Structure & Reactivity (A2 Only)
Chemistry: AQA A Level, topic 6.4.1
Structure and Reactivity of Aromatic Compounds
Benzene has a cyclic and planar structure of formula C6H6. Its reactivity is different from other unsaturated compounds.
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The structure of benzene
- Benzene has delocalised p-electrons.
- This makes benzene aromatic and more stable than its non-delocalised equivalent cyclohexa-1,3,5-triene.
- Benzene has a hexagonal planar structure with its p-orbitals above and below the plane of the ring.
- To be aromatic, the p-orbitals must overlap. This is why the structure is planar - so the p-orbitals can overlap.
Structural representation
- To represent the mobility of double bonds in benzene, we draw benzene with a ring (as shown above).
- The skeletal structure is an average structure to show the double bonds moving around the whole ring.
Substitution reactions
- Benzene's stable structure affects its reactivity.
- Benzene reactions are typically substitution, NOT addition.
- This is different from alkenes which normally undergo addition reactions.
- Substitution is favoured because it re-establishes the original structure - and so retains the very stable aromatic structure.
Evidence for the Structure of Benzene
Many pieces of evidence have been used to prove the structure of benzene.
Structure of benzene
- For years, chemists argued over what the best structure was to represent benzene.
Bond lengths
- All carbon-carbon bonds have been shown to be the same length.
- This length (0.139 nm) is somewhere between the C-C and C=C bond lengths.
Bond angles
- The structure has been proved to have a bond angle of 120o at each carbon.
- This means the structure must be planar.
Enthalpy of hydrogenation
- The enthalpy of hydrogenation for benzene is more positive than for cyclohexa-1,3,5-triene.
- Hydrogenation is a form of addition reaction.
- So this reaction breaks the aromaticity in the benzene ring, which is why it is unfavourable.
- This reflects how benzene is more stable due to aromaticity.
Test yourself on Structure & Reactivity (A2 Only)
Why is benzene planar?
Where do the p-orbitals in benzene lie?
What are the three pieces of evidence for the structure of benzene?