Explain benzene shape and delocalised bonding
Connect benzene's planar hexagonal shape and sp² hybridisation with σ bonds and a delocalised π system.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 29. Follow the formula, naming and optical-isomerism limits stated in the syllabus. Benzene-ring displayed formulas are not expected; use the supplied convention for aromatic skeletal formulas. Do not extend the optical-isomerism scope to naming diastereoisomers or meso compounds. Written practice is self-checked
Before you begin
- Use covalent formula conventions, functional groups, reaction terminology, and the AS organic chemistry foundations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the approximate bond angle around an sp²-hybridised carbon in benzene?
What you will learn
- Describe benzene as planar and hexagonal
- Relate sp² hybridisation to the σ-bond framework
- Explain the delocalised π system from overlapping p orbitals
Build the σ framework
Each benzene carbon is sp² hybridised. Three sp² orbitals form σ bonds: two to adjacent ring carbons and one to hydrogen or a substituent. The arrangement is trigonal planar around each carbon, so the ring is planar and the bond angles are approximately 120°.
A regular six-carbon ring gives benzene its hexagonal skeletal outline. The σ bonds form the framework; they are distinct from the π electrons discussed next.
Delocalised π electrons extend around the ring
Each sp² carbon has one unhybridised p orbital perpendicular to the ring plane. Sideways overlap of the six p orbitals creates a delocalised π system above and below the ring.
The π electrons are shared around the ring rather than confined to three isolated double bonds. Use a consistent aromatic representation; the syllabus does not require drawing benzene as a displayed formula.
Pause and explain
How does benzene form a delocalised π system?
Worked example
Explain why benzene is planar and has a delocalised π system.
Show the worked solution
- Each carbon is sp² hybridised, giving a trigonal planar σ-bond arrangement.
- The six carbon atoms form a planar ring.
- One p orbital on each carbon overlaps sideways to make a delocalised π system above and below the ring.
Answer Benzene is planar because its sp²-hybridised carbons form a trigonal planar σ framework; perpendicular p orbitals overlap to form a delocalised π system around the ring.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Separate the roles of σ and π bonding in benzene.
Give me a hint
State where each framework lies and which orbitals form it.
Compare my reasoning
- sp² orbitals make the in-plane σ framework.
- Unhybridised p orbitals overlap sideways.
- The resulting π electron density is above and below the ring.
The σ bonds form the planar ring framework; sideways overlap of p orbitals creates a delocalised π system above and below it.
Look for these in your work
- σ role stated
- p-orbital overlap stated
- π location stated
Why is a 109.5° angle a poor description of each benzene carbon?
Give me a hint
Link the angle to the hybridisation and geometry.
Compare my reasoning
- Benzene carbon is sp² hybridised.
- Its three σ-bond directions are trigonal planar.
- The approximate angle is 120°, not tetrahedral 109.5°.
The 109.5° angle describes tetrahedral sp³ geometry; benzene's sp² carbons are trigonal planar at about 120°.
Look for these in your work
- Hybridisation identified
- Geometry distinguished
- Angle corrected
A student explains benzene's extra stability using three isolated C=C bonds and no shared π electrons. Correct the model at syllabus level.
Give me a hint
Distinguish the σ skeleton from the shared p-orbital system.
Compare my reasoning
- The six-carbon ring has a σ framework.
- Each carbon contributes a p orbital.
- Their overlap forms a delocalised π system rather than three isolated pairs.
Benzene has a planar σ framework and a delocalised π system formed by overlap of six p orbitals; its π electrons are shared around the ring rather than confined to three isolated double bonds.
Look for these in your work
- σ framework included
- p orbitals included
- Delocalisation explained
Common mistakes
- Calling the π electrons localised between three fixed pairs only
- Mixing up σ bonds in the ring plane with the π system above and below
- Using tetrahedral geometry for sp² carbon
- Treating aromatic skeletal notation as proof of isolated double bonds
What can you explain now?
What is the approximate bond angle around an sp²-hybridised carbon in benzene?
Compare with the explanation
120°
Three electron regions in a trigonal planar arrangement give about 120°.
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Make it stick
How do sp² hybridisation and p-orbital overlap explain benzene's shape and bonding?
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