Aromatic Chemistry
Aromaticity is one of the most fundamental properties in organic chemistry. Aromatic compounds exhibit exceptional thermodynamic stability and characteristic reactivity, most notably electrophilic aromatic substitution (EAS).
Criteria for Aromaticity: Hückel's Rule
A monocyclic planar ring system is aromatic if it satisfies all four Hückel criteria:
1. Planar and cyclic: all ring atoms lie in the same plane. 2. Fully conjugated: each ring atom has a p orbital perpendicular to the plane. 3. (4n+2) rule: the number of π electrons is 4n+2 where n is a non-negative integer (0, 1, 2, …). 4. Delocalised lone pair or vacancy: heteroatoms (N, O, S) can contribute their lone pair.
| System | π electrons | n | Aromatic? |
|---|---|---|---|
| Benzene (C₆H₆) | 6 | 1 | Yes |
| Cyclopentadienyl anion (C₅H₅⁻) | 6 | 1 | Yes |
| Cycloheptatrienyl cation (tropylium, C₇H₇⁺) | 6 | 1 | Yes |
| Cyclobutadiene (C₄H₄) | 4 | — | Antiaromatic |
| Pyridine (C₅H₅N) | 6 | 1 | Yes (N contributes 0 π e⁻; lone pair in plane) |
| Pyrrole (C₄H₄NH) | 6 | 1 | Yes (N contributes 2 π e⁻) |
EAS Mechanism: General Scheme
EAS proceeds in two steps:
1. σ-complex (Wheland intermediate / arenium ion) formation: the electrophile E⁺ attacks the ring, breaking the aromatic delocalisation. A cationic intermediate is formed with positive charge delocalised over the ring (ortho and para positions relative to the attack site). 2. Restoration of aromaticity: a base (often the counter-ion) removes a proton from the attacked carbon, restoring the π system.
Overall: Ar−H + E⁺ → Ar−E + H⁺ (substitution, not addition).
Canonical reactions: - Nitration: HNO₃/H₂SO₄, electrophile NO₂⁺ (nitronium ion). - Halogenation: Cl₂/FeCl₃ or Br₂/FeBr₃, electrophile X⁺ (halogenium activated by Lewis acid). - Sulfonation: fuming H₂SO₄, electrophile SO₃. - Friedel-Crafts alkylation: R−X/AlCl₃, electrophile R⁺. - Friedel-Crafts acylation: R−COCl/AlCl₃, electrophile RC≡O⁺ (acylium).
Directing Effects of Substituents
Substituents on the ring direct the incoming electrophile and either activate or deactivate the ring:
| Substituent type | Effect on ring | Orientation |
|---|---|---|
| σ and π donors (−OH, −NH₂, −OR, −alkyl) | Activating | ortho/para |
| Halogens (−F, −Cl, −Br, −I) | Weakly deactivating (−I effect > +M effect) | ortho/para |
| Electron withdrawers (−NO₂, −CF₃, −COOH, −CHO) | Strongly deactivating | meta |
Mechanistic reasoning: a donor substituent stabilises ortho and para σ-complexes (positive charge on the carbon bearing the substituent) more than the meta σ-complex → ortho/para products predominate.
Heteroaromatic Compounds
Pyrrole and pyridine illustrate two contrasting cases:
- Pyrrole: the nitrogen lone pair is part of the π system (aromatic 6π). It is a very weak base (conjugate acid pKa ≈ −3.8). EAS is very fast (hyperactivated ring), predominantly at position 2.
- Pyridine: the nitrogen lone pair is in the plane (sp² orbital) — N acts as a strong electron-withdrawing substituent that deactivates the ring. EAS is slow and occurs preferentially at the meta position relative to N (position 3).
Aromatic chemistry is ubiquitous in dyes, pharmaceuticals, conducting polymers, and functional materials.