Aromaticity

The problem with Kekulé

Kekulé's structure, cyclohexa-1,3,5-triene, predicts alternating short C=C and long C–C bonds and alkene-like addition reactions. Experiments disagree:

• Bond lengths: X-ray diffraction shows all six C–C bonds are 0.139 nm, between C–C (0.154 nm) and C=C (0.134 nm).
• Enthalpy of hydrogenation: cyclohexene gives –120 kJ mol^{-1}; three times that predicts –360 kJ mol^{-1} for Kekulé benzene, but benzene releases only about –208 kJ mol^{-1}. Benzene is roughly 150 kJ mol^{-1} more stable than expected: the delocalisation (resonance) energy.
• Reactivity: benzene resists addition and undergoes substitution, which keeps the ring intact.

The delocalised model

Each carbon is sp^2 with bond angles of 120°, forming σ bonds to two carbons and one hydrogen. The remaining p orbital on each carbon overlaps sideways with both neighbours, giving a ring of π electron density above and below the plane. The six π electrons are delocalised over all six carbons, drawn as a circle inside the hexagon.

Consequences

Electrophilic substitution (nitration with HNO_3/H_2SO_4, Friedel–Crafts acylation with RCOCl/AlCl_3, halogenation with Br_2/FeBr_3) needs a strong electrophile, because the delocalised π system is less electron-dense than an alkene's localised π bond. After the electrophile attacks, loss of H^+ regenerates the stable aromatic ring.

Phenol and phenylamine react much more readily (phenol decolourises bromine water at room temperature, giving 2,4,6-tribromophenol) because a lone pair on O or N is partly delocalised into the ring, increasing its electron density.

Hückel's rule (extension)

A ring is aromatic if it is cyclic, planar, fully conjugated and has 4n + 2 π electrons. Benzene, with 6 π electrons, fits for n = 1.

Hückel's rule

A monocyclic, planar, fully conjugated system with 4n + 2 π electrons is aromatic; one with 4n π electrons is antiaromatic. Hückel MO theory explains why: for a ring of N p orbitals the energies are

giving a single lowest level then degenerate pairs (the Frost circle). 4n + 2 electrons exactly fill a closed shell of bonding MOs; 4n leave two electrons unpaired in a degenerate pair (or, after distortion, a small HOMO–LUMO gap), destabilising the system.

Applying the rule

• Ions: cyclopropenyl cation (2 π e), cyclopentadienyl anion (6), cycloheptatrienyl (tropylium) cation (6) are aromatic. This is why cyclopentadiene is unusually acidic (pKa ~16) and tropylium bromide is an ionic salt.
• Antiaromatic: cyclobutadiene (4 π e) is highly reactive and rectangular; the cyclopentadienyl cation is destabilised.
• Non-aromatic: cyclooctatetraene (8 π e) escapes antiaromaticity by adopting a tub shape, behaving as a polyene; its dianion (10 π e) is planar and aromatic.
• Annulenes: [18]annulene is aromatic; [10]annulene is not planar because inner hydrogens clash.
• Polycyclics: naphthalene and anthracene are aromatic but with less resonance energy per ring than benzene; reactions occur at specific positions that preserve one intact benzene ring.

Heteroaromatics

Count which lone pairs are in the π system:

• Pyridine: N contributes one electron to the π system; its lone pair is in an sp^2 orbital in the plane, so pyridine is basic (pKaH 5.2) and electron-poor (resists electrophilic substitution, undergoes nucleophilic substitution at C2/C4).
• Pyrrole: the N lone pair is part of the sextet, so pyrrole is barely basic and very electron-rich (electrophilic substitution at C2). Furan and thiophene are analogous, with one O or S lone pair in the ring and one in plane.
• Imidazole has one pyrrole-type and one pyridine-type N, making it a good base and nucleophile (as in histidine).

Experimental criteria

• Energetic: resonance energies from hydrogenation or isodesmic reactions.
• Structural: bond-length equalisation (HOMA index).
• Magnetic: the π electrons sustain a diatropic ring current in a magnetic field, deshielding protons outside the ring (benzene at 7.3 ppm) and shielding any inside it. In [18]annulene inner protons appear near –3 ppm. Antiaromatic rings show paratropic currents. NICS (nucleus-independent chemical shift) calculations quantify this: negative values indicate aromaticity.

Reactivity

Electrophilic aromatic substitution proceeds through a Wheland (σ-complex) intermediate, restoring aromaticity by proton loss. Substituents direct by stabilising this cation: donors (–OH, –NH_2, alkyl) activate and direct ortho/para; acceptors (–NO_2, –CO_2R) deactivate and direct meta; halogens deactivate but direct ortho/para.