AC3: Aromatics Flashcards
What are the 3 pieces of evidence that the structure of benzene is different to the Kekule model
- Doesn’t undergo typical addition reactions of alkenes / doesn’t decolourise bromine
- All the carbon to carbon bonds are the same length
- Hydrogenation of benzene is less exothermic than expected by comparing it to alkenes
In benzene, how many sigma bonds does each carbon form
3 sigma bonds
In benzene, how many electrons does each carbon have in each orbital and what is the orbital
1 electron in the p orbital
In benzene, what is the system called when the orbital overlap
delocalised pi- electron system
Why does benzene have low reactivity
Delocalisation spreads out the pi-electrons more than they would be in a localised pi-bond, hence electron density is lower in aromatic compounds- so less attractive to electrophiles
Why are the bond lengths the same size in benzene
The pi-electrons are distributed evenly around the ring, hence all of the carbon-carbon bond lengths are the same
Why is the hydrogenation of benzene less exothermic than expected
Delocalisation lowers the energy of the pi-electrons (they are in a more stable state than electrons in a localised pi-bond). This means that reactions that break up the delocalised pi-electron system are less favourable than might have been expected.
What is an electrophile
An electron pair acceptor
Are aromatic or alkenes are more reactive
Alkenes since theur pi-electrons are localised so they have a higher electron density
Do aromatics tend to react by addition or substitution
Substitution since the delocalised pi-system is still present in the product
What are the conditions needed to nitrate the benzene ring
- concentrated nitric acid
- concentrated sulfuric acid
What is the name of the mechanism to nitrate the benzene ring
Electrophilic substitution
What are the 3 steps of the mechanism to nitrate the benzene ring
- Formation of the electrophile
- Reaction with the electrophile
- Regeneration of the catalyst
When nitrating the benzene ring, what is the catalyst
The sulfuric acid
What type of reaction occurs between an aromatic compound and a halogen
Substitution