Question

The attacking electrophile during nitration of aromatic compound in case of mixed acid is (A) \(^{\oplus} \mathrm{NO}\) (B) \(\mathrm{NO}_{2}-\stackrel{\oplus}{\mathrm{O}} \mathrm{H}_{2}\) (C) \(\stackrel{\oplus}{\mathrm{NO}_{2}}\) (D) \(\mathrm{NO}_{2}^{-}\)

Short answer

The attacking electrophile during nitration of aromatic compounds in the case of mixed acid is \(\stackrel{\oplus}{\mathrm{NO}_{2}}\) (nitronium ion).

Step-by-step solution

Identify electrophiles

Electrophiles are species with a positive charge or electron-deficient, which can accept an electron pair. Let's look at the given options: (A) \(^{\oplus} \mathrm{NO}\): has a positive charge. (B) \(\mathrm{NO}_{2}-\stackrel{\oplus}{\mathrm{O}} \mathrm{H}_{2}\): has a positive charge. (C) \(\stackrel{\oplus}{\mathrm{NO}_{2}}\): has a positive charge. (D) \(\mathrm{NO}_{2}^{-}\): has a negative charge. We can eliminate option (D) because it has a negative charge and thus cannot be an electrophile.

Consider nitration of aromatic compounds

Nitration of aromatic compounds involves the replacement of a hydrogen atom with a nitro group (-\(\mathrm{NO}_{2}\)), which is an electrophilic substitution reaction. In the case of mixed acid, nitric acid (\(\mathrm{HNO}_{3}\)) acts as a nitrating agent and sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)) acts as a catalyst. The reaction between nitric acid and sulfuric acid generates the attacking electrophile. Let's analyze the remaining options: (A) \(^{\oplus} \mathrm{NO}\): is not generated in the reaction between nitric acid and sulfuric acid. (B) \(\mathrm{NO}_{2}-\stackrel{\oplus}{\mathrm{O}} \mathrm{H}_{2}\): is unlikely, as nitric acid loses an \(\mathrm{OH}\) group and the electrophile must have a nitro group (-\(\mathrm{NO}_{2}\)). (C) \(\stackrel{\oplus}{\mathrm{NO}_{2}}\): is the nitronium ion, which is generated in the reaction between nitric acid and sulfuric acid.

Determine the attacking electrophile

Based on the analysis in step 2, the attacking electrophile in the nitration of aromatic compounds in the case of mixed acid is: (C) \(\stackrel{\oplus}{\mathrm{NO}_{2}}\) (nitronium ion)

Key concepts

Electrophilic Substitution

Electrophilic substitution is a type of reaction where an electrophile replaces a substituent in a compound, such as a hydrogen atom on an aromatic ring. Aromatic compounds are rich in electrons, making them attractive to positively charged species that need electrons, which are known as electrophiles.
In the nitration of aromatic compounds, the hydrogen atom on the benzene ring is replaced by a nitro group (\(-\mathrm{NO}_2\)).
The aromatic ring’s electrons attack the electrophile, forming a carbocation intermediate. This reaction stabilizes further by losing a proton, completing the substitution and retaining the aromaticity of the ring.

• This type of reaction is crucial in modifying aromatic compounds for industrial applications.
• The resulting nitro compounds are often used in the production of explosives, dyes, and pharmaceuticals.

Understanding electrophilic substitution helps comprehend why aromatic compounds are such versatile and useful chemicals in industrial processes.

Mixed Acid Reaction

A mixed acid reaction involves combining acids to create a more potent reaction mixture. For nitrating aromatic compounds, a mixture of concentrated nitric acid (\(\mathrm{HNO}_3\)) and sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) is used.

Here’s what happens:

• Sulfuric acid acts as a catalyst and protonates the nitric acid, enhancing its reactivity.
• This process effectively concentrates nitric acid, creating a condition suitable for forming nitronium ions, the actual attacking species in nitration.

The role of the mixed acid in organic chemistry is essential because it:

• Ensures efficient generation of electrophiles, such as the nitronium ion.
• Provides conditions that allow electrophilic substitution to occur efficiently on aromatic rings.

By understanding mixed acid reactions, we can harness powerful reagents for the modification of organic molecules.

Nitronium Ion

The nitronium ion (\(\stackrel{\oplus}{\mathrm{NO}_2}\)) is a key intermediate in the nitration of aromatic compounds. It is formed when nitric acid reacts with sulfuric acid in the mixed acid system.

Here's how the nitronium ion is generated:

• In the presence of sulfuric acid, nitric acid loses a hydroxyl group forming water, and the remaining \(\mathrm{NO}_2\) gains a positive charge.
• This makes the nitronium ion a strong electrophile, ready to accept electrons from the aromatic ring.

The nitronium ion is crucial because:

• It directly participates in the electrophilic substitution, enabling the transfer of the nitro group onto the aromatic ring.
• This substitution forms new compounds that are central to various applications, including the creation of plastics and pharmaceuticals.

Understanding the formation and role of the nitronium ion provides insight into handling nitration reactions effectively in both academic and industrial settings.