<<–2/”>a href=”https://exam.pscnotes.com/5653-2/”>p>world of electrophilic and nucleophilic aromatic substitution reactions.
Introduction
Aromatic compounds, characterized by their stability and unique reactivity due to the delocalized pi-electron system, undergo substitution reactions where an atom or group on the aromatic ring is replaced by another. These reactions can be broadly classified into two types: electrophilic aromatic substitution (EAS) and nucleophilic aromatic substitution (NAS).
Key Differences: EAS vs. NAS
| Feature | Electrophilic Aromatic Substitution (EAS) | Nucleophilic Aromatic Substitution (NAS) |
|---|---|---|
| Attacking Species | Electrophile (electron-deficient species) | Nucleophile (electron-rich species) |
| Aromatic Ring | Acts as a nucleophile (electron-rich) | Acts as an electrophile (electron-deficient due to EWGs) |
| Mechanism | Two-step: (1) Formation of arenium ion, (2) Deprotonation | Varies: Addition-Elimination (most common), Benzyne, SRN1 |
| Substituent Effects | Electron-donating groups (EDGs) activate, EWGs deactivate | Electron-withdrawing groups (EWGs) activate, EDGs deactivate |
| Examples | Nitration, Halogenation, Friedel-Crafts Alkylation/Acylation | Reaction of chlorobenzene with NaOH under harsh conditions (SNAr) |
Advantages and Disadvantages
| Reaction Type | Advantages | Disadvantages |
|---|---|---|
| EAS | – Versatile for introducing a wide variety of functional groups | – Often requires catalysts (e.g., Lewis acids) |
| – Mild reaction conditions in many cases | – Regioselectivity can be challenging (mixture of ortho/para products) | |
| NAS | – Useful for introducing specific functional groups under certain conditions | – Typically requires harsh reaction conditions (high temperature/pressure) |
| – Can provide access to otherwise difficult-to-synthesize compounds | – Limited substrate scope (often requires activated aromatic rings) |
Similarities
- Both involve the replacement of an atom or group on the aromatic ring.
- Both proceed through intermediates that disrupt the aromaticity of the ring, which is later restored.
- Both are influenced by the nature and position of substituents on the aromatic ring.
FAQs
1. Why are EWGs required for NAS?
Electron-withdrawing groups (EWGs) like nitro
64 288 64S117.2 64 74.6 75.5c-23.5 6.3-42 24.9-48.3 48.6-11.4 42.9-11.4 132.3-11.4 132.3s0 89.4 11.4 132.3c6.3 23.7 24.8 41.5 48.3 47.8C117.2 448 288 448 288 448s170.8 0 213.4-11.5c23.5-6.3 42-24.2 48.3-47.8 11.4-42.9 11.4-132.3 11.4-132.3s0-89.4-11.4-132.3zm-317.5 213.5V175.2l142.7 81.2-142.7 81.2z"/> Subscribe on YouTube