Reference Reaction Summary Tables Electrophilic Aromatic Substitution
Electrophilic Aromatic Substitution
Estimated reading time: 2 min
In this section
- Overview
- Substitution and Elimination (SN1, SN2, E1, E2)
- Addition to Alkenes and Alkynes
- Nucleophilic Addition to Aldehydes and Ketones
- Nucleophilic Acyl Substitution (Carboxylic Acid Derivatives)
- Enols, Enolates, and Carbon–Carbon Bond Formation
- Electrophilic Aromatic Substitution
- Addition vs. Substitution at Carbonyls — A Direct Comparison
- Cross-References
| Reaction | Typical Reagents/Conditions | Electrophile Generated | Product |
|---|---|---|---|
| Halogenation | X₂, FeX₃ (Lewis acid catalyst) | X⁺ (halogenium, activated by the Lewis acid) | Aryl halide |
| Nitration | HNO₃, H₂SO₄ | NO₂⁺ (nitronium ion) | Nitrobenzene derivative |
| Sulfonation | SO₃, H₂SO₄ (fuming sulfuric acid) | SO₃ (electrophilic sulfur) | Benzenesulfonic acid (reversible — can be removed later by treatment with dilute aqueous acid) |
| Friedel-Crafts alkylation | R–X, AlCl₃ | R⁺ (carbocation, or a polarized complex) | Alkylbenzene; carbocation rearrangements possible; over-alkylation possible since the product is more electron-rich than the starting arene |
| Friedel-Crafts acylation | R–COCl, AlCl₃ | Acylium ion (R–C≡O⁺) | Aryl ketone; no rearrangement (acylium is resonance-stabilized); no over-acylation (the ketone product deactivates the ring) |
Directing and Activating Effects (Chapter 17)
| Substituent Type | Examples | Effect on Reactivity | Directs To |
|---|---|---|---|
| Strong activators | –OH, –NH₂, –OR | Strongly activating (lone pair donates into the ring by resonance) | Ortho/para |
| Weak activators | Alkyl groups (–CH₃, etc.) | Mildly activating (inductive electron donation, hyperconjugation) | Ortho/para |
| Deactivating, o/p-directing | Halogens (–F, –Cl, –Br, –I) | Deactivating overall (inductively electron-withdrawing) but still ortho/para-directing (lone pairs can still donate by resonance) | Ortho/para |
| Deactivating, meta-directing | –NO₂, –C≡N, –COOH, –CHO, –C(=O)R, –SO₃H | Strongly deactivating (withdraw electron density by resonance and induction, with no lone pair to donate back) | Meta |
Why halogens are the exception to the activating/directing correlation: halogens withdraw electron density inductively (deactivating the ring overall) but still possess lone pairs that can donate into the ring by resonance at the ortho/para positions specifically — enough to direct substitution there, even though the net effect on the ring is deactivating. See Chapter 17 for the full resonance argument.