Concept:Electrophilic aromatic sulfonation relies on a neutral, heavily electron-deficient molecule rather than a positively charged ion as its primary electrophile.
Formula:$$ 2\text{H}_2\text{SO}_4 \rightleftharpoons \text{H}_3\text{O}^+ + \text{HSO}_4^- + \text{SO}_3 $$
Solution:- In concentrated or fuming sulfuric acid, an auto-ionization/dehydration equilibrium produces free sulfur trioxide (\( \text{SO}_3 \)).
- In \( \text{SO}_3 \), the highly electronegative oxygen atoms pull electron density intensely away from the central sulfur atom.
- This renders the sulfur atom highly electron-deficient, making the neutral \( \text{SO}_3 \) molecule a powerful electrophile capable of attacking the benzene ring.
Why other options are incorrect:- \( \text{H}_2\text{SO}_4 \) is the bulk solvent/reagent, not the active electrophile.
- \( \text{HSO}_4^- \) is an anion and acts as a base, not an electrophile.
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