Experiment 7 Recrystallization - Purification of Crude Benzoic Acid and Phenanthrene PDF

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Lab Report Organic Chemistry...

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Experiment 7: Recrystallization -- Purification of Crude Benzoic Acid and Phenanthrene INTRODUCTION The purpose of this experiment is to purify the extracted benzoic acid and phenanthrene isolates from experiment 6. The two compounds will be recrystallized in hexanes or ethanol-water to remove any impurities encountered during the extraction process. The recrystallization technique depends on the solubility principles of the solute and solvent when hot and cold.

FIGURE 1. PERTINENT MOLECULES.

PHYSICAL DATA Compound

Molecular Weight (g/mol)

Melting Point (°C)

Boiling Point (°C)

Density (g/mL )

Solubility

Hazards

Benzoic acid

122

121-123

249-250

1.26

s OH, eth, bz, chl, ace, c, dis; ss pet.eth, hex

mild irritation if inhaled, in contact with skin, or eyes; moderately toxic; slight fire hazard

Chloroform-d

120

-64.0- -63.0

60-62

1.49

msc organic solvents, ethyl acetate, and acetone

severe irritation if in contact with eyes and skin; toxic if ingested or inhaled; corrosive

Ethanol

46.1

-116- -114

78-80

0.789

mcs H2O, eth, EtOH, chloroform: s bz

toxic/poisonous; flammable; irritant when in contact with eyes or skin, hazardous if ingested or inhaled

Hexanes

86.2

-96.0- -94.0

68-69

0.659

vs EtOH; s eth, chloroform; mcs OH,eth; i H 2O

flammable; hazardous if ingested, or leads to irritation if in contact with skin or eyes

Phenanthrene

178

99.2-101

338-340

1.18

s toluene, CCl4, poisonous if ingested; bz, CS2, EtOH, eye and skin irritant; irritant if inhaled ace; i H2O

Percent Recovery To find the percentage of benzoic acid or phenanthrene purified from the recrystallization technique.

percent recovery=( 100) PROCEDURE

amount isolate d initial amount

1. Clean two NMR tubes with acetone and set aside for later use. 2. With the previously isolated benzoic acid, place 0.10 g of the sample into an Erlenmyer flask with 3 mL of hexanes. 3. Lightly mix the benzoic acid and hexanes in the flask and then heat to boiling point of the hexanes (69 °C) on a hot plate. - For steady boiling add boiling chips to solution if needed. 4. Add 1 mL of hexane solvent until all benzoic acid is dissolved. 5. If the solution appears colored add Norite (activated charcoal) pellets to the solution until it turns clear. - To remove the Norite, change to a beaker and then carefully pour the clear solution back into the flask on the hot plate. 6. Remove the flask from the hot plate and place it with a lid on a paper towel on the counter. Wait until the solution comes to room temperature. - If no crystals begin to form: scrape the bottom or side of the flask, or boil away approximately half of the solution and then cool again 7. Place the flask on ice after around 15 minutes off of the hot plate and observe the complete formation of the crystals 8. Separate the crystals using vacuum filtration. Wash off the isolated crystals with cold hexanes to remove impurities, and filtrate for around 5 min to remove all mother liquid - Vacuum filtration procedure explained in experiment 6 9. Move the crystals to a weight-boat and set aside to dry 10. Take the weight, melting point, and NMR spectra of benzoic acid to determine the percent recovery and impurities. 11. Mix 0.1g of phenanthrene with 3 mL of hot ethanol in an Erlenmyer flask. 12. Add small amounts of water until the solution becomes cloudy, then add small amounts of hot ethanol until the solution becomes clear, then remove from the hot plate. 13. Let the flask cool to room temperature for 15 minutes and take 6 mL ethanol with 2 mL water in an Erlenmyer flask and cool on ice. 14. Move the phenanthrene crystals on ice and let cool 15. Separate the crystals using vacuum filtration. Wash off the isolated crystals with the cold ethanolwater solvent to remove impurities, and filtrate for around 5 min to remove all mother liquid 16. Move the crystals to a weight-boat and set aside to dry 17. Take the weight, melting point, and NMR spectra of phenanthrene to determine the percent recovery and impurities.

DISCUSSION OF RESULTS The objective of this preparative lab was to purify the crude benzoic acid and phenanthrene samples collected from experiment 6. This was accomplished through the process of recrystallization using hexanes, ethanol, and water as solvents. These specific compounds were used because of their solubility properties with benzoic acid and phenanthrene under specific temperatures. Recrystallization of the sample was dependent on the compound being soluble in the solvent at high temperatures and insoluble at low temperatures.

In the case of this experiment, benzoic acid was soluble in hot hexanes and insoluble in cold. Specifically at 65 °C--approximately the boiling point of hexanes-- 0.103g of benzoic acid fully dissolved in the solution with 7 ml of hexanes; ideally any contaminants would exhibit different solubility properties and thus remain in a solid phase, separated from the sample. Once the solution was removed from the hot plate, it was set aside to cool and develop crystals; however, the experimenter mistakenly placed the erlenmeyer flask directly on ice from the hot plate. The solution was on ice for approximately 30 seconds before properly moved to the benchtop. Through observation of the crystals, this error caused a disruption in formation. Cooling the solution too quickly impacted the quality and size of the crystals because the drastic change in temperature lead to precipitation rather than a steady development. The rapid evolvement of the solution caused possible impurities to be taken up and trapped within the crystalline structure. The crystals were expected to be long and thin, however, the observed crystals were more clustered and small. Due to time constraints, the process was unable to be replicated to obtain more reliable results, so the experiment continued with the deformed benzoic acid crystals and possible impurities. Vacuum filtration was used to isolate the crystals from the cooled solution, as previously done in experiment 6. Filtration proceed for 5 minutes to separate all the hexanes from the solid isolate, then the crystals were set aside to dry. The same general process was performed on phenanthrene, but due to its solubility, a mixture of ethanol and water was used. Phenanthrene required both solvents because ethanol would not fully satisfy the solubility characteristics at boiling point and room temperature needed to purify the crystals. If ethanol alone was used, then the yield of crystals would be smaller because a significant amount of phenanthrene would remain soluble in the cold ethanol. With the addition of water, phenanthrene was “forced out” of the solution in crystal form because of its insolubility. To dissolve 0.099g of the crude phenanthrene, 7 mL of hot ethanol and 1 mL of water was used. The solution was heated to 75 °C and let to boil until the solution became clear. Unlike the previous benzoic acid solution, the phenanthrene solution expressed a light yellow color suggesting impurities within the compound. To remove the contaminants 0.10g of Norite (activated charcoal) chips were added. These helped bind to the impurities and fix the discoloration. The solution was then set on the benchtop until cooled to room temperature and then moved to ice; during this period, small crystals began to form on the side of the erlenmeyer flask. Contrary to the benzoic acid crystal formation, the phenanthrene was able to cool at a steady rate and the crystals appeared to be more pure and clear. Again vacuum filtration was used to isolate the crystals as done with benzoic acid. After given time to dry, both the benzoic acid and phenanthrene crystals were weighed. The purified benzoic acid recovered was 0.051g from the 0.103g initially, which produced a 49.51% recovery. The purified phenanthere recovered was 0.040g from 0.099g initially, which produced a 40.40% recovery.

0.051 g = 49.51 % 0.103 g 0.040 g percent recoverybenzoic acid =(100) = 40.40 % 0.099 g

percent recoverybenzoic acid =(100)

Product loss occurred due to experimenter error specifically when transferring the compounds to different containers and instruments. During the boiling stage, both benzoic acid and phenanthrene solution was spilled onto the hot plate along with undissolved solid sticking to the side of the flask and being discarded into the dirty glassware area. With vacuum filtration, product was lost in the solution filtrate because some sample passed through the seam of the filter paper and funnel into the side-arm flask below. Crystal loss also occurred when the crystals were moved from the Buchner funnel to the weigh

boat. A third of the crystals spilled off the filter paper when being scraped off. All of these factors were large contributors to the low percent recovery. Compound

Initial Amount (g)

Isolated amount (g)

Percent Recovery (%)

Melting Point (°C)

NMR Impurities

*Crude Benzoic Acid

0.255

0.224

87.84

121.1

CDCl3 Acetone

Purified Benzoic Acid

0.103

0.051

49.51

123.6

CDCl3 Acetone

*Crude Phenanthrene

0.255

0.300

117.64

100.3

CDCl3 Acetone Dichloromethane

Purified Phenanthrene

0.099

0.040

40.40

104.8

CDCl3 Acetone

FIGURE 2. ISOLATE WEIGHT AND PURITY DATA. *Crude benzoic acid and phenanthrene data obtained from Experiment 6: Extraction data.

The purified samples were analyzed by NMR and melting point to determine purity, as done with the crude samples in experiment 6. The observed melting point of the purified benzoic acid was 123.6 °C. In comparison to the crude benzoic acid, the purified sample had a 2°C higher melting point, however, both data points were in the literature range suggesting that each were fairly pure. The crystalized benzoic acid sample as well had an NMR spectra very similar to the crude sample, with only chloroform and acetone as the impure peaks (figure 3 and 4). This chloroform and acetone contamination were negligible in respect to the purity of the crystals because those chemicals are involved in preparing the NMR spectra and were added to the compound after recrystallization. Thus considering both melting points and spectra, the isolated benzoic acid contained no impurities. The cooling mistake made in the beginning of the experiment did not appear to have an effect on the purity of the benzoic acid as initially expected. The observed melting point of the crystallized phenanthrene was observed to be 104.8 °C, which was outside the literature range by 2°C and higher than the crude melting point by 3°C. This data suggests that there was some discrepancy between the purified and crude sample. Comparison of the NMR spectras supports the difference in molecular purity of the samples. Both phenanthrene samples contain the chloroform and acetone, which are negligible due to their involvement in NMR preparation, but dichloromethane only appears in the NMR of the crude phenanthrene at 5.30 ppm (figure 5 and 6). This shows how the recrystallization process removed the dichloromethane contaminant from the original organic solution and lead to the development of pure phenanthrene crystals. The main errors in this experiment occurred due to the experimenter and impatient practices. Specifically, when moving solutions from flask to flask, product was lost due to quick, inaccurate movements. More issues arose with the preparation of the NMR tubes when the glassware used was not given ample time to dry before used. This error was a main cause of the acetone impurities in the NRM spectra because it was used as a cleaning agent for the glassware. To improve results in future purifications, all glassware will be properly cleaned and given time to dry to avoid contamination of the compounds of interest. To prevent experimenter error and large product loss, all precautionary measures will be taken when transferring solutions. This includes the use of funnels for each movement of liquids and not discarding possible solute left over in the flask or beaker. Aside from the human error, the method of recrystallization was very effective at purifying the crude benzoic acid and phenanthrene.

FIGURE 3. CRUDE BENZOIC ACID NMR.

FIGURE 4. RECRYSTALLIZED BENZOIC ACID NMR.

FIGURE 5. CRUDE PHENANTHRENE NMR.

FIGURE 6. RECRYSTALLIZED PHENANTHRENE NMR....