Lab 6: Extraction

Objective: During this lab will be learning how to perform extraction techniques. This process will also help us to practice the techniques of washing, adjusting pH, and testing the purity of compounds using TLC.

Compounds of Study:

Benzoic Acid

Melting Point: (122 C)

Pka: (4.2)

Image from TCI America

4-t-Butylphenol

Melting Point: (99.5 C)

Pka: (9.88)

Image from Sigma-Aldrich

Dimethoxybenzene

Melting Point: (54-56 C)

Pka: (-4.5)

Image from Fisher Scientific

Pre-Lab:

Outline the steps activity #1, you do not need to write complete sentences but simply outline what you should be doing first, then second, etc.

1.) Dissolve 2.5 mg of 1:1:1 mixture of benzoic acid in 30 ml of ether.

2.) Transfer the mixture into a separatory funnel.

3.) Add 10 ml of water and 10 mL of saturated sodium bicarbonate solution.

4.) Mix the layers.

5.) After the layers have separated, drain the aqueous layer into a beaker.

6.) Add an additional 10 mL of water and 10 mL of bicarbonate solution to the funnel.

7.) Mix again and after separation, drain the aqueous layer into the same beaker.

8.) Precipitate the carboxylate from the aqueous solution.

9.) Adjust the pH then do solid filtration to isolate.

10.) Extract the weak acid with 1.5 sodium hydroxide solution.

11.) Wash the solution with 10 ml water and 10 ml saturated salt solution.

12.) Repeat the same pH adjustment and isolated steps.

13.) Dry the ether using magnesium/sodium sulfate.

14.) Perform TLC in the next lab.

2. In your own words describe the difference between an extraction step and a washing step.

The extraction process removes a product from a solution while washing removes any impurities from a solution.

3. If the partition coefficient is a large number, does this mean that the compound is more soluble in water or more soluble in nonpolar organic?

Compounds with large partition coefficients indicate that a compound is more likely to be soluble in an organic solution.

4. To decide if it is better to extract multiple times with a small amount of solvent or extract once with a large amount of solvent considering the following scenario:

suppose a reaction mixture afforded 300 ml of an aqueous solution containing 30 g of malononitrile (CH2(CN)2) which is to be isolated by extraction with ether. The solubility of malononitrile in ether at room temperature is 20.0 g per 100 ml, and water is 13.3 g per 100 ml.

A. What is the partition coefficient of malononitrile? Take a picture of your calculation and upload the picture. Show all work.

See image below.

B. What weight of malononitrile would be recovered by extraction with (a) three 100 ml portions of ether; (b) one 300 ml portion of ether? Take a picture of your calculation and upload the picture. Show all work.

See image below.

C. Which is better, to extract several times with small amounts of solvent or one time with a large amount of solvent?

It is better to extract several times with small amounts of solvent.

5. Why is it not necessary to record the exact amount of the organic solvent used in activity #2 but it is important to record the exact amount of organic solvent used in activity #1?

Activity #1 has many compounds and data points involved that are required to keep track of, particularly to monitor the pH levels of the solution. However, activity #2 has fewer moving pieces it is not necessary to record the exact amount of organic solvent used.

6. The mixture contains two acids - why doesn’t the bicarbonate solution react with the weakly acidic phenol compound?

Phenol is a weaker acid than the bicarbonate solution and because of that, it is unable to pull protons away from the bicarbonate solution.

7. Why is it important to wash an organic layer with water just after washing it with a strongly basic aqueous solution?

To fully remove any impurities that could be lingering behind in the organic layer.

8. At the end of the lab, you will analyze your three solid products by TLC. Refer to lab #1 and explain how you prepare a solid sample for TLC analysis.

The solid sample must be be broken down so it can be mixed with a dilutant so it can be converted into liquid so it can be spotted onto a TLC plate for analysis.

Problem 4.a

Problem 4.b

Activity #1 - Procedure:

Experimental:

Measure out 2.5 grams of 1:1:1 mixture of benzoic acid/4-t-butylphenol and dimethoxybenzene.
Added approximately 30 mL of ether, 10 mL of sodium bicarbonate, and 10 mL of water to the solution.
Combined the solution in a separatory funnel.
The separatory funnel was inverted about 30 times, with the pressure being released every 4-5 inversion by removing the stock cap.
The solution was left to settle until distinct layers formed.
The bottom/aqueous layer was drained into an Erlenmeyer flask.
Another 10 mL of water and 10 mL of sodium bicarbonate was added into the separatory funnel.
The separatory funnel was inverted about 30 times, with the pressure being released every 4-5 inversion by removing the stock cap as before.
The aqueous layer was drained into the same flask as before.
The pH of the aqueous layer was tested. The pH was determined to be 5.
Approximately 22 drops of 6 M HCl was added to the aqueous solution to precipitate benzoic acid.
The pH of the solution with the 6 M HCL was tested again. The pH was determined to be 3.
The solution was then placed in an ice bath to further encourage the precipitation of benzoic acid crystals.
10 mL of sodium hydroxide solution was added to the organic layer that remained in the separatory funnel.
Drained aqueous layer into a flask.
The pH of the aqueous layer was tested. Added approximately 36 drops of HCL to get a pH of 2.
The inversion process described above was repeated with a wash of saturated salt solution to ensure complete removal of the aqueous layer with the 4-t-butylphenol from the organic layer.
Once crystals had formed for each solution the mass of each crystal was obtained.
Separated the ether layer out of the separatory funnel, before adding magnesium sodium sulfate in order to dry out the ether solution.
The Ether layer underwent gravity vacuum filtration while swirling over low heat to form the solid product of the neutral compound to remove magnesium sodium sulfate from the ether layer and promote the formation of crystals.
Once the strong and weak acids were cooled, both the crystals were removed from the ice and vacuum filtrated.
Covered all three crystal sets with a chem wipe and placed them in a drawer to dry over the next week.
The following week melting point was then determined for each substance to assess the purity of the compounds collected.
The crystals were then tested for purity using TLC using a solvent of 9ml hexane and 1 mL ethyl acetate.

Activity # 2 - Results:

Observations: Three beakers with distinct crystals were collected. In the strong acid beaker, a sample of white, shiny, flake-like crystals was collected, in the weak acid beaker, a sample of white, powder-like crystals was collected, and in the neutral beaker, a sample of large, dull, clear-colored crystals was collected. For both the strong and weak acids, a greater amount of crystals was precipitated after the products were set in an ice bath to cool.

Results:

Strong Acid, Benzoic Acid: 0.29 grams

Percent Recovered: (0.29g /2.5g) *100=11.6%

Starting melting at 123.0 Celcius
Stopped melting at 124.9 Celcius

Weak Acid, 4-t-Butylphenol: 0.63 grams

Percent Recovered: (0.63g /2.5g)*100= 25.2%

Starting melting at 89.7 Celcius
Stopped melting at 91.6 Celcius

Neutral, Dimexothybenzene: 0.41 grams

Percent Recovered: (0.41g /2.5g)*100=16.4%

Starting melting at 50.8 Celcius
Stopped melting at 52.3 Celcius

TLC:

1- Pure Benzoic Acid:
- RF Value: (2.3/5.5) = 0.42
2- Our Benzoic Acid:
- RF Value: (1.8/5.5) = 0.33
3- Pure Weak Acid 4-t-butyl-phenol:
- RF Value: (2.2/5.5) = 0.40
4-Our Weak Acid - 4-t-butyl-phenol:
- RF Value: (2.0/5.5) = 0.36
5-Pure Dimethoxybenzene:
- RF Value: (3.3/5.0) = 0.66
6- Our Dimethoxybenzene:

RF Value: (2.7/5.0) = 0.54

Activity #2 - Procedure:

Experimental:

Measured 0.5 g of benzoic acid, 8 mL of DI water, and 8 mL of dichloromethane in a centrifuge tube.
Thoroughly mixed the contents of the tube through inverting approximately five times.
Allowed the contents of the tube to separate into clearly defined layers.
Used a pipette to transfer each layer into three separate, pre-weighted, flasks.
Calculated the partition coefficient of benzoic acid.
Disposed of the contents of each flask in appropriate lab containers.

Activity #2 - Results

Results:

Parton Coefficient:

Mass of the pre-weighed flask: 95.1 grams

Mass of the flask and water: 102.9 grams

Concentration of water = 102.9-95.1 = 7.8 grams/8 mL= 0.98 g/mL

Mass of pre-weighed flask: 111.4

Mass of flask, dichloromethane, and benzoic acid: 120.8 grams

Concentration of dichloromethane = 120.8-111.4= 9.4 grams/8 mL= 1.17 g/mL

Partition coefficient = (g/mL of organic solvent)/(g/mL of water) = (1.17/0.98)=1.19

Discussion:

Through analyzing the masses of each of the chemicals collected in this experiment, it is easy to determine that not all of the compounds were purified in this experiment. The lowest yield for this experiment is 11.6% for the strong benzoic acid, and the highest yield is 25.2% for the acid 4-t-butylphenol. In a perfect experiment, the % yields would add up to 100%, demonstrating that all the compounds were effectively extracted. The data collected indicates that this extraction method for our experiment worked best for the weak acid, as it had the highest yield of the three compounds.

By examining the melting points, we can see that the substances that were obtained contained impurities. A fully pure substance has a melting point within 3°C of the literature value. The benzoic acid obtained in the experiment had a starting melting point of 123.0 °C and an end melting point of 124.9 °C, while its literature melting point is 122.3°C. The dimethoxybenzene compound had a starting melting point of 50.8°C and an ending melting point of 52.3 °C, with a literature value of 54–56 °C. Benzoic acid and dimethoxybenzene had accurate melting points within 3 degrees of their literature values. However, the extracted 4-t-butylphenol had a starting melting point of 89.7°C and an ending melting point of 91.6 °C, with a literature value of 99.5°C. 4-t-butylphenol was not within an accurate melting point range, which signifies that it is impure and wasn't properly extracted.

We also verified the purity of the compound in the lab using TLC. Looking at the TLC plates collected, they show a certain level of purity within the compounds as well as impurities that remained. Examining spots 1 and 2 for benzoic acid, the pure compound had an Rf of 0.42, while the experimental compounds had an Rf of 0.33. This indicates that some of the compound was extracted, but impurities remained. For 4-t-butylphenol, spots 3 and 4, the pure compound had an Rf of 0.40 and an experimental Rf of 0.36. Looking at the Rf value and the spots, some of the compounds were accurately isolated, but a large impurity remained, which is indicated by the presence of two distinct spots on the plate. Lastly, dimethoxybenzene, spots 5 and 6, had a pure Rf of 0.66 and an experimental Rf of 0.54. According to these numbers and the various spots on the experimental line of the TLC plate, some of the pure compound was isolated, but many impurities remained. It is possible that during the drying out of the ether layer, not enough magnesium sodium sulfate was used to remove any water-soluble impurities, lending the TLC plate to indicate impurities found within our compound.

For the second activity of this experiment, the patron coefficient for benzoic acid was determined to be 1.19. This reflects the solubility of the substance in polar vs. non-polar compounds. We know that benzoic acid is more soluble in an organic solvent, which means its coefficient should be greater than 1, which is reflected in the data that was collected in our experiment today.

Conclusion: The extraction procedure performed in this experiment proved to be somewhat effective as some amounts of the desired chemicals were extracted from the compound with some impurities remaining in the final product.

Reflection: In this lab, I learned how to separate the components of a compound by extraction. I now have a better understanding of yet another way that chemicals can be separated from each other. In this lab, I was able to continue to practice identifying melting points as well as TLC. One thing that I would do differently if I were to do this lab again would more carefully wash each substance to ensure a higher yield of each substance and a more accurate purity of the given chemicals after extraction. This could be achieved through careful attention to each step in the washing process and adding additional washing steps in my laboratory procedure.

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