AC3: Aromatics
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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
For a halogen to react with an aromatic compound, what is needed
A halogen carrier (catalyst)
What are the 2 main halogen carriers
AlCl3 and FeCl3 , the Cl can be Br etc
What are the 3 steps in the mechanism for halogenating an aromatic compound
• The halogen carrier reacts with the halogen to produce an electrophile • Attack on the aromatic ring by the electrophile • The halogen carrier is regenerated by the releases H+
What is the name of the mechanism between an aromatic compound and a halogen
Electrophilic substitution
How is an alkyl group attached to the benzene ring / what type of reaction
By electrophilic substitution
To attach an alkyl group to the benzene ring, what type of catalyst is needed
Halogen carrier
What are the 3 steps in the mechanism when attaching an alkyl group to the benzene ring
• Halogen carrier generates an electrophile • Electrophile attaches aromatic ring • Catalyst is regenerated
Why is added an alkyl group to an aromatic compound important in organic synthesis
Because they form carbon to carbon bonds, allowing for the framework of organic molecules to extend
When you add an acyl chloride to an aromatic compound, what is the product
Aromatic ketone