Aromatic Chemistry Flashcards
What is the formula of a benzene ring?
C6H6
What did kekule propose the initial benzene ring structure was?
A ring with alternating single and double carbon bonds.
How many problems were there with kekule’s model?
There were three problems.
What was the first problem?
Typically alkenes undergo addition reactions with bromine therefore if benzene had 3 carbon-carbon double bonds it would turn bromine water colourless but it doesn’t.
What is the second problem with kekule’s model?
If benzene has alternating single and double carbon bonds, there should be different bond lengths. However, evidence shows that the bonds are perfectly identical with a hexagon shape.
Whats the third problem with kekule’s model?
The enthalpy of hydrogenation of a carbon-carbon double bond is -120 kj mol-1 so for a benzene ring it should be -360 kj mol-1 but it was less about -208 kj mol-1.
The enthalpy of hydrogenation is less exothermic than we expected.
How does the current model work?
Each carbon forms three sigma bonds, and the remaining p orbitals overlap with both neighbouring p orbitals, to form a pi system, with delocalised electrons.
What type of molecule will have delocalised electrons?
Any molecule with alternating single and double carbon bonds
Why are compounds with delocalised electrons more stable then ones without?
As the delocalised electrons are spread out so this decreases the electron density of them.
How do we name benzenes?
When benzene is the highest-priority functional group in a molecule, use the suffix ‘benzene’.
If the molecule contains a higher-priority functional group, we treat the benzene ring as a substituent. We call this substituent as a ‘phenyl’
