Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction given below, the product \(\mathrm{Y}\) is \(\mathrm{C}_{6} \mathrm{H}_{6} \stackrel{\mathrm{HNO}_{3}}{\mathrm{H}_{2} \mathrm{SO}_{4}} \mathrm{X} \frac{\mathrm{Cl}_{2}}{\mathrm{FeCl}_{3}} \mathrm{Y}\) (a) 4-nitrochlorobenzene (b) 3-nitrochlorobenzene (c) 1-nitrochlorobenzene (d) none of the above

Step-by-Step Solution

Answer

The product Y is 3-nitrochlorobenzene (b).

Step 1: Identify the Reaction Conditions

The first part of the reaction involves benzene (C_6H_6) with nitric acid (HNO_3) in the presence of sulfuric acid (H_2SO_4). This setup indicates a nitration reaction that typically introduces a nitro group (-NO_2) to the benzene ring, resulting in nitrobenzene.

Step 2: Form Intermediate Product X

The nitration of benzene produces nitrobenzene, C_6H_5NO_2. Therefore, the intermediate product X is nitrobenzene.

Step 3: Apply Chlorination Reaction

The next part of the reaction involves treating the intermediate X (nitrobenzene) with chlorine (Cl_2) in the presence of ferric chloride (FeCl_3). This is indicative of a Friedel-Crafts chlorination reaction, where one hydrogen atom on the benzene ring is substituted by a chlorine atom.

Step 4: Consider the Positions of Substitution

The nitro group is a meta-directing group due to its electron-withdrawing nature. This means the chlorine will add at the meta position relative to the nitro group on the benzene ring.

Step 5: Determine the Final Product Y

Since the chlorine substitutes at the position meta to the nitro group in nitrobenzene, the final product Y is 3-nitrochlorobenzene.

Key Concepts

Nitration of Benzene

Nitration of benzene is a classic example of electrophilic aromatic substitution. This process involves adding a nitro group \((-NO_2)\) to a benzene ring. The reaction takes place under acidic conditions, typically utilizing nitric acid \((HNO_3)\) and sulfuric acid \((H_2SO_4)\).
The role of sulfuric acid is to generate the highly reactive nitronium ion \((NO_2^+)\), which acts as the electrophile. This nitronium ion is formed by protonating nitric acid with sulfuric acid, making it more susceptible to attack. The benzene ring, rich in electrons, provides the nucleophile.
When the electrophile and nucleophile meet, the nitronium ion attacks the benzene, resulting in the formation of nitrobenzene \((C_6H_5NO_2)\). During this transformation, the aromaticity of benzene temporarily breaks. However, it is restored after the departure of a hydrogen ion, which completes the substitution process, and the stable nitrobenzene is formed.

Friedel-Crafts Chlorination

Friedel-Crafts chlorination is another type of electrophilic aromatic substitution. This reaction involves substituting a hydrogen atom on a benzene ring with a chlorine atom. The reaction is facilitated by a Lewis acid catalyst such as ferric chloride \((FeCl_3)\).
The process starts with the interaction between chlorine \((Cl_2)\) and the catalyst, generating a chloronium ion \((Cl^+)\), which acts as the electrophile. The catalyst helps in polarizing the chlorine molecule, creating a strong enough electrophile for the substitution.
The intermediate stage retains the positive charge on the benzene ring, disrupting its aromaticity momentarily. Like the nitration, this reaction also concludes with the loss of a hydrogen ion, restoring the aromatic nature. The result is chlorobenzene, where the chlorine takes the place of the hydrogen atom in the structure.

Meta-Directing Groups

Meta-directing groups are substituents on a benzene ring that direct incoming electrophiles to the meta position, relative to the existing substituent. Nitrogen dioxide \((-NO_2)\), from the nitro group, is a classic meta-director due to its electron-withdrawing characteristics.
These groups are characterized by their ability to pull electron density away from the ring, typically by resonance or induction. The effect of these groups is to make the ortho and para positions less favorable for further substitution, thereby creating a preference for addition at the meta position.
During a substitution reaction, the presence of a meta-directing group like \(-NO_2\) on benzene leads electrophiles such as \(Cl^+\) to attach at the meta position. This is what happens in reactions involving nitrobenzene, where additional substituents prefer the position that is one carbon away from the nitro group. This positioning effect is crucial in predicting and understanding the product outcome in aromatic substitution reactions.