The structure and stability of benzene

Thermochemical evidence (spec 18.1)

Hydrogenating cyclohexene (one C=C) releases about 120 kJ mol^-1. A Kekulé benzene with three C=C bonds would be expected to release about 3 x 120 = 360 kJ mol^-1. The measured value for benzene is only about 208 kJ mol^-1. Benzene is therefore about 150 kJ mol^-1 more stable than the Kekulé structure predicts: this is its delocalisation (stabilisation) energy. On an energy level diagram benzene sits about 150 kJ mol^-1 below the hypothetical cyclohexa-1,3,5-triene.

X-ray and infrared evidence

• X-ray diffraction shows benzene is a planar regular hexagon with all six C-C bonds the same length, 0.139 nm, between C-C (0.154 nm) and C=C (0.134 nm). Kekulé would give two different lengths.
• Infrared: benzene does not show the alkene C=C stretching absorption expected for isolated double bonds; its ring absorptions are different from those of alkenes.

The delocalised model (spec 18.2)

Each carbon uses three electrons in σ bonds (to two carbons and one hydrogen), with bond angles of 120°. The fourth electron is in a p orbital at right angles to the ring. The six p orbitals overlap sideways to form a ring of π electron density above and below the plane, containing six delocalised electrons. Benzene is drawn as a hexagon with a circle inside, or as the Kekulé structure.

Resistance to addition (spec 18.3)

Alkenes decolourise bromine water at room temperature by electrophilic addition. Benzene does not. Its π electrons are spread over six carbons, so the electron density between any two carbons is lower than in a C=C bond and it polarises bromine less. Addition would also destroy the stable delocalised system, so benzene prefers substitution, which keeps the ring intact. Benzene burns with a very smoky flame because of its high carbon to hydrogen ratio.

Worked example

The enthalpy change of hydrogenation of cyclohexene is -120 kJ mol^-1 and that of benzene is -208 kJ mol^-1. Find the delocalisation energy of benzene.

1. Expected for three C=C: 3 x (-120) = -360 kJ mol^-1
2. Actual: -208 kJ mol^-1
3. Difference = 360 - 208

Answer: 152 kJ mol^-1 more stable than the Kekulé structure

Check yourself

1. The enthalpy of hydrogenation of cyclohexene is -119 kJ mol^-1, and of benzene -207 kJ mol^-1. Calculate the delocalisation energy of benzene in kJ mol^-1, as a positive number.
   Show answer

   150 kJ mol^-1. 3 x 119 = 357; 357 - 207 = 150 kJ mol^-1.

2. What does X-ray diffraction show about the carbon-carbon bonds in benzene?
   • There are three short and three long bonds
   • They are all the length of a C=C bond
   • They are all the length of a C-C bond
   • They are all the same length, between a single and a double bond
   Show answer

   They are all the same length, between a single and a double bond. All six are 0.139 nm.

3. Why does benzene not decolourise bromine water at room temperature, although cyclohexene does?
   • Benzene has no π electrons
   • Its delocalised π electrons give lower electron density between carbons, so it cannot polarise bromine enough
   • Benzene is saturated
   • Bromine is insoluble in benzene
   Show answer

   Its delocalised π electrons give lower electron density between carbons, so it cannot polarise bromine enough. The localised C=C in cyclohexene is a region of high electron density; benzene's is spread out.

4. Which statement describes the bonding in benzene?
   • Three localised double bonds alternate with three single bonds
   • Each carbon forms four σ bonds
   • Six p orbitals overlap sideways to form a ring of delocalised π electrons
   • The ring is puckered with 109.5° angles
   Show answer

   Six p orbitals overlap sideways to form a ring of delocalised π electrons. Sideways overlap of p orbitals gives a delocalised π system.

5. What is the C-C-C bond angle in benzene?
   • 120°
   • 109.5°
   • 90°
   • 180°
   Show answer

   120°. A planar regular hexagon with three σ bonds per carbon.

6. Why does benzene burn with a smoky flame?
   • It contains sulfur impurities
   • It has a low boiling point
   • It has a high carbon to hydrogen ratio, so combustion is incomplete
   • It is a saturated compound
   Show answer

   It has a high carbon to hydrogen ratio, so combustion is incomplete. Unburnt carbon forms soot.

7. Why does benzene usually undergo substitution rather than addition?
   • Substitution keeps the stable delocalised ring
   • Addition is impossible for any molecule with π electrons
   • Benzene has no hydrogen atoms to replace
   • Substitution needs no catalyst
   Show answer

   Substitution keeps the stable delocalised ring. Addition would destroy the delocalisation energy.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.