Unit 17: Electrophilic Aromatic Substitution

EAS is the central reactivity pattern of benzene because it lets the ring react while preserving aromaticity; directing effects make it one of the most important selectivity problems in synthesis.

• Describe the general EAS mechanism and identify formation of the arenium ion as the key high-energy step.
• Predict how substituents alter reactivity and ortho/para versus meta orientation.
• Apply common reagent sets for nitration, sulfonation, halogenation, and Friedel–Crafts reactions.
• Recognize the main limitations of Friedel–Crafts alkylation and when acylation is preferable.

Checkpoint Questions

Q: Why does anisole react faster than benzene in nitration?

A: The methoxy group donates electron density by resonance, stabilizing the arenium ion and activating the ring.

Q: Why is nitrobenzene meta directing?

A: The nitro group withdraws electron density strongly, and ortho/para attack would place especially unfavorable positive charge next to the nitro-substituted carbon in key resonance forms.