Lab Report Nitration of Methylbenzoate

Nitration of Methyl Benzoate Experiment 11 Introduction: Nitration of a benzene ring involves the addition of a nitro group. The electrophile in this reaction is the nitronium ion.1 This nitronium ion is created by reacting nitric acid with sulfuric acid. There are two steps to this process. First, a proton from the sulfuric acid is added to the —OH group on the nitric acid. This forms water, which is a good leaving group. Water leaves as the oxygen with a negative charge forms a double bond with oxygen. Thus forming the nitronium ion. This mechanism is shown in figure 1. Nitration is important synthetically because it is one of the best ways to add an amino group to a benzene ring. This can be done by reducing the nitro group into an amino group through the process of hydrogenation.1 In this experiment, methyl benzoate is reacted with the nitronium ion to form methyl nitrobenzoate. In the first step, the benzene ring acts as a nucleophile and attacks the nitrogen in the nitronium ion. This leaves a positive charge on the benzene ring. Then sulfuric acid removes the hydrogen from the same carbon that the nitronium ion is bonded to. The electrons from the hydrogen bond are moved into the benzene ring, which reforms the conjugated system. In this reaction, the nitronium ion can be added to three different locations on the benzene ring: meta, ortho, and para. The meta addition is shown in figure 2. The ortho addition is shown in figure 3. The para addition is shown in figure 4. All chemicals used in this experiment are shown in table 1. Figure 1: Figure 1 shows the mechanism for the formation of the nitronium ion from the reagents nitric acid and sulfuric acid. ! Figure 2: Figure 2 shows the mechanism for the formation of methyl m-nitrobenzoate from the reactants methyl benzoate and nitronium ion. Figure 3: Figure 3 shows the mechanism for the formation of methyl o-nitrobenzoate from the reactants methyl benzoate and nitronium ion. Figure 4: Figure 4 shows the mechanism for the formation of methyl p-nitrobenzoate from the reactants methyl benzoate and nitronium ion. Table 1: Table of Reagents2 Compound Molecular Weight (g/mol) Boiling Point (°C) Melting Point (°C) Density (g/ cm3) 32.04 64.7 -97.8 0.79 methyl benzoate 136.15 199.0 -12.4 1.09 methyl m-nitrobenzoate 181.15 — 78.0-80.0 — methyl o-nitrobenzoate 181.15 — -13.0 — methyl p-nitrobenzoate 181.15 — 94.0-96.0 — nitric acid 63.01 83.0 -41.6 1.51 nitronium ion 46.01 — -9.3 — sulfuric acid 98.07 337.0 10.3 1.83 water 18.02 100.0 0.0 1.00 methanol Experimental: In this experiment, 0.6 mL of sulfuric acid and 0.3 mL of methyl benzoate were added to a 10 mL flask, then put into an ice bath. Then, in a separate vial, 0.2 mL of nitric acid and 0.2 mL of concentrated sulfuric acid were mixed together. The first flask was then transferred to a cold water bath, and the acidic mixture was added dropwise to the flask. The solution was allowed to sit at room temperature for 15 minutes. Next, 2 small chunks of ice were added to the flask and the solution was mixed until the ice melted. Then, 2 more chunks of ice were added and the process was repeated. A precipitate was noted at this step. Next, the solution was poured into a suction filtration apparatus. The precipitate was washed with 1 mL of water and 2 mL of methanol. Then, the white powder product was transferred to a pre-weighed flask and weighed on a scale. The percent yield was calculated. Then, melting point was taken and a sample was submitted for NMR analysis. Results: In this reaction, methyl benzoate and nitric acid were reacted to form methyl nitrobenzoate. The percent yield was calculated for the product. First the limiting reagent was determined, which was methyl benzoate. Then the theoretical yield was calculated. This is shown in equation 1. Next, the actual yield was calculated. This is shown as equation 2. Finally, the percent yield was calculated to be 18.32%. This is shown as equation 3. Next, the melting point was taken on the final product. The melting point was a range from 79.3°C to 83.1°C. The melting point data, along with the rest of the data collected for the product, are shown in table 2. Finally, proton NMR analysis was done on the final product. The proton NMR spectrum is shown in figure 5. The analysis of the proton NMR spectrum, including multiplicity, integration, and chemical shift, is shown as table 3. Equation 1: Equation 1 shows the two calculations to determine the limiting reagent. Methyl benzoate is the limiting reagent, and 2.20x10-3 mol is the theoretical yield. Equation 2: Equation 2 shows the calculation for the actual yield, which is 4.03x10-4. Equation 3: Equation 3 shows the formula and calculation for percent yield which is 18.32%. Table 2: Data for Methyl Nitrobenzoate Percent Yield (%) Melting Point (°C) Appearance 18.32 79.3-83.1 crystal white solid Figure 5: Figure 5 shows the proton NMR spectrum for the product, methyl mnitrobenzoate. This figure also shows the structure of the molecule with hydrogen peaks labeled accordingly. Table 3: Proton NMR Analysis for m-Nitrobenzoate Label Multiplicity Integration Shift (ppm) A 1 3 3.9 B 3 1 7.7 C 2 2 8.5 D 1 1 8.9 Discussion: In this experiment, nitric acid and methyl benzoate were reacted to form methyl nitrobenzoate. The nitric acid could have added to the ortho, para, or meta position. This forms three possible products: methyl o-nitrobenzoate, methyl p-nitrobenzoate, and methyl m-nitrobenzoate. The reagent methyl benzoate is the benzene species. The functional group on this benzene ring is an electron withdrawing group. This functional group deactivates the ring, and it is a meta-director. Electron withdrawing groups are meta directors because their secondary carbocation intermediates are the most stable, compared to the ortho and para positions. This indicates that the meta product will be the major product when a benzene ring is mono-substituted with an electron withdrawing group.3 Because of this, methyl m-nitrobenzoate is the major product over methyl o-nitrobenzoate and methyl p-nitrobenzoate. In this experiment, nitric acid was used to form the electrophile, the nitronium ion. The sulfuric acid aided in the formation of the nitronium ion by donating a proton to nitric acid. Methyl benzoate was the nucleophile in this reaction. Ice chunks were used in this reaction to help the product crystallize out of the solution. Water and methanol were added to remove impurities from the product. Ice baths and ice water baths were used throughout the course of this experiment to prevent over-nitration of methyl benzoate. Although the ring was deactivated, over-nitration could have occurred if the mixture was heated.4 The percent yield calculate for the product was 18.32%. A few errors occurred throughout the experiment that could have affected the percent yield. When inserting the nitric acid mixture dropwise into the flask, a few drops were spilled. This could have caused fewer molecules of the nitronium ion to react, thus causing a lower percent yield. When transferring the crystallized mixture into the suction filtration apparatus, some solid product stuck to the inside of the flask, as well as the suction filtration funnel. Both of these errors could have contributed to the percent yield error. The melting point for the product obtained was a range from 79.3°C to 83.1°C. The melting point of methyl m-nitrobenzoate is -13.00°C. The melting point of methyl p- nitrobenzoate is a range from 94.00°C to 96.00°C. The melting point of methyl mnitrobenzoate is a range from 78.0°C to 80.0°C.2 The melting point of the product obtained most closely matched the melting point for methyl m-nitrobenzoate. The melting point obtained is slightly higher and broadened. This is an indication of impurities present. These impurities could have been unreacted reagents, water, or methanol. Finally, proton NMR analysis was done on the final product, shown in figure 5. The most shielded peak, peak A, was a singlet; located at 3.9 ppm. The second most shielded peak, peak B, was a triplet, located at 7.7 ppm. The fact that this peak is a triplet indicates that it is beside two neighboring hydrogens. These two neighboring hydrogens are about equal in equivalency, and show up as peak C on the spectrum. This peak is located at about 8.5 ppm. The final peak, peak D, is the most de-shielded peak. It is a singlet located at 8.9 ppm. Because it is the most de-shielded, this indicates that it is between the two groups on the ring. Peak D and peak B are both indicators that the major product obtained in this reaction was methyl m-nitrobenzoate. Conclusion: The major product formed in this reaction was methyl m-nitrobenzoate. The product was confirmed to be fairly pure by obtaining the melting point. The product was also determined to be meta by analyzing the melting point as well as the proton NMR spectrum. Overall, this experiment lacked much room for improvement. Another experiment could be done with a benzene ring that had an electron donating group to observe the major products that would be produced. Which would most likely be para and ortho products.