Arenes
Chemistry: Cambridge International A Level, topic 3.2.10
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.
Benzene Structure
Benzene is the simplest aromatic compound. It has a unique and very important structure.
Structure
- Benzene consists of six carbon atoms bonded together in a hexagon.
- The molecular formula is C6H6
- The original structure was thought to have distinct alternating double bonds between the carbon atoms.
- This was called the ‘Kekulé’ structure, after the man who developed it.
The Kekulé structure
- The Kekulé structure is shown above.
- Importantly, the Kekulé structure suggests that the different carbon-carbon bonds are of different lengths and reactivities.
- Improvements on Kekulé’s theory have lead to an understanding that this isn’t the case.
Aromaticity
- If you were to draw the p-orbitals out on the ring to show the double bonds, you would see that every carbon looks equivalent.
- In fact, there are no discrete double bonds. Instead the electrons circulate in a ring formed of all six overlapping p-orbitals.
The delocalised picture
- The delocalisation of the p-electrons is best shown in the above diagram.
- This emphasises that every carbon is equivalent, and so all bond lengths are the same.
- Reactivity is also expected to be the same at each carbon.
- This is supported by lots of experimental evidence and is superior to the Kekulé structure.
Why learn the Kekulé structure?
- While the Kekulé structure implies several untrue things about benzene, it’s a handy and quick way of drawing the benzene ring in mechanisms.
- The takeaway is that the Kekulé structure is not an accurate depiction of benzene’s physical structure.
- You can use either the Kekulé structure or the delocalised picture in drawing mechanisms out - the Kekulé method is just much more time efficient.
Test yourself on Arenes
Why is benzene planar?
Where do the p-orbitals in benzene lie?
What is the molecular formula of benzene?
Which of the following are true about the structure of benzene?