1 Fischer Esterification Written by: Connor Lipscomb Reviewed by: Edited by: March 28, 2020 CH 238-8Q Introduction The purpose of the lab was to successfully identify an unknown alcohol through NMR analysis of a product obtained through Fischer esterification. Fischer esterification an organic reaction in which a carboxylic acid is converted into an ester. This reaction is done in the presence of alcohol and an acid., and its general form is illustrated in Figure 2. 2 Figure 1 - Fischer Esterification General Reaction The general mechanism for this reaction is illustrated by Figure 2. Figure 2 - Fischer Esterification General Mechanism Due to the reversible nature of this reaction, Le Chatelier’s Principle can be applied in order to maximize the results. For the purpose of this experiment, the reaction is going in the forward direction, and it therefore requires an excess of one of the reagents in order to reach equilibrium. In order to accomplish this, carboxylic acid is used in excess and the reagents 3 undergo a reflux. An NMR spectrum is also acquired for the final product and is used to identify the unknown alcohol that is used at the start. The reagents for this reaction can be seen in Table 1. Table 1 - Table of Reagents Compound MW (g/mol) BP (℃) MP (℃) D (g/cm3) Amyl alcohol 89.14 138 49 0.815 Isoamyl alcohol 88.15 131.1 -117.2 0.81 Propyl alcohol 60.1 97.2 -127 0.803 Butyl alcohol 74.12 117.7 -89.8 0.81 Isobutyl alcohol 74.12 107.8 -108 0.802 acid 60.05 117.9 16.6 1.051 H2SO4 98.08 337 10.31 1.8302 NaHCO3 84.007 851 50 2.2 Na2SO4 142.04 1429 884 2.671 Glacial acetic Experimental To begin the procedure, a sand bath was plugged into a Variac and placed onto a stirring hot plate. 0.7 mL of unknown #3 were added to a 5 mL round bottom flask. This was followed by 1.4 mL of glacial acetic acid as well as 3 drops of concentrated H2SO4. A small spin bar was also added to the flask. The round bottom flask was fitted with a straight tube and placed into the sand bath. The straight tube was then secured to a ring stand and wrapped with a wet paper towel. Once the reflux apparatus was successfully established, the reaction could begin. 4 The voltage of the Variac was increased until the temperature of the sand bath measured 155℃. As the solution began to boil, the mixture began to condense back down into the flask. To make sure this process of condensation continued, the paper towel wrapped around the tube was periodically changed out for a fresh, moist one. The process of refluxing the mixture continued for 65 minutes. Once the time was up, the round bottom flask was removed from the sand bath and given time to cool to room temperature. After sufficiently cooling, its contents were transferred to a conical tube via pipette. 1.0 mL of 5% NaHCO3 was also added to this tube, and its were mixed until the bubbling stopped. The aqueous layer was then removed, and the process repeated twice more, each with the addition of 1.0 mL of 5% NaHCO3. A pipette was then used to transfer the organic layer to a clean sample vial. A small amount of Na2SO4 was used to dry the solution, and the vial was dried for five minutes. Once all water had been removed from the solution, it was submitted for NMR analysis. Results Once the reaction was successfully completed, the product was dried and submitted for IR and NMR analysis. The results of these two analyses are found in Figure 3 and Figure 4 respectively. 5 Figure 3 - IR Spectrum of Unknown #3 The first peak on the IR spectrum in Figure 4 is a strong peak at roughly 2950 cm-1. This indicates the presence of sp3 hybridized carbon hydrogen bonds that are present in nearly all organic compounds. Another strong peak occurs at 1750 cm-1, indicating the presence of an ester. These are the only two peaks of interest. 6 Figure 4 - NMR Spectrum of Unknown #3 There are four primary peaks across the NMR spectrum, disregarding the standard peak at 0.0 ppm. At 4.0 ppm, there is a 2H triplet, and this is peak is the furthest downfield. Next comes a 3H singlet, followed by a 2H sextet. These occur at 2.0 ppm and 1.7 ppm respectively. Finally, there is a 3H triplet at 1.0 ppm, and this finishes the spectrum as it is the furthest upfield. Thanks to the data provided by the two spectrums, the final product of the reaction could be drawn as illustrated in Figure 5. Figure 5 - Final Product, Propyl Ethanoate Discussion Le Chatelier’s principle was the primary factor that needed to be taken into account when the Fischer esterification reaction was performed. As the yield of product was an acceptable amount for NMR analysis, it is clear that Le Chatelier’s principle was illustrated perfectly by this reaction. Because one of the reactants, the carboxylic acid, was added in excess, the equilibrium shifted towards the products. This allowed a suitable amount of product to be synthesized in order to analyze it via NMR and IR. 7 The IR and NMR data in Figure 3 and Figure 4 respectively were critical in identifying the product found in Figure 5. The first peak in the IR spectrum at 2950 cm-1 indicated the presence of the four sp3 hybridized carbons that are present on the molecule, while the second peak at 1750 cm-1 was indicative of the ester that was present on the molecule. Once the identity of the specific atoms on the molecule were discovered, the NMR spectrum was then used to identify their relative positions and number of hydrogens. The 3H singlet that appears at 2.0 ppm was identified as the lone methyl group attached to the carbonyl carbon, as it was relatively downfield compared to the other molecules. The 2H triplet that was furthest downfield at 4.0 ppm was directly attached to the oxygen molecule of the ester. This made finding where the other two sp3 hybridized carbons were placed on the molecule simply a matter of their position on the NMR spectrum. The 2H sextet found at 1.7 ppm was attached to the CH2 group found at 1.7 ppm. The 3H triplet was the furthest up-field on the spectrum and was therefore the furthest away from the ester group and at the end of the propyl chain. Once the molecule was identified, finding the identity of unknown #3 was simply a matter of working backwards. Using the general reaction and mechanism from Figure 1 and Figure 2 respectively, the identify of the unknown alcohol was simply the chain attached to the non-carbonyl oxygen with an -OH group replacing the ester. This means that the identity of unknown #3 was propyl alcohol, also called propanol. Therefore, the results of this reaction support what should have occurred: Fischer esterification of propyl alcohol. Conclusion In closing, the primary purpose of this lab was achieved with quality results. The goal of this experiment was to properly determine the identity of an unknown alcohol through Fischer esterification and NMR analysis. The Fisher esterification of unknown #3 yielded propyl 8 ethanoate, according to the results of the NMR and IR analyses. By backtracking, the identity of unknown #3 was determined to be propyl alcohol. Therefore, the primary goals of this experiment were indeed completed. 9 References 1. Amyl alcohol-1-13C. https://pubchem.ncbi.nlm.nih.gov/compound/Amyl-alcohol-1-13C (accessed March 24, 2021). 2. Isoamyl alcohol. https://pubchem.ncbi.nlm.nih.gov/compound/Isoamyl-alcohol (accessed March 24, 2021). 3. Propanol. https://pubchem.ncbi.nlm.nih.gov/compound/Propanol (accessed March 24, 2021). 4. 1-Butanol. https://pubchem.ncbi.nlm.nih.gov/compound/263 (accessed March 24, 2021). 5. Isobutanol. https://pubchem.ncbi.nlm.nih.gov/compound/Isobutanol (accessed March 24, 2021). 6. Acetic acid. https://pubchem.ncbi.nlm.nih.gov/compound/Acetic-acid (accessed March 24, 2021). 7. Sulfuric acid. https://pubchem.ncbi.nlm.nih.gov/compound/Sulfuric-acid (accessed March 24, 2021). 8. Sodium bicarbonate. https://pubchem.ncbi.nlm.nih.gov/compound/Sodium-bicarbonate (accessed March 24, 2021). 9. Sodium sulfate. https://pubchem.ncbi.nlm.nih.gov/compound/Sodium-sulfate (accessed March 24, 2021).
