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Nitration of Methyl Benzoate: A Lab Report

Nitration of Methyl Benzoate: A Laboratory Report

Abstract

The major aim of the experiment was to substitute the hydrogen atoms on the methyl benzoate ring with a nitro group to obtain a pure product of methyl 3-nitrobenzoate after recrystallization. Nitration of Methyl benzoate follows the principles of an electrophilic aromatic substitution reaction. As such, the attachment of the nitrogen group depends on the type of pre-existing functional groups on the ring. In this case, the existence of the carbomethoxy group determines whether the nitrate group is ortho-para directed or meta directed. The reaction proceeds as shown below. After completion of the reaction, the experimental yield of Methyl 3-nitrobenzoate is determined and compared with the theoretical yield. Besides, the melting point of the product was determined. A detailed laboratory report for the experiment is presented herein.

Figure 1: The structural formula for the nitration of methyl benzoate (Drawn using ChemDraw Ultra, version 12.0).

Introduction

Nitration reactions involve nitro group addition or substitution with pre-existing functional groups such as methyl in organic compounds. In ortho-para electrophilic aromatic substitution reactions, electron-donating functional groups, which are attached to the ring, are used as the directors while electron-withdrawing ones are responsible for meta directed substitution reactions. The figure below illustrates the difference in these types of reaction products. Furthermore, in meta directed products, the substituent is attached to the third Carbon in the ring while in ortho-para, the attachment is in the second or the fourth carbon atom.

Figure 2: Illustration for ortho,para, and meta directed substitutes in electrophilic aromatic substitution reactions (Drawn with ChemDraw Ultra, version 12.0).

Nitration of methyl benzoate begins with the formation of a nitronium ion (NO2+) from the reaction between sulfuric acid and nitric acid. Since the ion is positively charged, it is more attracted to a neutral carbon atom on the ring where it substitutes the hydrogen atom. The ion is formed via the interaction of the two strong acids as shown in the reaction below.

Figure 3: Reaction equation for the formation of the nitronium ion (Drawn with ChemDraw Ultra, version 12.0).

Sulfuric acid then protonated methyl benzoate to form resonance intermediaries that have a high density of electrons at the meta position (Williamson & Masters, 2010). The nitronium ion then attacks the third carbon to form an arenium ion intermediate, which undergoes proton transfer to the basic bisulfate ion to give methyl 3-nitrobenzoate as the end product of the electrophilic reaction. In meta directed electrophilic aromatic substitution reactions, the electron withdrawing group, in this case the ester, deactivates the benzene ring for the reaction making the ortho and para positions to be positively charged. The two positions further destabilize the carbon that is next to the carbomethoxy group. This leaves the mata position, which is neutral, to be more reactive compared to the two positively charged carbons. Experimentally, the reagents must be kept in a cool environment to protect the nitro group from being removed. Through recrystallization process, the resultant product is purified (Williamson & Masters, 2010). 

As a laboratory precaution, it is important to avoid contact with the two strong acids, sulfuric and nitric. In case of an accident that leads to spillage of these chemicals, it is advisable to neutralize with sodium bicarbonate powder and immediately clean up (Williamson & Masters, 2010). The practical must also be carried out in the presence of at least one skilled laboratory technician who must be informed in case of accidents and other risky instances. Protective wear is also essential equipment during the experiment. For instance, the addition of nitric acid to organic substances can react rapidly and is a highly exothermic reaction. As such, the experiment was carried out with a keen adherence to the laboratory rules and cautions involving the reagents used.

Figure 4: A balanced chemical reaction for the nitration of Methyl Benzoate (Drawn with ChemDraw Ultra, version 12.0).

Results and Calculations

Table 1: The amount of reactants used in the experiment (Provided in the experiment procedure).

SubstanceAmount usedMolecular weights
Nitric acid0.2 mL63.01 g/mol
Sulfuric acid0.2 mL98.079 g/mol
Additional Sulfuric acid 0.6 mL98.079 g/mol
Methyl benzoate0.3102g136.14792 g/mol
Methanol0.5 mL32.04 g/mol

Table 2: Amount of substances produced after the experiment (experimental results).

SubstanceAmount produced
Crude methyl 3-nitrobenzoate 0.5 g
Pure methyl 3-nitrobenzoate0.0733g
Theoretical melting point of methyl 3-nitrobenzoate78 0C
Melting point range of methyl 3-nitrobenzoate produced77.3-80.3 0C
Theoretical yield methyl 3-nitrobenzoate0.0022784042 moles
Actual yield of methyl 3-nitrobenzoate produced0.0004049723 moles
Percentage Yield 17.76 %

From the equation:

The mole ratio for methyl benzoate: methyl 3-nitrobenzoate is 1:1

Amount of methyl benzoate used       = 0.3102g

Moles of methyl benzoate used          =

                                                = [0.3102 g ÷ 136.14792 g/mol] moles

                                                = 0.0022784042 moles of C6H5COOCH3

Because of the mole ratio of 1:1, the number of moles of methyl benzoate equals the number of moles of methyl 3-nitrobenzoate produced.

Therefore, the number of moles of C6H4COOCH3NO2 = 0.0022784042.

But from the experiment, 0.0733 grams of methyl 3-nitrobenzoate was produced.

Moles of C6H4COOCH3NO2 =

                                                = [0.0733g ÷ 181.14 g/mol] moles

                                                = 0.0004046593 moles of C6H4COOCH3NO2 produced

% yield = Percentage yield = { } ×100%

                                    = {0.0004046593 ÷0.0022784042} × 100%

                                    = 0.1776064580 × 100%

                                    = 17.760645806%

                                    = 17. 76%

From nitric acid point of view, the number of moles used can be calculated as shown below.

Moles of nitric acid =

Mass of HNO3used = 0.2 mL, which translates to 0.2 grams since 1 mL= 1g.

Moles                          = [0.2g ÷ 63.01g/mol] moles

                                    = 0.0031740993 moles of HNO3

Since the mole ratio of HNO3: C6H4COOCH3NO2is 1:1, the possible yield of methyl 3-nitrobenzoate is 0.0031740993 moles. Therefore, methyl benzoate is considered as the limiting reagent of the experimental reaction. Therefore, the theoretical yield is calculated by considering the least possible number of moles of the product, which is as a result of the limiting reagent’s contribution. The theoretical yield for the experiment is 0.0022784042 moles of C6H4COOCH3NO2.

Discussion and Conclusion

In the reaction for nitration of methyl benzoate, the nitro group substitutes the hydrogen atom at the meta position of the benzene ring. This is because the carbomethoxy functional group in the methyl benzoate compound is an electron-withdrawing group. The reaction took place in a fume hood as a precaution due to the corrosive and reactive nature of strong sulfuric and nitric acids. In the identification of the experimental product, the melting point was determined. The range of 77.3-80.3 0C is within the theoretical value for melting methyl 3-nitrobenzioate, which should range between 78-800C according to literature works. The slightly lower melting point of 77.30C is due to persistent impurities that were not excluded during crystallization and recrystallization process using 0.5 mL of methanol. It is possible that there were side reactions that led to the formation of ortho and para directed substituents in the product despite being unfavorable compared to meta positioned major product. These bi-products might have been formed as isomers from the side reaction. This is the likely reason for a slightly higher value of the melting point range with a deviation of 0.30C.

The ester was used as an electron-withdrawing group in the electrophilic aromatic substitution reaction that led to nitration of methyl benzoate compound with a nitric acid. The ester deactivated the benzene ring against reacting with the nitonium ion, which was formed from the interaction between sulfuric and nitric acids. Ortho and para positions attained positivity that was least preferred by the ion during the electrophilic attack. The most favorable position, the third carbon atom from the carbomethoxy position in the ring was thus preferred for substitution. The hydrogen atom was substituted with the nitronium ion to for methyl m-nitrobenzoate compound as the end target product.

The major product of the experimental reaction is methyl 3-nitrobenzoate. This is because of the registered melting point, which is within a closer range with the theoretical value and slightly lower i.e. 77.3-80.30C as compared to the expected 78-800C. Ortho-para directed methyl nitrobenzoate products are expected to show a higher melting point than the expected range. This is because of the activation of the aromatic ring by the electron donating groups e.g. the halogen functional groups, which leads to the formation of a highly negative center as a site for reaction with the positively charged electrophiles. Eventually, it requires more energy to break the bonds between the atoms in an ortho-para directed product due to relatively stronger electronegative forces.

The percentage yield for the experimental reaction was recorded as 17.76%. This is a low value; hence the procedure used cannot be used for commercial production of methyl 3-nitrobenzoate. The method used to purify the end product is effective and efficient. This is because of the melting point value that was almost reproduced. The range attained is a characteristic feature for methyl m-nitrobenzoate with a slight deviation. Despite the workability of the recrystallization method employed in the experiment, slight variations in melting point ranges indicate the presence of persistent impurities. The experiment was successfully carried out, proving the hypothesis that the reactants used and the conditions would lead to nitration of methyl benzoate to methyl 3-nitrobenzoate via an electrophilic aromatic substitution reaction.

References

Williamson, K. L., & Masters, K. M. (2010). Macroscale and microscale organic experiments.Boston, Mass: Cengage Learning

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