2.2.13Additional lessons
Benzene (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 37 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Structure of Benzene
Benzene is the standard example of a delocalised ring structure.
Formula
- Formula The molecular formula is C₆H₆, with six carbon atoms in a flat ring.
Shape
- Shape The ring is planar with all bond angles at 120°.
Bond lengths
- Bond lengths All six C–C bonds are 140 pm, between a single bond at 154 pm and a double bond at 134 pm.
Kekulé structure
- Kekulé structure Alternating single and double bonds can be drawn, but they do not match the equal bond lengths.
Section 2 of 5Delocalised Electrons
Delocalised Electrons
Sigma framework
- Sigma framework Each carbon forms three sigma bonds: two to carbon neighbours and one to hydrogen.
p orbitals
- p orbitals The fourth electron on each carbon sits in a p orbital perpendicular to the ring.
Overlap
- Overlap These six p orbitals overlap sideways to form a continuous ring of electron density above and below the plane.
Representation
- Representation Benzene is drawn as a hexagon with a circle inside to show the delocalised electrons.
Section 3 of 5Evidence for Delocalisation
1
X-ray diffraction shows all C–C bonds in the ring have the same length of 140 pm.
2
Cyclohexene releases about 120 kJ mol⁻¹ when hydrogenated, so three isolated C=C bonds would release about 360 kJ mol⁻¹.
3
Benzene actually releases only about 208 kJ mol⁻¹ when hydrogenated to cyclohexane.
4
The difference of about 152 kJ mol⁻¹ shows benzene is more stable than the Kekulé structure suggests.
5
Benzene prefers substitution reactions to addition, because addition would destroy the stable delocalised ring.
Section 4 of 5Consequences
Stability
- Stability The delocalisation energy makes benzene resistant to attack compared with an alkene.
Bromine test
- Bromine test Benzene does not decolourise bromine water as alkenes do.
Substitution
- Substitution Hydrogen atoms are replaced by other groups while the ring remains intact.
| Property | Alkene C=C | Benzene ring |
|---|---|---|
| Bond length / pm | 134 | 140 for every bond |
| Reaction with bromine water | Decolourises | No reaction |
| Typical reaction | Addition | Substitution |
Section 5 of 5Exam Checklist & Tips
2.2.13 – Key Points for the Exam
- Delocalised electrons Electrons shared over several atoms rather than between two.
- Kekulé structure A structure of benzene with alternating single and double bonds.
- Planar Lying entirely in one flat plane.
- Delocalisation energy The extra stability gained through delocalisation of electrons.
Common Mistakes
- Saying benzene has alternating single and double bonds that swap quickly.
- Giving the bond length as 154 pm or 134 pm instead of 140 pm.
- Claiming benzene decolourises bromine water.
- Forgetting that every carbon also forms one C–H sigma bond.
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