CbD - Benzene Flashcards
What are aromatic compounds derived from?
Benzene
What do arenes or aromatic compounds contain?
A benzene ring
What are the 2 structures of benzene?
The Kekule structure
The Delocalised structure
What is the formula for benzene?
C6H6
What was the first structure of benzene?
Kekule’s structure
What did Kekule’s structure propose?
- Carbons were arranged in a planar ring.
- The single and double bonds continuously flipped between the carbons.
What were the problems with Kekule’s structure?
- The observed value for the enthalpy of hydrogenation of benzene.
- (Un)reactivity of benzene with bromine water.
- Regular shape (all bonds are the same length).
What model was developed after Kekule’s?
The delocalised model.
Why was the delocalised model developed?
To better explain benzene’s properties.
Explain the delocalised model
Carbons arranged in a ring.
No alternating single and double bonds - instead, each carbon donates an electron from its p-orbital. The p-orbitals combine to form a ring of delocalised electrons.
What happens if you add bromine to benzene?
- If you add bromine to benzene nothing happens and the colour remains orange (rather than undergoing an electrophilic addition reaction like with an alkene).
- You need to use a catalyst to get benzene and bromine to react at all, but even then it is very slow and they react by electrophilic substitution rather than addition.
How does the delocalised structure explain why benzene reacts via substitution?
- The electrons in the delocalised ring have more room than if they were squeezed into localised double bonds. They can get further away from each other, spreading out the negative charge, so the molecule is more stable.
- An addition reaction would need to take electrons from the stable delocalised ring to form new bonds. Substitution reactions don’t do this - a hydrogen atom just gets swapped for something else and the stability of the delocalised electrons is preserved.
How do enthalpy changes give more evidence for delocalisation?
The experimental enthalpy of hydrogenation of benzene is far less exothermic than expected.
Energy is put in to break bonds and energy is released when bonds are made. So more energy must have been put in to break the bonds in benzene than would be needed to break the bonds in the Kekule structure.
This difference indicates that benzene is more stable than the Kekule structure would be. This is thought to be due to the delocalised ring of electrons.
How did X-ray diffraction studies show that the Kekule structure of benzene was incorrect?
They have shown that all the carbon-carbon bonds in benzene are the same length - between the length of a single and double bond (rather than having 2 separate bond lengths).
What are the predictions from the Kekule model about the shape of benzene?
Asymmetrical hexagonal planar.
