Aromaticity & Electrophilic Aromatic Substitution

Hückel's rule — the four conditions

A ring is aromatic only if all of these hold:

• Cyclic
• Planar
• Fully conjugated (a continuous ring of p orbitals — every ring atom contributes a p orbital)
• Contains (4n + 2) π electrons where n = 0, 1, 2… (i.e. 2, 6, 10, 14…)

Worked classifications

• Benzene — 6 π e⁻ (n=1): aromatic.
• Cyclopentadienyl anion — 6 π e⁻: aromatic. (Neutral cyclopentadiene is not; the carbanion completes the sextet.)
• Tropylium cation (cycloheptatrienyl⁺) — 6 π e⁻: aromatic.
• Cyclobutadiene — 4 π e⁻: antiaromatic (very unstable).
• Pyridine — 6 π e⁻; the N lone pair is in an sp² orbital in the plane, NOT in the π system → aromatic and basic.
• Pyrrole — the N lone pair IS donated into the ring to make 6 π e⁻ → aromatic; that is why pyrrole N is weakly basic.
• Furan — O donates one lone pair into the ring → 6 π e⁻, aromatic.

Electrophilic aromatic substitution (EAS) — mechanism

• Step 1: electrophile E⁺ attacks the π system → arenium ion (σ-complex / Wheland intermediate), a resonance-stabilised carbocation. This is the slow, rate-determining step.
• Step 2: a base removes H⁺ from the sp³ carbon → aromaticity is restored (substitution, not addition).

Common EAS reactions & their electrophiles

• Nitration: HNO₃ / H₂SO₄ → electrophile is NO₂⁺ (nitronium ion).
• Halogenation: Cl₂ or Br₂ with a Lewis acid (FeCl₃, FeBr₃) → X⁺ equivalent.
• Sulfonation: fuming H₂SO₄ / SO₃ → SO₃/HSO₃⁺; notably reversible.
• Friedel–Crafts alkylation: R–X / AlCl₃ → R⁺ (carbocation; prone to rearrangement).
• Friedel–Crafts acylation: RCOCl / AlCl₃ → acylium ion RCO⁺ (no rearrangement; gives a ketone).

Directing & activating effects

• Activators = ortho/para directors (electron-donating groups, EDG): −NH₂, −OH, −OR, −NHCOR, −R (alkyl). They raise ring electron density and stabilise the arenium ion at o/p positions.
• Deactivators = meta directors (electron-withdrawing groups, EWG): −NO₂, −C≡N, −SO₃H, −COOH, −COR, −CHO, −NR₃⁺. They pull density out and direct the next group meta.
• Halogens are the exception: −F, −Cl, −Br, −I are ortho/para directing but deactivating (inductive withdrawal deactivates; lone-pair resonance directs o/p).

30-second recall table