Liquid/Liquid Extraction of an Unknown Mixture
Nhi “Titi” Tran
CHEM 213W Section 004
Purpose
Liquid-liquid extraction is an essential technique used in organic chemistry to separate products from a reaction mixture by exploiting its various properties, such as its acidity. Overall, this then allows organic chemists to obtain a purer product. For the purpose of this experiment, liquid-liquid extraction was utilized to isolate a neutral, amine, and carboxylic acid component from an unknown mixture in order to determine their identities by performing melting point and 1H NMR analyses on each individual element.
Results, Discussion, and Conclusions
Unknown sample 207 was analyzed in this experiment. Using acid/base chemistry, certain components were moved to the aqueous layer at each step in order to separate the amine, carboxylic acid, and neutral parts from each other. First, the basic amine was protonated with 5% HCl, a strong acid. This protonation step led to the ionization of the amine component, causing it to migrate into the aqueous layer where it was more soluble.1 This left only the neutral and carboxylic acid components in the organic layer since both do not react with the acid, and thus remained uncharged. Then, the addition of 1M NaOH, a strong base, resulted in the carboxylic acid being deprotonated.1 This ionized form of the acid was then soluble in the aqueous layer.1 Since the NaOH does not react with the neutral component, this part remained in the organic layer, and was further dried using anhydrous sodium sulfate.
For the amine component, the melting point ranged from 103.8-105.1°C. Out of the two possibilities, this melting point range correlated to 4’-aminoacetophenone, since that compound’s accepted melting point is 103-107°C.2 Because the experimental melting point was much higher than that of 3’-aminoacetophenone (94-98°C), the basic component could not have been that compound.3 The narrow melting point range also indicates that there was little, if any, contamination. Thus, 1H NMR was not performed for the amine component. The percent recovery of the 4’-aminoacetophenone was 55.265%. The remaining amount that was not recovered may have been leftover in the first test tube with the neutral component if not all of the amine was pipetted out in that step. Another possibility for the imperfect percent yield may have been a result of not fully precipitating out the amine when 1M NaOH was added. Despite the litmus test showing that the solution was basic, if not all of the protonated amine was deprotonated by the base, then some of it might have remained in the aqueous solution in the charged form and discarded as the filtrate.
For the carboxylic acid component, the identity of this compound was determined to be 3-methoxybenzoic acid based off of the observed melting point, as well as from evidence derived from 1H NMR analysis. The melting point obtained for the compound was 105.6-106.2°C, which fell in the actual melting point range (105-107°C) of 3-methoxybenzoic acid.4 The 1H NMR data (Figure 1, Supplemental Information) showed multiplet peaks at 7.08-7.80 ppm with an integration of 4.56, all of which were representative of the four aromatic hydrogen’s in 3-methoxybenzoic acid. The slightly larger integration value may have resulted from contamination with other two components, which all contained aromatic hydrogen’s as well. More evidence of contamination was seen with the small peaks at 1.25 and 2.60 ppm, which may have been traces of the ether solvent (used during extraction) and residual amine/neutral components, respectively.1 The singlet at 11.0 ppm was consistent with the hydrogen bonded to the carboxyl group in the compound. The integration for this peak (0.57) was a little lower than the expected number because hydrogen-deuterium exchanges may have occurred between the carboxylic acid hydrogen and the deuterated NMR solvent, CDCl3.5 The main distinction between benzoic acid and 3-methoxybenzoic acid would be the presence of a singlet at 3.2-3.8 ppm that had an integration of three. Since the spectrum displayed this peak at 3.82, it indicated that the unknown did contain the methoxy group. Overall, 51.317% of the acid component was recovered. Similar to the amine component, the rest of the carboxylic acid may have been lost if it was not fully precipitated when concentrated HCl was added, or if not all of the ionized acidic component was extracted from the organic layer in the initial steps of the extraction.
With the neutral component, the melting point range observed was 50.9-52°C. This experimental melting point was most consistent with the reported melting point of 4-acetylbenzonitrile (56-59°C).6 Since the observed melting point was depressed, this was characteristic of contamination.1 The source of contamination is likely from either the acidic or basic components of the original mixture if they were not fully separated in the previous steps, or from leftover anhydrous sodium sulfate that may have been accidentally transferred to the watch glass along with the neutral component. When looking at the 1H NMR data (Figure 2, Supplemental Information), the two doublets, both with an integration of two, at 7.78 and 8.06 ppm corresponded to the two pairs of aromatic hydrogen’s in the para-substituted benzene ring of 4-acetylbenzonitrile. The total aromatic hydrogens indicated from the integration of the doublets confirmed that the compound could not have been fluorenone or else the spectrum would have shown an integration value closer to eight. In addition, because there was a singlet methyl peak at 2.65 ppm that was distinguishing for a ketone group, the neutral must have been 4-acetylbenzonitrile rather than fluorenone. As mentioned with the depressed melting point, there was most likely contamination with the neutral component. This was further confirmed when additional peaks at 1.15, 1.67, and 6.64 ppm were observed, which may have resulted from leftover amine and carboxylic acid. Additionally, residual peaks for acetone and the CDCl3 NMR solvent were present at 2.17 and 7.26 ppm, respectively.1 The neutral component’s percent recovery was 64.147%. Some of the neutral may have been combined with the acidic or basic component if part of the organic layer was extracted along with the aqueous layers, resulting in a lower percent recovery.
To conclude, based off of the melting point and 1H NMR analyses completed, the three components extracted from mixture 207 was determined to be 4’-aminoacetophenone, 3-methoxybenzoic acid, and 4-acetylbenzonitrile. The main sources of error in this experiment, which led to contamination and low percent recoveries, were from imprecise pipetting techniques during the extraction phase and incomplete precipitation of the amine and carboxylic acid components. This experiment could be improved with better laboratory techniques when pipetting the aqueous layers to different test tubes. Being more thorough with the isolation steps would decrease the likelihood that any of the three components would be contaminated, preventing residual peaks and depressed melting points. In addition, the percent recoveries could be improved if there was a more precise way to measure the pH of the solutions. If the amine layer was not basic enough, or if the carboxylic acid solution was not acidic enough, some of those components could have remained as the ionized form in the aqueous layer and was lost during vacuum filtration. Therefore, the next time this experiment is carried out, numerous litmus tests could be performed to confirm the acidity/basicity of the solution before filtering out the crystals.
References
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(5) Reed, D. R.; Kass, S. Hydrogen–deuterium exchange at non-labile sites: A new reaction facet with broad implications for structural and dynamic determinations. Journal of the American Society for Mass Spectrometry, 2001, 12, 1163-1168.
(6) 4-Acetylbenzonitrile. SDS No. 154393 [Online]; Sigma-Aldrich: St. Louis, MO, June 27, 2014. http://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=US&language=en&productNumber=154393&brand=ALDRICH&PageToGoToURL=http%3A%2F%2Fwww.sigmaaldrich.com%2Fcatalog%2Fproduct%2Faldrich%2F154393%3Flang%3Den (accessed Sep 15, 2017).