Aromatic compounds

Resistance to reaction - benzene does not decolourise Br2 at room temperature

Benzene does not contain C=C bonds
Less reactive than expected

Bond lengths -
All C to C bonds the same
C to C bond length is between that of the C-C and C=C

Benzene does not contain isolated, alternating C-C or C=C bonds

Enthalpy change for hydrogenation (3H2) is less exothermic than enthalpy change predicted for triene

Benzene does not contain 3 C=C bonds - not a triene
Benzene is more stable than expected due to resonance

Delocalised structure of benzene

Cyclic molecule - closed ring of 6C
Trigonal planar
Sigma bonds between the C atoms and between the C-H atoms - single direct overlap of orbitals directly between bonding atoms
Pi system above and below plane of C ring - double sideways overlap of p orbitals on each C atom of C ring
Pi system consists of 6 delocalised electrons which are mobile across all 6C atoms of benzene ring