Question

\(p\)-Nitrotoluene on further nitration gives (A) 3,4 -dinitrotoluene (B) 2,4 -dinitrotoluene (C) 2,6 -dinitrotoluene (D) 2,4 -dinitrobenzoic acid

Short answer

When p-Nitrotoluene undergoes further nitration, the ortho/para directing methyl group directs the new nitro group to one of the ortho positions, leading to the formation of 2,4-dinitrotoluene. Therefore, the correct answer is (B) 2,4 -dinitrotoluene.

Step-by-step solution

Identify the structure of p-Nitrotoluene

p-Nitrotoluene is an aromatic compound with a methyl group (CH3) and a nitro group (NO2) attached para to each other on the benzene ring. The structure is as follows: 

Analyze the activating/deactivating nature of substituents

In electrophilic aromatic substitution (EAS), the substituents present on the benzene ring can be activating or deactivating. The methyl group (CH3) is an activating group, which means it donates electron density to the ring and makes the compound more reactive towards electrophiles. The nitro group (NO2) is a deactivating group, which means it withdraws electron density from the ring and makes the compound less reactive towards electrophiles. Since the activating group is more powerful than the deactivating group, the activating group will control the direction of the incoming electrophile.

Determine the directing effect of the methyl group

The methyl group (CH3) is an ortho/para directing group, which means it directs the incoming electrophile to add either ortho or para to itself. In this case, since the nitro group is already in the para position with respect to the methyl group, the ortho position is favored.

Predict the product of further nitration

When p-Nitrotoluene undergoes further nitration, the methyl group directs the new nitro group to one of the ortho positions to itself, leading to the formation of 2,4-dinitrotoluene.

Identify the correct answer from the given options

From the predicted product, we can determine that the correct answer to this exercise is: (B) 2,4 -dinitrotoluene

Key concepts

Nitration

Nitration is a key electrophilic aromatic substitution reaction where a nitro group \((\text{NO}_2)\) is introduced into an aromatic ring. This process involves using a mixture of concentrated nitric acid \(\text{HNO}_3\) and sulfuric acid \(\text{H}_2\text{SO}_4\), which generates the powerful electrophile, nitronium ion \(\text{NO}_2^+\). This ion attacks the electron-rich benzene ring, leading to substitution.

• The reaction typically happens under controlled conditions because the benzene ring is stable, and adding a nitro group without excessive reaction can be challenging.
• Nitration increases the electron-withdrawing property of the compound, which can affect the reactivity for further reactions.

Whether the nitro group attaches at the ortho, meta, or para position depends on other substituents present on the aromatic ring. These substituents determine subsequent reactive sites.

Activating/Deactivating Groups

In the context of electrophilic aromatic substitution, groups attached to a benzene ring can be classified as either activating or deactivating depending on how they alter the ring's reactivity.

• Activating groups, like the methyl group \((\text{CH}_3)\), donate electron density through resonance or induction, making the ring more reactive toward incoming electrophiles.
• Deactivating groups, such as the nitro group \((\text{NO}_2)\), withdraw electron density from the ring, rendering it less reactive.

The relative strength of these groups determines the overall reactivity. In p-nitrotoluene, while the \(\text{NO}_2\) group deactivates the ring, the \(\text{CH}_3\) group ultimately influences the final site of nitration because it is an ortho/para director and stronger than the \(\text{NO}_2\) group.

Ortho/Para Directing Groups

The methyl group \(\text{CH}_3\) on p-nitrotoluene is an excellent example of an ortho/para directing group. Such groups direct the incoming electrophile to the ortho and para positions relative to themselves due to their electron-donating nature.

• Ortho positions are directly adjacent to the substituent.
• Para position is directly across the ring from the substituent.

Since the nitro group is already para to the methyl group, further nitration will favor an ortho position, which gives rise to compounds such as 2,4-dinitrotoluene rather than affecting already substituted sites. Understanding this directing ability is crucial for predicting the outcomes of substitution reactions in aromatic chemistry.

Aromatic Compounds

Aromatic compounds are a fascinating class of organic molecules characterized by their cyclic ring structure and resonance stability. This structure causes the π-electrons in the ring to be delocalized.

• They are distinguished by their ability to engage in electrophilic aromatic substitution (EAS) reactions, allowing functional groups to be added without breaking the aromatic ring.
• This unique stability and reactivity stem from Huckel's rule, which implies that having \(4n + 2\) π-electrons creates a stable aromatic system (where n is an integer).

This concept of aromaticity is crucial for understanding why certain reactions like nitration are feasible without disrupting the aromatic system. The benzene's stability and planarity make it a platform for exploring complex substitution reactions that are foundational in synthetic chemistry.