Shapes of aromatic organic molecules; σ and π bonds

Full Notes

Benzene and its structure has been covered in more detail here.
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Shape and Bonding in Benzene

Benzene (C₆H₆) has a planar, cyclic structure with six carbon atoms arranged in a regular hexagon.

• All C–C bond lengths are equal, approximately 0.140 nm — intermediate between a single and double bond.
• Bond angles are 120°, giving trigonal planar geometry.

Hybridisation and σ Bonds

Each carbon in benzene is sp² hybridised:

Three sp² hybrid orbitals form σ bonds:

• Two with adjacent carbon atoms.
• One with a hydrogen atom.

These σ bonds create a strong hexagonal framework.

Delocalised π Electron System

Each carbon atom has one unhybridised p orbital perpendicular to the plane of the ring.

These p orbitals overlap sideways with adjacent p orbitals and form a delocalised π system (a ring of electron density above and below the plane).

• This delocalisation gives benzene its exceptional stability (known as aromatic stability or resonance energy).

Consequences of Delocalisation

• Benzene does not behave like an alkene despite being unsaturated, it does not undergo addition reactions easily.
• It undergoes electrophilic substitution to preserve the delocalised π system.
• The delocalisation of electrons also contributes to the planar shape and equal carbon-carbon bond lengths.

Summary

• Benzene’s structure is a perfectly flat hexagon.
• All carbon atoms are sp² hybridised.
• The π electrons are delocalised over the entire ring, stabilising the molecule.
• This structure is common in many aromatic compounds, including substituted benzenes and polycyclic aromatics.