Organic Chemistry Lab 1 PDF

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Purification of a Solid: Crystallization and Melting Point Danielle Stern Partner: 02/12/2020 Department of Chemistry, Binghamton University, Binghamton NY 13901

Introduction The main purpose of this experiment was to determine the identity of an unknown organic solid acid through executing recrystallization and determining the melting point. Additionally, a thermometer correction was performed with Benzoic Acid in order to obtain an accurate melting point for the unknown acid. Recrystallization is a critical technique utilized to purify organic solids through the removal of their impurities. In the purification process, a specific solvent is combined with an organic compound and the mixture is heated, which enables the compound to dissolve and produce a saturated solution. The subsequent cooling of the solution activates the formation of crystals composed of the pure solid. Several factors underlie the mechanisms of the recrystallization process, but solubility is the most influential component throughout because it is the primary method of distinguishing and segregating the impurities from the solid. A crucial element involved in this procedure is determining what solvent should be used based on the organic solid being purified. It is imperative that the solid has a high solubility at cold temperatures and a low solubility at hot temperatures. These are the conditions necessary to induce dissolution upon boiling the mixture and induce precipitation of crystals upon cooling the mixture as well as avoiding the loss of product. Another detail to take into consideration is the polarity of the solvent. The solute and the solvent should have polarities close to one another to prevent experiencing resistance when dissolving the solute. After

selecting a particular solvent, the organic solid can be measured out and then the solute and solvent can be combined in an Erlenmeyer flask. After this the solvent in the mixture is heated to a boil in order to dissolve the solute into the solvent. While heating the solution, sometimes a nucleation site does not form, which suppressed the gas bubbles and there is a potential for superheated areas to eject bubbles, suddenly. This occurrence is known as a bump and the utilization of boiling sticks can keep this from happening. The sticks have their own nucleation sites, which allow bubbles to form on their surface and provide more even heating of solutions. Following the dissolution of the solid, the solution may still contain colored impurities that will not dissolve. To remove these materials, a decolorizing charcoal, such as Norit, can be added to the solution. The decolorizing charcoal will be able to adsorb the impurities because it is drawn to conjugated, colorful compounds and possesses a large surface area. The solution should be boiled again and then examined to see if the color is still there, which would indicate having to add more charcoal and reheating again. Now that there is charcoal in the mixture, the solution must be filtered into a new flask using hot gravity filtration. Fluted filter paper should be used because it has a greater surface area than other filter papers, which can quicken the filtration process. It is also important to add boiling water to the flask that is receiving the solution and that the flask continues to boil as solution is filtered into it. This is essential because if the temperature drops too much then the solute could begin to crystallize and that solid would be lost. After this filtration all the insoluble impurities should no longer be present in the solution. The solution should then be removed from heat and left to cool until it is room temperature. When the solution in the flask has reached room temperature it should be inserted into an ice bucket to recrystallize. It is very important that time is allotted for the solution to cool to ensure the formation of crystals that are bigger and more pure. The crystal lattices of the impure

substances were shattered when they dissolved in the boiling solution and the organic solid can form new crystals without the impediment of the contaminants. When slow cooling is enacted it increases the purity of the solid because impure material will not be able to integrate itself into the new crystals and it can then go back to the solution. However, when fast cooling occurs the impurities may become surrounded by the solid crystals and not be able to leave. After the crystals have finished forming, perform suction filtration with a Buchner funnel to separate the solvent from the pure crystals. Soluble impurities will remain with the solvent and be separated from the organic solid. Rinse the crystals with cold solvent in order to remove any impurities that may still be attached to them. After filtration dry and weigh the product to determine the amount of solid recovered. In order to evaluate the purity of the solid acid and to identify it, the melting point must be determined. Melting point can be a very effective way to identify an unknown compound and assess the purity of a compound. The melting point of a compound being depressed or presenting a broader range is indicative of impurities. Usually pure compounds will have a range of about 1 - 2°. Before the melting point of the unknown solid acid can be determined, the melting point of Benzoic Acid must be measured in order to determine a thermometer correction. This is significant because a thermometer’s accuracy cannot be assumed. The instrument being implemented to determine melting point is the SRS MPA161 DigiMelt. To find the melting point of Benzoic Acid a small amount of the solid is inserted into a capillary tube and the tube is placed within the sample holder. The DigiMelt can be programmed with the maximum and minimum temperature range as well as the rate that the temperature increases. When recording temperature, provide the temperature of the initial melting as well as when the substance is a clear liquid. The known and observed melting points of Benzoic Acid can be compared to determine the thermometer correction. Then the DigiMelt can be utilized to

find the melting point of the unknown solid acid. Since the melting point of the compound is unknown a fast run will be applied first in which the temperature increases at rapid rates. The result of the fast run can be used to determine the maximum and minimum of the slow run. After applying the thermometer correcting the melting point range can be utilized to identify the solid acid. Procedure Recrystallization and Identification of an Unknown Organic Solid Acid 1. Measured 3.06 g of unknown solid acid #25 on scale 2. Placed measured sample into 250 mL Erlenmeyer flask 3. Added 50 mL of distilled water into the flask 4. Swirled the flask around 5. Placed the flask on a hot plate and added amount of heat for boiling the solvent 6. Inserted the boiling stick consistently into the flask throughout heating 7. Added excess distilled water to the flask to dissolve solid acid 8. Increased heat and let flask heat for 10 minutes 9. Removed the flask from the hot plate 10. Added small amount of Norit to the flask 11. Swirled the flask around 12. Placed the flask on the hot plate again 13. Let the flask heat for 7 minutes 14. Removed the flask from the hot plate 15. Added large amount of Norit to the flask 16. Swirled the flask around

17. Placed the flask on the hot plate again 18. Let the flask heat for 7 minutes 19. Placed second Erlenmeyer flask with small amount of distilled water on the hot plate for water to boil in preparation for hot gravity filtration 20. Added stemless funnel and fluted filter paper on second flask 21. Removed the first flask (unknown/Norit/water) from the hot plate 22. Tilted the flask and examined the color of the solution to ensure the solution was colorless 23. In small increments added solution from the first flask into the second flask with the boiling water to slowly filtrate 24. During filtration the second flask remained on the hot plate 25. Simultaneously, the first flask was alternatively moved onto and off of the hot plate 26. Removed both the flasks from the hot plate 27. Allowed the flask with the solution to sit and cool for 25 minutes 28. Filled large graduated cylinder with ice to make ice bath 29. Checked to make sure the flask was at room temperature 30. Submerged the flask into the ice bath and added more ice to surround it 31. Let the flask sit for 15 minutes 32. While recrystallization took place retrieved filter flask and inserted adapter and Buchner funnel into it, and put filter paper in funnel 33. Retrieved ring stand and clamp to hold the filter flask in place 34. Attached suction hose to filter flask and trap 35. Turned on sink to start suctioning

36. Removed recrystallization flask from ice bath and swirled solution 37. Poured solution into filter funnel 38. Rinsed material remaining in flask with cold water and poured it into the funnel 2 times 39. Used stirring rod to remove excess crystals in the flask 40. Dried crystals by continuing suction filtration 41. Used paper towel to absorb excess solvent on solid acid 42. Air dried crystals for 10 minutes 43. Weighed final crystal product 44. Inserted small amount of product into 1 capillary tube by pressing open end of tube into crystals 45. Placed capillary tube inside plastic tube with the closed end facing down to pack sample 46. Placed capillary tube in one of the sample slots inside the heating chamber for DigiMelt #8 47. Looked into eyepiece to make sure I could see sample in the tube 48. Pressed start temp button and used the buttons labeled 2 and 3 to set my starting temperature to 100℃ and then pressed start temp button again 49. Pressed ramp rate button and used the buttons labeled 2 and 3 to set my starting temperature to 20℃/min and then pressed ramp rate button again 50. Pressed stop temp button and used the buttons labeled 2 and 3 to set my starting temperature to 250℃ and then pressed stop temp button again 51. Pressed start/stop button to preheat DigiMelt #8 52. When the Ready light turned on pressed start/stop to begin melting 53. Recorded temperature when sample first began to melt

54. Recorded temperature when sample finished melting 55. Press start/stop button to turn off the heat 56. Inserted small amount of product into 3 capillary tubes by pressing open end of tubes into crystals 57. Placed capillary tubes inside plastic tube with the closed end facing down to pack sample 58. Placed each capillary tube in one of the sample slots inside the heating chamber for DigiMelt #8 59. Looked into eyepiece to make sure I could see each sample in their tubes 60. Pressed start temp button and used the buttons labeled 2 and 3 to set my starting temperature to 120℃ and then pressed start temp button again 61. Pressed ramp rate button and used the buttons labeled 2 and 3 to set my starting temperature to 5℃/min and then pressed ramp rate button again 62. Pressed stop temp button and used the buttons labeled 2 and 3 to set my starting temperature to 170℃ and then pressed stop temp button again 63. Pressed start/stop button to preheat DigiMelt #8 64. When the Ready light turned on pressed start/stop to begin melting 65. Recorded temperature when each sample first began to melt 66. Recorded temperature when each sample finished melting 67. Press start/stop button to turn off the heat 68. Applied thermometer correction to melting points 69. Identified the solid acid Determination of Melting Point - Thermometer Correction

1. Inserted small amount of Benzoic Acid into 3 capillary tubes by pressing open end of tubes into sample 2. Placed capillary tubes inside plastic tube with the closed end facing down to pack sample 3. Placed each capillary tube in one of the sample slots inside the heating chamber for DigiMelt #8 4. Looked into eyepiece to make sure I could see each sample in their tubes 5. Pressed start temp button and used the buttons labeled 2 and 3 to set my starting temperature to 100℃ and then pressed start temp button again 6. Pressed ramp rate button and used the buttons labeled 2 and 3 to set my starting temperature to 5℃/min and then pressed ramp rate button again 7. Pressed stop temp button and used the buttons labeled 2 and 3 to set my starting temperature to 140℃ and then pressed stop temp button again 8. Pressed start/stop button to preheat DigiMelt #8 9. When the Ready light turned on pressed start/stop to begin melting 10. Recorded temperature when each sample first began to melt 11. Recorded temperature when each sample finished melting 12. Press start/stop button to turn off the heat 13. Compared observed value to expected value to calculate thermometer correction Table of Reagents Compound

Molecular Weight (g/mol)

Chemical Formula

Water

18.015

H 2O

Molecular Structure

Melting Point (℃)

Safety

0

Not toxic, neutral, not flammable

Benzoic Acid

122.12

C7H6O2

122

Toxic, need fume hood, irritant to eyes and skin

Adipic Acid

146.14

C6H10O4

152

Irritates eyes and skin, need fume hood

Phthalic Acid

166.14

C8H6O4

207

Need fume hood, major damage to skin, eyes, and organs

Salicylic Acid

138.12

C7H6O3

159

Combusts, needs fume hood, severe eye damage

p-Aminobenzoic Acid

137.14

C7H7NO2

187

Needs fume hood, toxic, irritates skin

2-Chlorobenzoic Acid

156.57

C7H5ClO2

142

Damages skin and eyes, need fume hood

Norit

12.01

C

3652

Irritates eyes, need fume hood,

C

Data Tables

Compound

Fast trial M.P. (o C)

Slow Trial M.P. (o C)

Avg. of Slow Trial (o C)

Unknown 25 wtI= 3.06 g wtF= 0.62 g

146.0

149.2 151.6

149.2 151.8

149.2 152.2

150.4

150.5

Benzoic Acid

122.4

120.6 123.8

120.6 123.8

120.6 123.8

122.2

122.2

Overall Avg. of Slow Trial ( o C)

Avg. of Slow Trial w/ M.P. Corr. (o C)

150.7

150.53

150.73

122.2

122.2

- 0.20

Observations ● The unknown solid acid is a pale chalky substance with a yellowish tint. ● When the distilled water was added to the sample the solution was a clear yellow color. ● Despite the high setting of the hot plate it took a long time for the solution to boil and it was a weak boil with few bubbles ● Bubbles seemed to form specifically close to the stick ● While the solution was heating up before the solute dissolve it settled in the center of the flask in clumps ● When the Norit was added a white foam would form on the solute and then disappear. ● The Norit dispersed in the solution as individual dark grey pellets ● The first addition of Norit was a very small amount and it didn’t make much of an impact on the color of the solution even after reheating it ● After adding an amount 4-5 times the size of the initial addition and reheating it there was a change ● As the flask heated the solution became somewhat grey ● After removing the flask from the hot plate and examining the solution after the carbon had settled the solution was almost completely clear with a slight yellow tint ● The little bit of water in the second flask prepared for the hot gravity filtration came to a strong boil ● After the solution was filtered the resulting mixture still had a slight yellow tint ● The wet charcoal was contained both inside the filter paper and in the first Erlenmeyer flask ● While the solution was cooling to room temperature crystals had not begun to form

● There was no evidence of lost crystals in the filter, first flask, or anywhere else ● The crystals that formed in the ice bath were very small and clumped together within the solution ● The suction filtration made the product into a chalky, white powder ● The solid lost a significant amount of its substance ● In the DigiMelt the substance appeared to crumble when it initially began melting ● When the substance was completely melted it presented as a clear liquid Calculations To calculate the thermometer correction, use the following equation: Thermometer correction = Observed Benzoic Acid M.P. - Theoretical Benzoic Acid M.P. Thermometer correction = 122.2o C - 122.4o C = - 0.2o C To apply the thermometer correction to the M.P. of a sample, use the following equation: Corrected M.P = Observed M.P. - Thermometer correction Corrected M.P. of overall avg. M.P. of unknown solid acid in slow trial = 150.53o C - (-0.2o C) = 150.73o C To calculate the percent recovery of the unknown solid acid, use the following equation: Percent yield of unknown solid acid = (Observed weight of unknown solid acid in grams) / (Theoretical weight of unknown solid acid in grams) x 100% Percent yield of unknown solid acid = 0.62 g / 3.06 g x 100% = 20.26% Discussion The objective of this laboratory exercise was perform recrystallization on a contaminated organic solid to generate pure crystals and then to determine the melting point of that product in order to identify the solid acid. Along with that in order to obtain an accurate melting point for

the unknown solid, Benzoic Acid was utilized to obtain a thermometer correction. During the purification process the unknown solid was dissolved in a boiled solvent in order to remove the impurities. Insoluble impurities were removed through the addition of Norit, a decolorizing charcoal that adsorbed colorful, impure material. The charcoal and impurities were then filtered out through the utilization of hot gravity filtration. Then, when the cooling of the solution triggered the recrystallization of the solid, it led to the separation of soluble impurities that remained with the solvent. The suction filtration process was able to isolate the solid crystals from the liquid solvent, which eliminated the majority of impurities that had not already been dispelled from the organic solid. Despite attempts to retain as much solid as possible throughout the procedure, there were many scenarios when it was difficult to prevent the loss of solid. For example every single time the solution was transferred from one vessel some solute is likely to get lossed. Also, when the suction filtration occurred some pure solid was probably lost in the liquid solution. On top of that during the hot gravity filtration, the loss of some of the heat contact could have led to some solute recrystallizing and that product being lost. Another factor may have been rinsing the flask, this is done in order to loosen crystals, but instead those crystals may have redissolved. One element which may have been particularly impactful in the loss of yield is the utilization of too much solvent. Even though solvent provides essential parts the recrystallization the more solvent there is the more likely it is that pure solids will get stuck in the solvent. When executing this experiment it is important to be conscious of the amount of solvent being used. During the recrystallization process the pure solid diminished from 3.06 g to 0.62 g, a 20.26% yield, which is extremely poor. Following the formation of the crystals the DigiMelt found the melting point of the sample. When finding the melting point initially a fast trial was used in order to gain a general idea of the melting point in order to know where to focus

when performing the slow trials. In the fast trial the solid acid exhibited a melting point of 146.0℃. This information led to designating the range of 120℃ - 170℃ for the slow trials. The slow trial used 3 samples and for each of them the melting began at 149.2, but each of them finished melting at a slightly different time...