Problem 133 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Nitrobenzene on further nitration produces: [Manipal 1999| (a) \(a\)-dinitrobenzene (b) \(m\)-dinitrobenzene (c) \(p\)-dinitrobenzene (d) all of these

Step-by-Step Solution

Answer

(b) m-dinitrobenzene

Step 1: Identify Directing Effects

Nitrobenzene contains a nitro group (-NO2) already attached to the benzene ring. The nitro group is a meta-directing group, which means that it deactivates the ortho and para positions and directs new substituents to the meta position.

Step 2: Predict the Nitration Position

Since the nitro group is meta-directing, further nitration of nitrobenzene will most favorably occur at the meta position relative to the existing nitro group. This leads to the formation of meta-dinitrobenzene.

Step 3: Conclude the Most Likely Product

Based on the directing effects identified, the most likely product when nitrobenzene undergoes further nitration is meta-dinitrobenzene.

Key Concepts

Nitrobenzene

Nitrobenzene is a fundamental compound in organic chemistry, characterized by the presence of a nitro group (-NO2) bonded to a benzene ring. This combination significantly affects the chemical nature of the molecule. Nitrobenzene's nitro group is influential for directing future reactions and influences how substituents attach to the benzene ring.

• The nitro group is strongly electron-withdrawing.
• It makes nitrobenzene less reactive compared to benzene.
• Its presence affects the positions where additional groups are likely to attach.

In the context of further nitration reactions, the nitro group within nitrobenzene plays a pivotal role in determining the sites on the benzene ring that are susceptible to chemical attack by an incoming electrophile.

Meta-Directing Groups

In electrophilic aromatic substitution reactions, meta-directing groups like the nitro group play a crucial role. These groups determine where new functional groups will be added to an aromatic compound by affecting the electron density distribution on the benzene ring.

• Meta-directing groups withdraw electrons from the ring, particularly making the ortho and para positions less reactive.
• As a result, they increase the reactivity of the meta position relative to the other positions.

The nitro group is a classic example because of its strong electron-withdrawing properties, which deactivates the ring toward further substitution. This results in preferentially directing newly added groups to the meta position relative to itself. Understanding the nature of meta-directing groups is essential in predicting and controlling the outcomes of substitution reactions in aromatic chemistry.

Nitration Reaction

The nitration reaction is a common type of electrophilic aromatic substitution. It involves the introduction of a nitro group into an aromatic ring and is a crucial process in industrial organic synthesis.
The reaction begins with the generation of the electrophile, the nitronium ion ( NO2^+ ), which is formed from a mixture of concentrated nitric acid and sulfuric acid.

• The aromatic compound reacts with the nitronium ion, resulting in the formation of a new C-NO2 bond.
• In nitrobenzene, the existing nitro group guides this reaction to occur at the meta position due to its meta-directing nature.

This results in the formation of meta-dinitrobenzene when nitrobenzene undergoes further nitration. Understanding the process of nitration and the influence of existing groups on the aromatic ring helps predict the structure of the resulting product and optimize various synthetic pathways.