Question

Out of benzene, \(m\) -dinitrobenzene and toluene which will undergo nitration most easily and why?

Short answer

Toluene undergoes nitration most easily due to its activating methyl group.

Step-by-step solution

Understanding the Reactants

We are considering three compounds: benzene, \(m\)-dinitrobenzene, and toluene. We need to determine which one undergoes nitration most easily. Nitration is an electrophilic substitution reaction where a nitro group (\(\text{-NO}_2\)) is added to an aromatic ring.

Evaluating Benzene

Benzene is an aromatic compound with a stable ring structure that can undergo nitration. However, it does not have any activating groups that would make nitration particularly easy.

Evaluating \(m\)-dinitrobenzene

\(m\)-dinitrobenzene has two nitro groups already attached to the benzene ring in a meta position. Nitro groups are deactivating, meaning they actually make the ring less reactive towards electrophilic substitution reactions like nitration.

Evaluating Toluene

Toluene has a methyl group attached to the aromatic ring. The methyl group is an electron-donating group, making the ring more reactive towards electrophiles. This activates the ring and facilitates nitration.

Conclusion and Comparison

Comparing all three compounds, we can conclude that toluene will undergo nitration most easily due to the presence of the activating methyl group, which increases the ring's electron density, making it more reactive compared to benzene and especially \(m\)-dinitrobenzene, where nitro groups deactivate the ring.

Key concepts

Electrophilic Substitution

Electrophilic substitution is a fundamental reaction mechanism in organic chemistry, particularly for aromatic compounds. Aromatic compounds, like benzene, possess a highly stable ring structure due to delocalized electrons. During electrophilic substitution, an electrophile replaces a hydrogen atom in the aromatic ring. The stability of the ring, however, is slightly disrupted during this process, forming a high-energy intermediate called an arenium ion.
To encourage this reaction, the stability provided by aromaticity should be preserved or compensated for.

• Common electrophiles involved in these reactions are positively charged or polarized species.
• The reaction consists of two main steps: the formation of the arenium ion, and the deprotonation to restore aromaticity.
• Nitration, which introduces a nitro group (\(-\text{NO}_2\)) into the ring, is a classic example.

Understanding how substituents already present on an aromatic ring can impact its reactivity during electrophilic substitution is vital. These substituents can either activate the ring to facilitate the reaction or deactivate the ring to slow it down.

Activating and Deactivating Groups

When considering the nitration of aromatic rings, it's crucial to understand the role of substituents as activating or deactivating groups. Activating groups increase the rate of electrophilic substitution by donating electron density to the ring, making it more attractive to an electrophile. In contrast, deactivating groups withdraw electron density, hindering substitution.
The influence of these groups is due to their electron-donating or electron-withdrawing nature:

• Activating Groups: Typically have lone pairs or \(\sigma\) bonds adjacent to the ring, like the methyl group in toluene. These groups increase electron density, enhancing reactivity.
• Deactivating Groups: Often possess \(-\text{NO}_2\) or C=O group functionalities, such as in \(m\)-dinitrobenzene. They pull electron density away, reducing the ring's reactivity.

The presence of these groups is key when predicting the outcome of reactions. For example, in the problem, toluene, with its activating methyl group, undergoes nitration more readily than benzene or \(m\)-dinitrobenzene.

Aromatic Ring Reactivity

Aromatic ring reactivity is greatly influenced by the electronic effects of substituents already attached to the ring. The position of these groups and their electron-donating or withdrawing abilities define both the rate and the position of electrophilic attack.

• Substituents that increase electron density generally activate the ring, leading to faster reactions and favoring specific substitution sites.
• Substituents that withdraw electron density tend to deactivate the ring, slowing down reactions and altering substitution patterns.

In the context of the nitration example, toluene's methyl group increased electron density, activating the ring at ortho and para positions due to resonance and hyperconjugation effects.
In contrast, the nitro groups in \(m\)-dinitrobenzene significantly withdraw electron density, deactivating the ring and lowering the likelihood of further nitration. Understanding these factors aids in predicting product distribution and reactivity in aromatic compounds.