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2.2.12 (HL)—Benzene (C₆H₆)

Syllabus
First assessment 2025
Objective
2.2.12
Level
HL

Benzene as a Resonance Example

HL only

Benzene is an important resonance example: alternative ring structures place the double bonds in different positions, while the actual π electrons are delocalized across the ring.

each contributor has a six-carbon ring with three alternating double bonds; all three double bonds shift between contributors; each contributor has exactly six carbon-bound hydrogens; the structures are joined by a resonance arrow rather than an equilibrium arrow.
six parallel carbon p orbitals overlap sideways in the upper panel; the lower panel has continuous pi density above and below the ring plane; the six-carbon sigma framework and six hydrogens are preserved; the delocalized model does not show localized alternating pi bonds.

Use the resonance drawings as representations of one delocalized bonding system, not as separate rapidly changing molecular forms.

Benzene's six equal C–C bonds and greater stability than a localized triene support a delocalized π system above and below the ring. Use the two Kekulé drawings or a circle as representations of the same molecule, never as an equilibrium between two species.

Interpreting Benzene Resonance

HL only

[ 2 ]

The Styrene molecule is a derivative of benzene.

State both a chemical and physical reason structure A is a better representation of benzene.

Chemical reason:
Physical reason:

The Covalent Model Summary

Retrieve the covalent pathway: shared pairs and bond order lead to geometry, polarity and molecular polarity; structure determines network properties, IMF behaviour and chromatography; HL representations extend to resonance, formal charge, sigma/pi bonds and hybridization.

Check the representation first, then count domains, apply geometry, identify polarity or forces, and connect the structure to the requested property or HL bonding description.