: 220
Aromaticity requires four criteria: cyclic, planar, fully conjugated, and (4n+2) pi electrons. Aromatic species are exceptionally stable; anti-aromatic species (4n pi electrons, planar, cyclic, conjugated) are extremely unstable. Key aromatic species for JEE: benzene (6 pi e-), naphthalene (10), tropylium cation C7H7+ (6), cyclopentadienyl anion C5H5- (6), cyclopropenyl cation C3H3+ (2), pyridine (6 — N lone pair NOT in pi system), and pyrrole (6 — N lone pair IS in pi system). Anti-aromatic species: cyclobutadiene (4 pi e-), cyclopropenyl anion (4), cyclopentadienyl cation (4). Non-aromatic: cyclooctatetraene (8 pi e-, adopts tub shape to AVOID anti-aromaticity), cyclohexane (not conjugated). Aromaticity profoundly impacts acidity and stability: cyclopentadiene is unusually acidic (pKa ~15) because its conjugate base is aromatic. Tropylium cation is easily formed because losing one electron from cycloheptatriene gives an aromatic system. Conversely, cyclopropenyl anion is extremely difficult to form because it would be anti-aromatic. The concept of aromaticity extends to heterocyclic compounds — pyridine (6 pi electrons, basic due to sp2 lone pair in plane) and pyrrole (6 pi electrons including N lone pair, very weak base).