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Step-by-Step Explanation
1. Understanding the σ-Complex (Arenium Ion)
During electrophilic aromatic substitution, the attacking electrophile ($E^+$) forms a transient intermediate with the benzene ring called the σ-complex (also known as the arenium ion). This intermediate is crucial in determining the overall energy and stability of the reaction pathway.
2. Influence of Substituents
A substituent on the benzene ring can either donate or withdraw electron density from the ring. A strongly electron-withdrawing group (EWG), such as the nitro group ($-NO_2$), decreases the electron density on the ring. As a result, the formation of the positively charged σ-complex is less stabilized in the presence of EWGs.
3. Comparison of Nitrobenzene vs. Benzene
Nitrobenzene ($-NO_2$ group present): The $-NO_2$ group withdraws electron density from the ring, making the intermediate σ-complex less stable. This effect applies whether the nitro group is in the ortho, meta, or para position.
Benzene (no substituents): In unsubstituted benzene, no strong electron-withdrawing or electron-donating group is present. The ring retains more electron density, stabilizing the positively charged σ-complex more effectively.
4. Lowest Energy σ-Complex
Because the intermediate formed from benzene does not suffer from the electron-withdrawing effect of a substituent, it remains at a lower overall energy compared to any σ-complex formed in nitrobenzene. Therefore, the σ-complex with unsubstituted benzene is the most stable (lowest energy) among the given options.
5. Conclusion
Hence, the correct answer is the structure where the electrophile attacks pure benzene (option labeled in the question as the correct answer). Its σ-complex is the most stable due to the absence of the $-NO_2$ group.
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