Describe the nitration process of methyl benzoate and discuss the regioselectivity of the reaction.

Nitration is a chemical process that involves the introduction of a nitro group (-NO2) into an organic molecule. In the case of methyl benzoate, nitration is typically carried out using a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4). The reaction is an example of an electrophilic aromatic substitution (EAS), where the aromatic ring of methyl benzoate acts as a nucleophile and is attacked by an electrophilic nitronium ion (NO2+).

The process of nitration of methyl benzoate can be broken down into the following steps:

1. Generation of the Electrophile: The first step in the nitration process is the generation of the nitronium ion, which is the active electrophile. This is achieved by mixing concentrated nitric acid with concentrated sulfuric acid. The sulfuric acid protonates the nitric acid, leading to the formation of the nitronium ion and water:

$${\text{HNO}}_3+2{\text{H}}_2{\text{SO}}_4\to {\text{NO}}_2^{+}+{\text{HSO}}_4^{-}+{\text{H}}_3{\text{O}}^{+}+{\text{HSO}}_4^{-}$$

2. Formation of the σ Complex (Arenium Ion): The aromatic ring of methyl benzoate, being electron-rich, reacts with the nitronium ion. A π electron pair from the benzene ring is donated to the nitronium ion, forming a carbocation intermediate known as the σ complex or arenium ion. This step is reversible.

$${\text{C}}_6{\text{H}}_5{\text{COOCH}}_3+{\text{NO}}_2^{+}\to {\text{C}}_6{\text{H}}_4{\text{COOCH}}_3({\text{NO}}_2{)}^{+}$$

3. Deprotonation: The σ complex loses a proton to reform the aromatic system, yielding the nitrated product. The proton is typically abstracted by the bisulfate anion (HSO4-) that was formed in the first step.

$${\text{C}}_6{\text{H}}_4{\text{COOCH}}_3({\text{NO}}_2{)}^{+}\to {\text{C}}_6{\text{H}}_4{\text{COOCH}}_3{\text{NO}}_2+{\text{H}}^{+}$$

4. Regioselectivity: The regioselectivity of the nitration of methyl benzoate is influenced by the electron-withdrawing ester group (-COOCH3). This group is meta-directing and deactivating, meaning that it decreases the electron density of the benzene ring and directs incoming electrophiles to the meta position relative to itself. Therefore, the major product of the nitration of methyl benzoate is meta-nitromethyl benzoate.

$${\text{C}}_6{\text{H}}_5{\text{COOCH}}_3+{\text{NO}}_2^{+}\to {\text{C}}_6{\text{H}}_4({\text{COOCH}}_3)({\text{NO}}_2{)}_{\text{meta}}+{\text{H}}^{+}$$

The overall reaction can be summarized as:

$${\text{C}}_6{\text{H}}_5{\text{COOCH}}_3+{\text{HNO}}_3\stackrel{{\text{H}}_2{\text{SO}}_4}{\to }{\text{C}}_6{\text{H}}_4({\text{COOCH}}_3)({\text{NO}}_2{)}_{\text{meta}}+{\text{H}}_2\text{O}$$

In conclusion, the nitration of methyl benzoate involves the generation of a nitronium ion electrophile, the formation of a σ complex, and the deprotonation to yield the nitrated product. The regioselectivity of the reaction is influenced by the meta-directing effect of the ester group, leading predominantly to the formation of meta-nitromethyl benzoate.