NUCLEOPHILIC AROMATIC SUBSTITUTION
Nucleophilic Aromatic Substitution (NAS) is a reaction where a nucleophile replaces a leaving group on an aromatic ring. In simple terms, a nucleophile (electron-rich species) attacks the aromatic ring and pushes out the leaving group.
This reaction happens mainly when the aromatic ring has electron-withdrawing groups that make the ring electron-poor. A common leaving group in these reactions is a halide such as chloride or fluoride.
Conditions Required for Nucleophilic Aromatic Substitution
- A strong nucleophile must be present. It donates electrons to bond with the carbon of the ring.
- The aromatic ring must be electron-deficient. This is usually due to an electron-withdrawing group like NO₂, CN, or COOH.
- The leaving group is typically a halide (Cl⁻, Br⁻, F⁻).
- The nucleophile attacks the carbon where the leaving group is present.
Example
1-Chloro-4-nitrobenzene reacts with sodium methoxide to form 1-methoxy-4-nitrobenzene:
Cl-(C₆H₄)-NO₂ + CH₃ONa → CH₃O-(C₆H₄)-NO₂ + NaCl
Mechanism of Nucleophilic Aromatic Substitution
There are several possible mechanisms for NAS, but the most common one is the Addition–Elimination Mechanism.
This mechanism is somewhat similar to electrophilic aromatic substitution, but the intermediate formed is an anion instead of a carbocation.
Steps in the Addition–Elimination Mechanism
- Addition Step: The nucleophile attacks the carbon attached to the leaving group, forming a negatively charged intermediate (Meisenheimer complex).
- Elimination Step: The leaving group is expelled, and aromaticity is restored.
Electron-withdrawing groups such as nitro (NO₂) strongly activate the ring by stabilizing this anionic intermediate. That is why NAS works better when such groups are present.
In simple words: The nucleophile first attacks, forms a temporary anionic structure, and then the leaving group leaves.
Detailed Notes:
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