Acid-Base Liquid-Liquid Extraction

Acid-Base Liquid-Liquid Extraction Introduction: A common technique practiced in organic chemistry is liquid-liquid extraction, which is used to separate organic or aqueous compounds from a mixture of compounds. Liquid-liquid extractions work based on three principles: molecule polarity and intermolecular forces, “like-dissolves-like”, and density1 . Substances that have similar polarities, or the presence of or lack of electronegativity, and intermolecular forces will typically dissolve into each other when mixed, such as when mixing two polar or two nonpolar substances together1 . Additionally, compounds with dipole-dipole interactions will have intermediate polarity and therefore dissolve in either polar or nonpolar solvents1 . Density is used to determine the position of the separate layers in liquid-liquid extraction1 . The solution with a greater density will flow to be the bottom layer while the solution with less density will be lighter and flow up to be the top layer1 . Acid-Base extraction is a specific type of liquid-liquid extraction used to separate liquids based on their acid-base properties. In acid-base extraction, either a Brønsted-Lowry acid or base is added to a solution to make at least one of the components charged, allowing it to separate from the organic solution into a separate aqueous layer solution2 . Brønsted-Lowry acids are substances that donate protons, or H+ ions, to Brønsted-Lowry bases, which are compounds that accept protons2 . Once the Brønsted-Lowry acid has donated its proton, it becomes a conjugate base and becomes more negatively charged2 . The Brønsted-Lowry base will accept the proton and therefore become more positively charged2 . The proton that is donated usually is a hydrogen atom that is attached to a highly electronegative atom. This addition of a charge to compounds allows them to utilize the “like dissolves like '' rule and become more polar, and therefore dissolve in polar, aqueous based solvents, separating them from organic layers2 . Knowing the concept of how Brønsted-Lowry acid-base reactions work, acid-base liquid-liquid extractions utilize the changes in charge to dissolve the newly charged compound from the original organic solution into a polar solution and separating the two compounds. In this experiment, the acid-base liquid-liquid extraction technique was practiced and then used to separate a mixture of two organic compounds by adding a strong Brønsted-Lowry base and acid to charge a compound in the mixture, and to later bring it back to neutralization once the solutions are separated2 . To understand how the liquid-liquid extraction process worked, the properties of Sudan Orange and Sudan Blue were investigated and a mixture of the two dyes in a hexane solution was separated with Brønsted Lowry acids and bases and liquid-liquid extraction with a separatory funnel. After practicing the extracting technique, the following week, a mixture of two organic compounds dissolved in diethyl ether was separated using a separatory funnel, NaOH, and HCl to deprotonate and neutralize the acidic compound to separate it and solidify it out of the solution. Once separated, each of the compounds were filtered and solidified using suction filtration, ‘drying’ with sodium sulfate, and evaporation techniques2 . Percent yield calculations were also used to compare amounts of the compounds obtained in the lab to the amounts expected. Finally, melting point determination with a melting point apparatus was used to assess the purity of the solid compounds obtained in the lab by comparing their melting points to the literature values of the original compounds (Table 1). Table 1: Reagents 3,4 Compound Amount to be Used (g) Amount Used (g) Melting Point (℃) Boiling Point (℃) Density (g/mL) Molecular Weight (g/mol) Hexane ~37 ~37 -95.3 68.7 0.66 86.18 HCl 4 or 49 4 -114.2 -85.1 1.036 36.46 Water 4 or 49 4 0.0 100.0 1.00 18.02 NaOH 4 or 49 49 323.0 1388.0 1.083 40.00 Sudan Orange ~31 ~31 145.0 407.5 1.24 214.22 Sudan Blue ~31 ~31 121.0 568.7 1.12 294.35 0.15 0.15 55.0-58.0 182.0 1.30 195.053 0.15 0.15 242.0 N/A 1.60 197.119 Experimental4,5: In week one of the experiment, three test tubes were obtained. In each test tube, 2 milliliters of sudan orange were added. Then, 2 milliliters of 1 M HCl were added to test tube 1, 2 milliliters of distilled water were added to test tube 2, and 2 milliliters of 1 M NaOH were added to test tube 3. The initial locations of the hexane standards and aqueous layers were noted. Next. the tubes were capped and shaken and the solutions inside were allowed to separate again. The final locations of the layers were noted in Table 2. Next, the same procedure was repeated with sudan blue. NaOH, was determined to be the best solution for separation. Next, 25 milliliters of a standard mixture of solution made of sudan blue and sudan orange dissolved in hexane was obtained along with 45 milliliters of the selected solution chosen earlier, NaOH. With the stopcock closed, 25 milliliters of the green dye solution was added to a 125 milliliter separatory funnel followed by 15 milliliters of the NaOH. The funnel was inverted and swirled to mix the solutions with the stopper in place. The solution was then vented by holding the stopper in place with the funnel upside down and opening the stopcock to release the built up air. This process was repeated three times to mix the solutions. Next the funnel was secured onto a ring stand and placed into an iron ring and the solutions were allowed to separate. A clean beaker was then placed below the funnel, the stopper was removed, and the stopcock was opened to drain out the bottom layer of the mixture. The draining was stopped before any of the first layer was allowed to drain. The colors of the two separated layers were then examined, compared to the standard dyes, and noted. The entire extraction process was repeated two more times, each time with 15 milliliters of NaOH added and each was drained into the same beaker as the first extraction. The colors of the solutions were examined and noted at the end of each extraction. During the second week of this experiment, the structures of the components of a mixture were analyzed and determined as either neutral, acidic, or basic. 15.0 milliliters of the mixture was obtained and placed into a 125 milliliter separatory funnel, along with 10 milliliters of a selected aqueous solution chosen based on the structure of the components of the mixture. In this experiment, NaOH was selected. The mixture was then extracted three times according to the procedure from week one of the experiment, and the bottom aqueous layer was collected into a 200 milliliter beaker. For the next part of the second week’s experiment, the aqueous solution obtained in the 200 milliliter beaker was neutralized with HCl. HCl was added to the solution until a constant precipitant was formed and litmus paper indicated the solution to be acidic. Isolation of the solid was then performed through suction filtration. The solid was then dried by pulling air through the suction funnel for a few minutes. Once dry, the solid was then collected, weighed, and retained for later. Next, the organic top layer was placed into a 50 milliliter Erlenmeyer flask. To ‘dry’ the organic solution, anhydrous sodium sulfate was added to the flask until it formed a free flowing clump at the bottom of the flask. The remaining liquid was then transferred to a 50 milliliter beaker. The beaker was then placed into a 45 ℃ warm water bath, and the ether solution was allowed to boil until the liquid was completely evaporated. The beaker was then removed from the bath and allowed to cool to form solid crystals. These crystals were then obtained, weighed and retained. For both the solid obtained from the aqueous layer and the solid obtained from the organic ether solution layer, the percent yield was calculated. For the final part of the experiment, using a melting point apparatus, the melting points of each solid were determined. The plateau was set to 20 ℃ below the literature melting point for each melting point determination and the ramp was set to 2 ℃. The experimental melting point was compared to the literature point. Results: Table 2.  Locations of Hexane Layers When Mixed with Aqueous Solutions Compound 1 M HCl (Aqueous) Distilled Water (Aqueous) 1 M NaOH (Aqueous) Sudan Orange (Hexane) Bottom (slightly miscible) Top (slightly miscible) Bottom (distinct) Sudan Blue (Hexane) Completely Miscible Top (slightly miscible) Top (distinct) The hexane solutions were mixed with three different aqueous solutions to determine which solution allowed for the clearest separation and would provide the clearest distinction between the two dyes when separated. Table 3: Extraction Observations During Week One Extraction Number Top Solution Bottom Solution 1 Light Blue Orange 2 Blue Orange 3 Dark Blue Orange After each extraction, the colors of the separated layers were noted and compared to the original sudan orange and sudan blue solutions. For part one of the week one experiment, it was found that 1 M NaOH allowed for the best separation and clearest distinction between the two dyes, since it allowed sudan orange to be the top layer while allowing sudan blue to be the bottom layer (Table 2). For this reason, NaOH was selected to be the aqueous solution to be used in the extraction procedure. For the second part of the week one experiment, clear separation was found between the two dyes. With each extraction, the top dye solution became closer to the original unmixed color for sudan blue while the bottom layer remained orange in the receiving beaker. This allowed the top layer to be determined as sudan blue while the top layer was determined to be sudan orange. Compound 1 Compound 2 Figure 1: 10 g of each of these two compounds were dissolved per 1 L of diethyl ether Figure 2: Deprotonated Molecule present in the aqueous layer during extraction. 1L Grams of Each Solid in Solution= (15 mL)( 1000 mL )(10 g 1L )= 0.15 g of compound Eq. 1 Percent Yield= (grams obtained/0.15 g original)*100 Eq. 2 Table 4: Data for Solid Compounds Obtained Compound Solid Mass Obtained (g) Percent Yield (%) Melting Point Range (℃) 1 0.11 73.33% 55.9-59.0 2 0.043 28.67% 242.6-242.9 For each solid compound obtained from the organic and aqueous layers, the mass was measured to allow for a percent yield calculation (Eq. 2) from the original amount of each in the solution (Eq. 1). The melting point was also measured to assess the purity of each compound. In the week two experiment, compound two was determined to be a Brønsted-Lowry acid due to the donatable hydrogen, while compound one was determined to be neutral due to it not having a donatable hydrogen, or an area where one could be readily accepted (Figure 1). Compound two’s properties determined NaOH to be the best solution used for deprotonation. HCl was chosen as the neutralizing solution for the aqueous layer due to the presence of a negative oxygen, which can readily attach a hydrogen (Figure 2). In the second part of the week two experiment, there was 0.11 g of compound one obtained; a 73.33% yield from the original 0.15 g in solution (Eq. 1, Table 4). There was 0.043 g of compound two obtained; a 28.67% yield from the original 0.15 g in solution (Eq. 1, Table 4). The melting point range of compound one was 55.9℃-59.0℃, while the melting point range of compound two was 242.6℃-242.9℃(Table 4). This allowed both compounds to be determined as relatively pure based on their close proximity to their literature melting points (Table 1). Discussion: In this experiment, acid-base liquid-liquid extraction was used to separate and determine the properties of different mixtures. There are three principles that liquid-liquid extraction relies on: molecular polarity and intermolecular forces, “like-dissolves-like”, and density1. In the first week, NaOH was determined to be the best solution for separation based on its polar properties. NaOH and HCl are relatively polar but NaOH readily accepts H+ protons while HCl donates protons. This solution provided the best division for the dye extraction because of the polar forces in NaOH and the dyes. The mixture produced an aqueous layer and organic layer with the denser layer being at the bottom. In the second week, the acid base properties were examined. Compound one was determined to be the less dense, organic layer and compound two was determined to be the aqueous layer that was extracted first. The given mixture had an acid therefore NaOH was utilized to neutralize the acid. The mixtures were then extracted to separate the compounds in the mixture. The neutral solution dissolved into the diethyl ether organic layer and the acid dissolved into the polar aqueous layer. After the solutions were solidified, the compounds melting point was taken to confirm the identities of the mixture. The experimental melting point of compound one was 55.9℃-59.0℃(Table 4) which was close to its theoretical melting point of 55℃-58℃. It was determined that the extracted compound one was very pure since its experimental melting point was close to the theoretical melting point. The experiment melting point of compound two was 242.6℃-242.9℃(Table 4) which was close to its theoretical melting point of 242℃ so it was determined to be very pure. Possible sources of error include not allowing the solution to dry completely in the extraction. This could influence the amount of material collected and the overall quality. Another source of error could have been choosing the wrong solution to neutralize the acid or base. Conclusion: This experiment determined that liquid-liquid extraction is a useful method to separate acid base solutions and determine distinct properties of the mixture. This lab demonstrated how like dissolves into like which demonstrate the relative polarity of molecules. This lab also demonstrated how density affects the extraction. The layer that was denser dispensed first since that layer was the bottom layer. The lab also demonstrated how Bronsted-Lowry Acid and Bases would react in organic and aqueous layers. Liquid-liquid extraction can be used in other fields, such as forensic science6 , to separate and identify unknowns, but in this experiment, this liquid-liquid extraction was used to separate compounds and test the purity of each of the separated compounds. Purity was measured by drying the compounds and testing their melting points using a melting apparatus. To avoid error, the procedure should include a detailed protocol for drying the compounds completely. Drying methods include allowing the compound to boil off solvent longer, run the suction funnel longer, or using filter paper to compress and absorb excess solvent.