Why is benzene so unreactive?
• Benzene is exceptionally stable. Heats of hydrogenation • *Why is benzene so stable? Aromaticity • *Resonance is only part of the answer. • *Cyclobutadiene is very unstable in spite of resonance. • • *The cyclopropenium ion is surprisingly stable. • • *The cycloheptatrienylium ion is very stable but the corresponding anion is very unstable. • • • • *The cyclopentadienide ion is very stable, but the corresponding cation is very unstable. • • • • *An odd number of delocalized pairs of electrons (4n+2 π electrons) in a continuous ring of atoms with an unhybridized p orbital on each atom is exceptionally stable – Aromatic. • *Delocalized electrons can include unshared pairs. • Pyrrole is aromatic. • • Continuous ring can include carbocation or carbanion. • *There can be more than one ring involved as long as there is an unhybridized p orbital on each atom. • Naphthalene is aromatic • is not aromatic. • is aromatic. • *Need to be sure that orbitals are available for delocalization of unsh
