Aromatic compounds, which are planar cyclic rings with (4n+2)π electrons, will not undergo simple addition reactions like those of alkyl substances. However, in the presence of an electrophile, aromatic compounds will undergo electrophilic aromatic substitution. In this type of reaction, two π electrons from the aromatic ring serve for the ring to act as a nucleophile and attack an electrophile. For nitration, this nucleophile is NO2+, which is produced by reacting nitric and sulfuric acids. After the nucleophile adds, the ring has lost aromaticity.
Therefore, the deprotonated acid in solution can pull off a hydrogen from the same carbon that the nitro group has added to, allowing the electrons from that bond to go back into the ring to reproduce aromaticty. There are three possible positions on a benzene ring that a nucleophile could add to, referred to as the ortho, para, or meta positions. In this experiment, the nucleophile will primarily add into the meta position. This is because the starting material is methyl benzoate, as opposed to just benzene.
The ester group of methyl benzoate is capable of participating in the resonance of the ring. This withdraws electron density from the benzene ring, and is said to be deactivating. Deactivating substituents destabilize the carbocation intermediates formed from substitutions to the ortho or para positions. This decreases the reaction rate for those positions. Therefore, the meta position is the most reactive one.
The mechanism for this reaction is
The product is not likely to undergo further titration at the reaction conditions provided. Generally, polynitrated products do not form except under harsh conditions. For example, the third nitration of toluene in the production of TNT only happens in boiling concentrated sulfuric acid. Although the theory states that there will only be one product, methyl m-nitrobenozate, there are three possible products, so characterization is necessary. This will be done through melting point determination and infrared spectroscopy. Materials and Methodology
Refer to Part B of Experiment 12.2 in Schoffstall, page 306-307. Compound
Results and Observations
The mass of product was 1.1915g and had a melting point of 75-76 °C. Since methyl benzoate was the limiting reactant, 0.008 moles or 1.4492g of product were expected. The percent yield was 82.2% yield. After recrystallization, the mass was 0.158g, reducing the yield to 11%. Literature and experimental IR spectra follow.
Discussion and Conclusion
Comparison of melting points shows that the predominant species formed was, in fact, methyl m-nitrobenzoate. Peaks at 1351 cm-1 and 1527 cm-1 on the experimental IR spectrum indicate NO2 stretching. Peaks around 3100 indicate sp2 C-H stretching, along with overtone bands from 2000-1667 cm-1 indicate aromaticity. A peak at 1716 cm-1 indicate C=O stretching of an aldehyde. These factors strongly indicate the product was the expected meta compound. Electrophilic substitution can have several products due to the resonance contributors of the resulting σ complex of substitution. Different substituents on the ring can direct substitution to certain positions on the ring, and this makes it possible to predict the product based on whether the existing substituent is so called activating or deactivating. Deactivating substituents direct substitution to the meta position, while activating substituents direct to the ortho/para positions. This happens because the activating or deactivating nature of the substituent means that it can supply or withdraw electrons to or from the ring, respectively. Aromaticity is much more stable than other cyclic hydrocarbon substances, and so if it is possible compounds will preferentially form aromatic rings. Therefore it is possible to use an aromatic substance and knowledge of electrophilic substitution reactions to predict and create substituted aromatic compounds.
Cite this Nitration of methyl benzoate
Nitration of methyl benzoate. (2016, Jul 21). Retrieved from https://graduateway.com/nitration-of-methyl-benzoate/