Concept:In Friedel-Crafts acylation, the reaction proceeds through a distinct, non-aromatic cationic intermediate before restoring aromaticity.
Solution:- Propanoyl chloride reacts with a Lewis acid to form an acylium electrophile (\( \text{CH}_3\text{CH}_2\text{C}^+=\text{O} \)).
- When the benzene pi ring attacks this electrophile, a resonance-stabilized carbocation is formed within the ring.
- This intermediate—where one carbon becomes \( \text{sp}^3 \) hybridized holding both an H atom and the new acyl group—is called a \( \sigma \)-complex, arenium ion, or benzenonium ion.
Why other options are incorrect:- The acylium ion is the electrophile, but the actual intermediate product formed with benzene is the \( \sigma \)-complex.
- A phenyl radical belongs to free radical mechanisms, not electrophilic substitution.
- A simple primary carbocation is not formed in aromatic acylation.
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