Arenes Flashcards
What is an arene?
A chemical compound with a benzene ring
Benzene structure
A molecule with a hexagonal ring of 6 carbon all are bonded to 1 hydrogen
Other electron per carbon atom is delocalised above and below ring of the plane
Pi system in benzene
One unused electron on each C atom in a p orbital, perpendicular to the plane of the ring.
The Six p electrons are delocalised in a ring structure above and
below the plane of carbon atoms so spread out in a pi system
Where is electron density in benzene?
Above and below the carbon ring
Bond angle in benzene
All 120 degrees
What is the Kekule structure for benzene
Alternating C=C in the ring, no pi system
Is the Kekule structure for benzene incorrect?
Yes
What evidence is against the Kekule structure?
Enthalpies of hydrogenation
Carbon-Carbon bond lengths
How do enthalpies of hydrogenation provide evidence against kekules structure?
Because there are 3 double bonds so the enthalpy of hydrogenation should be 3x that of cyclohexene that only has 1 double bond
But the value of hydrogenation is actually lower because there are NOT 3 DOUBLE BONDS BUT A PI SYSTEM
How do bond lengths from Xray chrystallography provide evidence against Kekule strucure?
All bond lengths and enthalpies in benzene is equal
If C=C was present then 3 Bonds would be shorter in length
Similarities between Kekule structure and delocalised structure
Both have electron density spread above and below the molecular plane
Both involve the overlap of p orbitals to form pi bonds
Differences between Kekule structure and delocalised structure
Electrons arent delocalised in Kekule but they are in pi system
Pi system cannot polarise halogen molecule (cant undergo electrophilic addition) but Kekule can
Delocalised undergoes electrophilic substitution, kekule doesnt
Why benzene is resistant to bromination, compared with alkenes
Electron density is delocalised so not enough charge in benzene so cannot polarise the Br-Br bond to form Br- ion and Br+ in electrophilic addition
