29.3 Shapes of aromatic organic molecules; σ and π bonds
- Syllabus
- 9701–2028–2029
- Topic
- 29.3
- Level
- A2
Learning objectives
sp² ring carbons create a planar σ framework and one delocalised π system
Each carbon in benzene is sp² hybridised. Its three sp² orbitals form three σ bonds in a trigonal-planar arrangement with bond angles of about 120°, so the six carbon atoms and their attached hydrogen atoms form a planar hexagonal σ framework.
| Bonding part | Orbitals and overlap | Position of electron density | Role in benzene |
|---|---|---|---|
| σ framework | end-on overlap of sp² orbitals for C–C and sp² with H 1s for C–H | along internuclear axes, in the molecular plane | fixes the six-membered planar skeleton; 6 C–C σ and 6 C–H σ bonds |
| delocalised π system | sideways overlap of one unhybridised p orbital on every carbon | continuous regions above and below the ring plane | spreads six π electrons over all six carbon atoms |
sp² hybridisation first provides coplanar σ-bond directions and leaves one p orbital perpendicular to the plane on each carbon. Because all six p orbitals are parallel and adjacent, they overlap around the whole ring; this continuous overlap produces the delocalised π system.
Delocalisation makes all six C–C bonds equivalent, with bond length and character intermediate between localised C–C single and C=C double bonds. Benzene is therefore not three independent alkene units.
The same test applies to other aromatic ring systems: the atoms participating in the conjugated ring must provide aligned adjacent p orbitals for continuous delocalisation. An attached substituent or non-ring atom is not automatically in the same plane unless its own bonding requires it.
Planarity enables parallel p-orbital overlap; it is misleading to say that delocalisation alone creates the sp² geometry. The circle in a benzene symbol represents the delocalised π system, not an extra atom, bond or electron shell.