Question

The correct order of increasing reactivity of benzene (I), anisole (II) and chlorobenzene (III) towards nitration is (A) II \(<\mathrm{I}<\mathrm{III}\) (B) \(\mathrm{III}<\mathrm{I}<\mathrm{II}\) (C) \(\mathrm{I}<\mathrm{III}<\mathrm{II}\) (D) III \(<\mathrm{II}<\mathrm{I}\)

Short answer

The correct order of increasing reactivity of benzene (I), anisole (II), and chlorobenzene (III) towards nitration is determined by analyzing the activating and deactivating substituent effects on the aromatic rings. Anisole (II) has an electron-donating OCH3 group, making it more reactive, while chlorobenzene (III) has an electron-withdrawing Cl group, making it less reactive compared to benzene (I). The correct order is Chlorobenzene (III) < Benzene (I) < Anisole (II), which corresponds to option (B).

Step-by-step solution

Identify the Substituents' Effect on Reactivity

First, let's look at the substituent groups attached to the aromatic rings in anisole (II) and chlorobenzene (III). In anisole (II), the OCH3 group is electron-donating, which makes the aromatic ring more electron-rich and hence more reactive towards electrophilic attacks like nitration. In chlorobenzene (III), the Cl group is electron-withdrawing, making the aromatic ring less electron-rich.

Analyze the Relative Reactivity towards Nitration

Now that we have identified the substituent effects, let's analyze the relative reactivity towards nitration for benzene (I), anisole (II), and chlorobenzene (III). Since nitration involves electrophilic aromatic substitution, electron-rich aromatic rings (activating substituents) will be more reactive than electron-poor rings (deactivating substituents). 1. Anisole (II): Due to the electron-donating OCH3 group, anisole (II) is more electron-rich and therefore more reactive towards nitration than benzene (I). 2. Chlorobenzene (III): The electron-withdrawing Cl group makes chlorobenzene (III) less electron-rich compared to benzene (I), thus making it less reactive towards nitration.

Determine the Correct Order of Reactivity

By analyzing the activating and deactivating substituent effects, we can now determine the correct order of increasing reactivity towards nitration: Chlorobenzene (III) < Benzene (I) < Anisole (II) This corresponds to option (B) in the given choices. Therefore, the correct answer is: (B) III < I < II

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a critical reaction type in organic chemistry, particularly when dealing with aromatic compounds like benzene. This process involves an electrophile, which has a strong desire for electrons, attacking an aromatic ring that has electrons to share. During the EAS process, the aromaticity of the ring is temporarily lost but is restored once the substitution is complete.

Imagine the process as a dance. The aromatic ring is like a stable platform where electrons are evenly distributed. An electrophile approaches, drawn by the electron cloud, and replaces one of the hydrogen atoms on the ring. This temporarily disrupts the stable electron cloud, but it quickly reforms, accommodating the new substituent.

In the case of nitration, which is a specific type of EAS, the electrophile is a nitronium ion ( NO₂⁺). This ion is hungry for electrons and approaches the aromatic ring, leading to the substitution of a hydrogen atom by a nitro group (NO₂). This reaction is typically facilitated by a mixture of concentrated nitric and sulfuric acids.

Substituent Effects on Reactivity

Substituent groups attached to an aromatic ring significantly influence its reactivity towards EAS reactions, such as nitration. These substituents can be broadly classified into two categories: electron-donating and electron-withdrawing groups.

Electron-donating groups (EDGs) like the OCH₃ group in anisole, push electron density towards the ring, effectively making the ring more attractive to electrophiles. These groups activate the ring for EAS by increasing its electron density. On the other hand, electron-withdrawing groups (EWGs) like the Cl in chlorobenzene pull electron density away from the ring, making it less attractive to electrophiles and consequently deactivating the ring for EAS.

• EDGs make the aromatic ring a better dancer, eager to interact more frequently.
• EWGs make the ring a reluctant dancer, less likely to engage in the dance of EAS.

During an EAS reaction, the activated ring with EDGs will react more readily compared to a deactivated ring with EWGs.

Electron-Donating and Electron-Withdrawing Groups

Let's dive deeper into the personalities of electron-donating and electron-withdrawing groups, which are like the social influencers of the aromatic ring community.

Electron-donating groups (EDGs), such as alkyl and methoxy groups, are like extroverts that bring more energy and electron density to the party, making the ring an appealing target for electrophiles. Common EDGs include groups like -OH, -OCH₃, and -NH₂. They often show this donating behavior through resonance or inductive effects, sharing their electrons with the rest of the aromatic circuit.

Electron-withdrawing groups (EWGs) are like introverts, pulling energy and electron density away, resulting in a more electron-deficient ring that's less appealing to electrophiles. These groups include nitro (-NO₂), cyano (-CN), and halogens (like Cl). Similarly, they exert their influence through resonance or inductive effects, but in contrast to EDGs, they withdraw electron density, stabilizing potential electrophilic attacks.

The nature of these groups is pivotal as they determine the ring's reactivity and the relative rate at which the EAS occurs. A ring donned with EDGs will engage in the EAS more swiftly and willingly compared to one shadowed by EWGs.