Electrophilic Aromatic Substitution: Nitration of Methyl Benzoate

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Sophomore (College 2nd year) ・Biology ・MLA ・2 Sources

Electrophilic fragrant substitution is an organic reaction whereby a hydrogen atom inside an aromatic compound is replaced the usage of an electrophile. The reaction mechanism occurs in two steps which starts offevolved with an electrophilic attack to form an arenium ion and after that deprotonation the use of a weak base. An electrophilic attack is a rate-determining step, therefore, must be performed using a compound with a excessive activation energy and strong ample to disrupt the aromaticity of the stable benzene ring (Topchiev).
In nitration of methyl benzoate, the acid mixture, concentrated sulfuric acid and nitric acid, is used to put together nitronium ion, an electron-rich aromatic ring, from the methyl benzoate used as an electrophile in the rate-determining step. This ion then attaches to the benzene ring through a selective attack on the nucleophilic pi-bond to form methyl 3-nitrobenzoate as the major product.

Aim of the experiment

Purposes of this experiment are to:
a) Study electrophilic aromatic substitution reactions (EAS)
b) Learn regioselectivity for EAS reactions.

Procedure

1.1.0 mL sample of concentrated sulfuric acid was placed in a clean, dry 6” test tube and then swirled in an ice bath for cooling to about 00C. 0.7 mL of methyl benzoate was then added carefully, and the mixture was shaken to produce one layer. The solution was then continuously cooled at 00C.
2. A mixture of 0.4 mL of concentrated nitric acid and 0.4 mL of concentrated Sulfuric acid was prepared in a separate 6" test tube, and the mixture cooled to 00C.
3. The cold mixture of the acids was added dropwise using a Pasteur pipet to the sold solution of the ester in the sulfuric acid. The adding was done while swirling the ester solution in an ice bath for 8.13 minutes. The temperature during the reaction was maintained at 00C throughout the reaction.
4. The mixture prepared in step 3 was swirled further for 7 minutes in an ice bath. The reaction mixture was then allowed to stand at room temperature with interval swirling for also ten minutes.
5. Another 6" test tube was two-thirds filled with crushed ice, and two drops of the reaction mixture added using a Pasteur pipet. Occasional swirling was done to the mixture until crystallization of the product started to form. The remaining reaction mixture was then added slowly and dropwise while swirling until all the ice melted.
6. Suction filtration using Hirsch funnel was then conducted to collect the solid crystals. The solid product was then rinsed carefully with 1.0 mL of cold tap water to remove traces of the acids.
7. The solid product was then washed using ice-cold 95% ethanol on the filter and after that air was blown to fasten the drying process.
8. The weight and melting range of the "crude yield" obtained were determined.
9. Further purification of the crude yield was performed, and weight and melting point determined.

Results and Calculations

Amount of Methyl benzoate used

0.756g

0.0056mol

Theoretical yield of major product

1.0144g

0.0056mol

Weight of crude product made

 

1.0297g

Weight of purified product made

 

1.0264g

% yield of major product ( from crude)

 

101.51%

Melting point of product

 

71.30C

Discussion

Sulfuric acid acts as a catalyst during nitration of methyl benzoate with nitric acid. Nitronium ion being an active electrophile attacks the electron-rich benzene ring and withdraw electrons, therefore, deactivating it. Maintain the reacting mixtures at very low temperatures ensures that the substitution only occurs at the meta position (Di Somma). Performing the reaction at a temperature higher than 00C allows the ester to be an ortho-para director. Possible products due to this would be methyl 2-nitrobenzoate, and methyl 4-nitrobenzoate, and methyl n, m-dinitrobenzoate.

The mechanism of this reaction occurs in three steps namely: generation of the electrophile; nucleophilic attack to the electrophile by nitronium; degeneration of the catalyst.

The last step in the substitution reaction is used to regenerate the sulfuric acid used as a catalyst. The concentrated nitric acid is used in excess to ensure that complete nitration of the methyl benzoate occurs. The weight of the pure solid crystals obtained is higher than that calculated theoretically because the solid crystals aren't completely dry. There is still some amount of water crystallization remaining despite the drying done.

Conclusion

The purposes of this experiment were achieved since the desired product was obtained. Sources of error in this experiment could be due to not following the steps provided in the procedure carefully, therefore, allowing other electrophiles present, e.g., hydroxyl or alkoxyl groups preferentially attack the aromatic benzene ring.

Works Cited

Di Somma, I., Marotta, R., Andreozzi, R., & Caprio, V. "Kinetic and safety characterization of the nitration process of methyl benzoate in mixed acid." Organic Process Research & Development, 16(12) (2012): 2001-2007.
Topchiev, A. V. Nitration of hydrocarbons and other organic compounds. Elsevier, 2013.

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