10. Aromatic Chemistry
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What structure are arenes based on?
A benzene ring structure
How many carbons and hydrogens does benzene contain?
6 of each
How was the bonding in a benzene ring originally thought to be?
A ring of six carbon atoms containing alternate single and double bonds
What is the IUPAC name of a cyclic structure with alternate single and double bonds?
Cyclohexa - 1, 3, 5 - triene
What is the molecular formula of cyclohexa - 1, 3, 5 - triene?
C6H6
What is the Kekule structure?
What the structure of benzene was originally thought to be; six carbon atoms with alternate single and double bonds
Do double and single bonds have the same length?
No
Why are double bonds shorter than single bonds?
The extra overlap of the π orbitals brings the atoms closer together and the bond is shorter
What was the evidence that benzene could not have the Kekule structure?
Analysis of benzene shows all bonds have the same length; but single and double bonds are different lengths
What are the length of the bonds in benzene?
0.142 nm - intermediate between single and double
What is the length of a C-C bond?
0.154 nm
What is the length of a C=C bond?
0.134 nm
What was the alternate suggestion to the Kekule structure for benzene?
Contained delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms
Describe the structure of benzene
• delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms • p-orbitals overlap sideways with both neighbours and the six electrons are delocalised above and below the ring
How can the delocalised electrons in benzene be represented?
As a circle within the hexagonal ring
What effect do the p-electrons in benzene have on stability?
They have a stabilising effect, meaning it is hard for an addition reaction to take place
What are arenes?
Hydrocarbons based on benzene
Why is it hard for an addition reaction to take place with benzene?
The 6 delocalised p electrons have a stabilising effect
What shape is benzene?
Planar
What are all the bond angles in benzene?
120°
What happens when benzene is added to water?
It doesn’t decolourise
Why doesn’t benzene decolourise when added to water?
There are no double bonds so it is difficult for an addition reaction to take place
What is more stable: benzene or cyclohexa - 1, 3, 5 - triene?
Benzene due to delocalised electrons
What are the two ways aromatic compounds can be named?
• based on benzene * based on phenyl