Purpose
The purpose of this experiment was to perform a liquid/liquid extraction in order to separate an unknown neutral component, unknown amine component, and unknown carboxylic acid component from a given three-component mixture. Extractions are an invaluable technique used in organic chemistry in order to isolate a product, since products are typically contaminated by unreacted starting-materials and byproducts. Extraction utilizes the inherent physical and chemical properties of these molecules to separate them, making it very useful in organic synthesis. The separated components in this experiment will be characterized by 1H NMR spectroscopy and melting point comparisons1.
Results and discussion
In this experiment, Unknown #3 was separated using extraction and the components were analyzed using melting point comparisons and 1H NMR spectroscopy. The extraction technique, also known as the “work up” of a reaction, relies on polarity differences between organic molecules in order to separate them into either an aqueous layer or an organic layer. The order of the solvents in the tube is determined by comparing the density of the two layers. In this case, ether was used as the organic solvent, which is less dense that water, causing it to separate as a layer on top of water1. Considering the principle of “like-dissolves-like,” the aqueous layer should contain water-soluble molecules, such as inorganic salts and charged molecules, and the organic layer should contain neutral and water-insoluble molecules, such as the organic product, starting materials, and organic byproducts. The given three-component mixture consisted of a neutral component, an amine component, and a carboxylic acid component. Using acid/base chemistry, the amine and carboxylic acid functionalities can be altered by adding solvents that alter the pH1. When extracting the amine group, a strong acid, such as the 5% HCl used in this experiment, will cause protonation of the neutral amine and conversion into an ionic water-soluble form, which then moves to the aqueous layer and can be removed and neutralized. When extracting the carboxylic acid, the same result can be attained with the addition of a base that results in deprotonation of the carboxylic acid, such as the 10% sodium bicarbonate used in this experiment. The neutral component cannot be altered with the addition of aqueous solvents; therefore, it will always remain in the organic layer1.
The neutral component in Unknown #3 was identified as 1,4-dimethoxybenzene. The experimental melting point of the neutral unknown was measured as 47.8-51.0°C. When compared to known melting points of the two possible neutral components, the measured melting point was closest to 1,4-dimethoxybenzene, which has a known melting point range of 54-56°C2. Fluorenone has a know melting point range of 82-84°C, which is much higher than the measured melting point3. The identity of the neutral component was confirmed using 1H NMR spectroscopy. As seen in Figure 1, there is a singlet found at 3.775 ppm with an integration value of 6.00. This corresponds with the chemical shift for 6 chemically identical ether hydrogens. There is another singlet found at 7.253 ppm with an integration value of 3.65, indicating the presence of 4 chemically identical aromatic hydrogens. 1H NMR spectroscopic analysis indicates that there is a plane of symmetry in this molecule (Figure 1). Structural analysis of fluorenone reveals the presence of 4 chemically distinct hydrogens and a predicted chemical shift to the aromatic region of 6.5-8.0 ppm1. 1H NMR spectroscopy, however, revealed a chemical shift at 3.65 ppm and only 2 chemically distinct hydrogens, providing further evidence that the identity of the neutral component is 1,4-dimethoxybenzene (Figure 1). The experimental melting point appears to be slightly depressed, which indicates the presence of soluble impurities. Soluble impurities cause the melting point of organic compounds to depress and broaden, because the impurities disrupt the crystalline structure, thereby weakening the attractive forces that hold the crystals together1. Figure 1 also reveals the presence of impurities around 7-8.5 ppm, the aromatic region, 4.5-5 ppm, the amine region, and 2.5 ppm, the ester hydrogen region1. These impurities correspond with the presence of carboxylic acid and amine contamination in the neutral component most likely due to improper pipetting and separation of the layers. The percent recovery of 1,4-dimethoxybenzene was 10%, which is very low. During the mixing of 5% HCl into tube 1, many emulsions formed; therefore, these emulsions could have resulted in loss of the component while pipetting. For better result in the future, more precaution should be taken during pipetting in order to extract as much layer as possible without leaving behind impurities.
The amine component in Unknown #3 was identified as 3’-aminoacetophenone. The identity of the amine component was determined exclusively through melting point comparisons. The experimental melting point of the unknown amine component was measured as 95.0-97.9°C. This melting point is within the known range for 3’-aminoacetophenone, 94-98°C4. The known melting point range for 4’-aminoacetophenone is 103-107°C, providing support that the true identity of the unknown amine is 3’-aminoacetophenone5. The percent recovery of 3’-aminoacetophenone was calculated to be 8%, which is very low. This could have resulted from pipetting error in separating the aqueous layer from the organic layer. As stated above, the 1H NMR of the neutral component revealed some amine contamination due to insufficient extraction (Figure 1). Furthermore, insufficient addition of 50% potassium carbonate during the isolation step could have resulted in a large amount of 3’-aminoacetophenone remaining in solution. When this solution was filtered using vacuum filtration, the non-precipitated amine was filtered and discarded, resulting in low recovery. Adding more 50% potassium chloride in order to precipitate more of the amine can increase the percent recovery.
The carboxylic acid component in Unknown #3 was identified as benzoic acid. The experimental melting point of the carboxylic acid was measured as 122.4-123.2°C. This measured melting point corresponds to benzoic acid, with a known melting point of 121-125°C6. 3-toluic acid has a known melting point of 107-113°C, which is much lower than the measured melting point7. The identity of the carboxylic acid component was confirmed using 1H NMR spectroscopy. Figure 2 reveals a weak singlet at 9.062 ppm with an integration value of 1.00, corresponding to a carboxylic acid hydrogen. Furthermore, there is a group of hydrogens ranging from 7.570-8.067 ppm, indicating the presence of an aromatic ring. Figure 2 also reveals a large peak resulting from acetone contamination at 2.187 ppm, due to improper cleaning of the NMR tube, and a small peak at 7.270 ppm from CDCl3 contamination. In the future, allowing the NMR tube to dry completely will prevent significant acetone contamination. Further evidence of the identity of the carboxylic acid component is the lack of a peak corresponding to an aromatic methyl group at 2.3-2.7 ppm, a characteristic of 3-toluic acid1. The percent recovery of benzoic acid was determined to be 25%, which is low but higher than the other component’s percent recoveries. The low percent recovery is most likely due to the addition of concentrated HCl too quickly, which resulted in foaming. Furthermore, it is possible that not enough HCl was added to acidify and precipitate the benzoic acid, resulting in loss of benzoic acid during vacuum filtration. In the future, carefully adding HCl in order to isolate the carboxylic acid component and repeatedly stirring and checking the pH of the solution will prevent substantial product loss through vacuum filtration.
Conclusion
Overall, the three components in Unknown #3 were identified as 1,4-dimethoxybenzene, 3’-aminoacetophenone, and benzoic acid. There were many sources of error in this experiment that can be avoided in future extractions in order to obtain purer products. First, ensuring the NMR tubes are completely dry by leaving them in the drying oven for at least 20 minutes will prevent acetone contamination. Also, adding enough base or acid when isolating the amine or carboxylic acid, respectively, will ensure that the components fully precipitate and are not filtered out through vacuum filtration as part of the solution. This will improve the percent recovery of each component. Lastly, better pipetting technique is essential in a good extraction. Ensuring that there are no emulsions and that enough of the layer is pipetted out can help prevent contamination and ensure a higher percent recovery.