Question

Attacking electrophile during nitration of aromatic compound in case of dilute \(\mathrm{HNO}_{3}\) is (A) \({ }^{\oplus} \mathrm{NO}\) (B) \(\mathrm{NO}_{2}-\stackrel{\Theta}{\mathrm{O}} \mathrm{H}_{2}\) (C) \(\mathrm{NO}_{2}\) (D) \(\mathrm{NO}_{2}^{-}\)

Short answer

The attacking electrophile during the nitration of an aromatic compound in the presence of dilute HNO₃ is \({ }^{\oplus} \mathrm{NO}_{2}\) (Option A).

Step-by-step solution

Recall the nitration process

Nitration of an aromatic compound, such as benzene, involves the replacement of one hydrogen atom by a nitro group (-NO₂). The nitration process requires an electrophile that can attack the aromatic ring, resulting in its substitution with the nitro group.

Formation of the electrophile in the presence of dilute HNO₃

In the nitration process, the electrophile is typically formed by the reaction of the concentrated nitric acid (HNO₃) and a strong acid such as concentrated sulfuric acid (H₂SO₄). The reaction forms the nitronium ion (NO₂⁺), which acts as the electrophile for the aromatic compound. In this scenario, we are dealing with dilute HNO₃, and it can be assumed that there is a small amount of H⁺ ions in the solution. The reaction between the dilute nitric acid and the H⁺ ions produces the same nitronium ion, although it may form at a slower rate compared to the concentrated form. Here, HNO₃ will lose its H⁺ ion and form NO₂⁺. The overall reaction can be represented as follows: HNO₃ + H⁺ → \({ }^{\oplus} \mathrm{NO}_{2}\) + H₂O The electrophile in this case is \({ }^{\oplus} \mathrm{NO}_{2}\).

Identify the correct option

Based on the information provided in Step 2, the electrophile attacking the aromatic compound in the presence of dilute HNO₃ is \({ }^{\oplus} \mathrm{NO}_{2}\). This corresponds to the option (A) \({ }^{\oplus} \mathrm{NO}_{2}\) . Therefore, the correct answer is: (A) \({ }^{\oplus} \mathrm{NO}\)

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry, especially pivotal for the synthesis of many aromatic compounds. This process involves the replacement of a hydrogen atom on an aromatic ring with an electrophile, a molecule that is attracted to electrons.

In the context of nitration, the electrophile is typically created by the reaction of nitric acid (HNO3) with a stronger acid like sulfuric acid (H2SO4), producing a highly reactive species known as the nitronium ion (NO2⁺). Dilute nitric acid can also participate in this reaction, albeit more slowly. The aromatic compound, with its electron-rich pi system, acts as a nucleophile and undergoes attack by the electrophile, leading to the formation of a sigma complex. Subsequently, a proton is lost to restore the aromaticity, resulting in the substitution of a nitro group (-NO2) on the aromatic ring.

Nitronium Ion

The nitronium ion ((plust)NO2) plays a crucial role as the electrophile in the nitration of aromatic compounds. It is a positively charged ion with a central nitrogen atom surrounded by two oxygen atoms, one of which carries a positive charge.

The formation of the nitronium ion from nitric acid requires a reaction with a proton donor or a stronger acid. In the case of dilute HNO3, the presence of water and the small amount of available protons means that the formation of the nitronium ion is less efficient than with concentrated acids. However, the powerful electron-withdrawing effect of the nitronium ion makes it an excessively strong electrophile that can attack the electron-rich aromatic rings to facilitate the nitration process.

HNO3 Reactivity

Reactivity of nitric acid (HNO3) is central to the nitration of aromatic compounds. Dilute and concentrated forms of HNO3 show different reactivity patterns due to their varying concentrations of H⁺ ions. While concentrated HNO3 is typically used in conjunction with sulfuric acid to produce the reactive nitronium ion, dilute HNO3 can react directly with existing H⁺ ions to form NO2⁺, although the reaction proceeds at a slower pace.

The reactivity of HNO3 is also governed by its capacity to act as an oxidizing agent, with the potential to introduce a variety of functional groups into aromatic systems. Understanding the reactivity levels of HNO3 is crucial for students preparing for competitive exams such as JEE, where mechanistic details can inform problem-solving strategies.

Organic Chemistry for JEE

Organic chemistry is an essential component of the Joint Entrance Examination (JEE) syllabus for students aspiring to enter engineering fields in India. The examination assesses a deep understanding of mechanisms, reactivity, and the ability to solve complex problems.

Nitration and electrophilic aromatic substitution are among the key reactions covered in JEE organic chemistry. A firm grasp of these reactions, including the behavior of the electrophiles, such as the nitronium ion, and the role of reagents like HNO3, is vital for success. Students are expected to understand both the conceptual framework and the practical implementation of these organic processes. Focusing on the step-by-step mechanism of nitration will not only help in achieving a high score in JEE but also build a strong foundation for future studies in organic chemistry.