Aromatic Hydrocarbons

Aromatic hydrocarbons center on benzene (c1ccccc1) and its derivatives. Benzene has six equal C-C bonds (1.39 Angstrom, intermediate between single 1.54 and double 1.34 Angstrom), with delocalized $\pi$ electrons above and below the planar ring. Kekule proposed alternating single-double bond structures, but the actual structure is a resonance hybrid with complete $\pi$-electron delocalization.

Aromaticity (Huckel Rule): A compound is aromatic if it is planar, cyclic, fully conjugated (every atom in the ring has a p-orbital), and has (4n+2) $\pi$ electrons (n = 0, 1, 2...).
For benzene, n = 1 gives 6 $\pi$ electrons. Anti-aromatic compounds have 4n $\pi$ electrons in a planar, cyclic, conjugated system (e.g., cyclobutadiene with 4 $\pi$ electrons). Cyclooctatetraene (8 $\pi$ electrons) is non-aromatic because it adopts a non-planar tub shape to avoid anti-aromaticity.

Electrophilic Aromatic Substitution (EAS) — General Mechanism: (1) Generation of electrophile (E+); (2) Electrophilic attack on the $\pi$ cloud forming the arenium ion ($\sigma$ complex / Wheland intermediate — a carbocation where aromaticity is temporarily lost); (3) Loss of H+ (rearomatization restoring the stable aromatic sextet). The five major EAS reactions are:

Halogenation: C₆H₆ + Cl₂/Br₂ → ArCl/ArBr + HX, using Lewis acid catalyst FeCl₃/FeBr₃/AlCl₃, which generates X+ electrophile. Nitration: C₆H₆ + HNO₃ (conc. H₂SO₄) → ArNO₂ + H₂O; electrophile = NO₂+ (nitronium ion) generated by H₂SO₄ protonating HNO₃. Sulfonation: C₆H₆ + fuming H₂SO₄ → ArSO₃H; electrophile = SO₃; this is the ONLY reversible EAS reaction (dilute H₂SO₄ + high temperature removes -SO₃H). Friedel-Crafts Alkylation: C₆H₆ + RCl → ArR + HCl (AlCl₃ catalyst); electrophile = R+ (carbocation); limitations include polyalkylation (product is more reactive than benzene) and carbocation rearrangement. Friedel-Crafts Acylation: C₆H₆ + RCOCl → ArCOR + HCl (AlCl₃ catalyst); electrophile = RCO+ (acylium ion); advantage — no polyacylation (the -COR product is deactivated) and no rearrangement of the acylium ion.

Directive Effects: Ortho-para directors are electron-donating groups that activate the ring: -OH (), -NH₂ (), -CH₃ (), -OCH₃, -NHR, -NR₂. They increase electron density at ortho and para positions via +M or +I effects. Critical exception: Halogens (-F, -Cl, -Br, -I) are o/p directors BUT deactivating — they withdraw electrons by -I effect (overall deactivating) but donate by +M effect to o/p positions (directing). Meta directors are electron-withdrawing groups that deactivate the ring: -NO₂ (), -CN, -CHO, -COR, -COOH, -SO₃H. They destabilize the arenium ion at o/p positions more than at meta through -M effect.

The key testable concept is that halogens are ortho-para directors despite being deactivating (unique among directing groups), and that Friedel-Crafts acylation avoids both rearrangement and polysubstitution — making it preferred over alkylation.