 1 of the experiment (macroscale) our substance had no visible impurities prior to filtration, though that may have been attributed to adding the maximum allowed quantity of H20 (25 mL) to the .49 g of substance. During our macroscale recrystallization, crystals were left to form in a room temperature vial over the course of the week and did not sit in a desiccator, rather, the crystals were collected by suction filtration with a Buchner funnel subsequently. The first part of the experiment yielded a 50% yield (.49 g original unknown, .246 g crystals) and melting point determination was used to identify the compound. Once slow and fast melts were conducted to narrow down ranges and determine the melting point, mixed melting points were performed to confirm our results. Experimentally, we found the melting point for our crystals between 167.2-169.0 degrees Celsius, which we found comparable to (and thus utilized in our positive mixed melting point test) the substance p-

Acetamidophenol (MP=169-172 degrees C). To test a ‘negative mixed melting point’ we used the substance Salicylimide (MP=140-144 degrees C) which yielded a final melting point of 142.6-144.4 degrees C. ‘Positive mixed melting point’ performed with p-Acetamidophenol confirmed our predictions that it was the identity of our unknown compound, as the final melting point was 167.9-169.9 degrees C. For part B of the experiment, the same ‘unknown’ compound was used, but procedure was performed at a microscale level – utilizing smaller glassware and a centrifuge. Likely due to a smaller compound to water ratio (.8 mg of ‘unknown compound’ aka p-Acetamidophenol), the compound dissolved quickly and evenly over heat in near boiling water, and efficiently crystallized inside the 2 mL Craig tube, We did not experience any crystallization within the filtration pipette, and no seed crystals or additional aid/ice bath had to be implemented to induce crystal formation. For the microscale recrystallization, some brownish coloration was observed in the pre-filtered solution that was resolved by adding charcoal chips to the heated vial for approximately 25 minutes – however, we once again did not encounter any visible insoluble or soluble impurities within the solution. Though the same substance was evaluated during part 2 of the experiment, use of centrifugation over vacuum filtration may have contributed to differences in experimental melting point yields. Of the original .80mg of substance dissolved, a .70mg yield of crystals was obtained after centrifugation by measuring the difference of the tare weight of the Craig tube; percent yield for this part of the experiment was 87.5%. Aside from some degree of malfunction in the laboratory’s gram scale, two minutes of centrifugation may have been less effective than a full hour of vacuum filtration at separating the solvent and the crystals. Thus, the relatively higher percent yield despite consistency in laboratory techniques might have been due to inadequately dried crystals. In this portion, only fast/slow melts were used to

determine/confirm the identity of our unknown substance and results were somewhat consistent. Microscale recrystallization of the compound produced crystals with a melting range of 167.0168.8 degrees C (near theoretical values given for p-Acetamidophenol). The slightly lower experimental melting point range found was similarly attributed to the crystals not being perfectly dried through the centrifugation separation technique used. Conclusion: Macroscale and microscale recrystallization techniques were utilized to isolate dry crystals from a given unknown compound, using H2O as a solvent. Vacuum filtration was used in macroscale recrystallization to recover purified crystals, whereas microscale recrystallization utilized a centrifuge (with solution in Craig tube) for the same purpose. The unknown compound was weighed before and after crystal formation in both parts of the experiment; resulting in a 50% yield using macroscale techniques, and 87.5% yield using microscale techniques. Fast and slow ramp rates were applied to determine the compound’s melting point, and we found that resultant melting point was slightly lower when examining the microscale sample; melting point for the macroscale recrystallized sample was 167.2-169.0 degrees C, and 167.0-168.8 degrees C for the microscale sample. To confirm the identity of the unknown compound, positive (with pAcetamidophenol) and negative (with Salicylamide) mixed melting points were performed using the crystal yield from the macroscale portion. The unknown sample was therefore identified as pAcetamidophenol, given its similar theoretical melting point of 169-172 degrees C.

Post lab Questions: 1. To find min amount of ethanol needed to recrystallize (values at ethanol’s BP) a. 2.0 g X / 1.4 g X = 1.429 x 10 mL ethanol = 14.29 mL MINIMUM From 78 degrees C -> cooling to 0 degrees C…

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b. .21 g X / 10 mL x 14.29 mL = .30 g left dissolved in solution (answer) c. 2.0 g - .3 g of X means that 1.7 g is the max amount of crystals that can be formed It is important to use a minimum amount of hot solvent when dissolving a solid to be purified so that significant amounts of solid will not remain dissolved after the solution has cooled. Adding too much solvent may result in also dissolving soluble impurities, which will then not be effectively filtered out during hot water filtration, resulting in impure crystal formation or no crystal formation at all. If we had not used a pre heated filter during the hot gravity filtration step, crystals may have formed in solution as it filtered through the funnel, potentially clogging the filter. As the solution cools down through a room temperature funnel, impure crystals can become trapped very quickly, this is also why it is important to pour the solution all together as fast as possible. Benzyl alcohol is a poor choice for the recrystallization of fluorenol because its boiling point is higher than the melting point of the solid we want to recrystallize. The fluorenol would dissolve far earlier than necessary in the process of heating a solution containing benzyl alcohol to near boiling point (205 degrees C) as its melting point range is 153-155 degrees C. How to remove… a. Sand/inorganic contaminant: Via hot water gravity filtration using a prewarmed filter b. Colored organic: Charcoal chips added in solution and subsequent hot gravity filtration c. Very soluble contaminant: Repeated filtration AFTER crystals have cooled and separated. Acetone is a very polar solvent that dissolves almost any organic compound, making it ideal for directly dissolving any organic impurities and cleaning glassware. Recrystallization solvents should dissolve a substance well at the solvent’s boiling point (acetone’s is 56 degrees C, lower than H2O), since acetone’s BP is so low, it may require a greater deal of attention and care on the hot plate. However, because its polarity is comparable to water (which we used in the experiment), it can be inferred that it would be an adequate solvent choice for recrystallization (boiling point could be a limiting factor in efficient carrying out of the experiment). It could be a potentially dangerous solvent/compound because it is highly flammable; considering the several steps involving heating/fire in the recrystallization process, it may be a fire hazard....