5.4.1 Arenes: Benzene Flashcards
What is the Kekulé structure of bonze?
Alternating single and double bonds that constantly flip between the two carbons.
Give two bits of evidence against the Kekulé structure of benzene.
X-ray diffraction showed all the carbon-carbon bonds in benzene are the same length and infrared studies have shown that none of the bonds are normal single or double bonds because they absorb energy of different frequencies.
Explain the delocalised model of benzene.
The p-orbitals of all six carbon atoms overlap to create π bonds. This creates two ring shaped clouds of electrons above and below the ring. The bonds are all equal in length and the p electrons are delocalised.
How are the delocalised rings represented in the skeletal formula of benzene?
A circle within the hexagon.
How do the delocalised electrons in benzene affect its stability?
They increase it.
The enthalpy change of hydrogenation of cyclohexene is -120kJ mol^1 what would you expect the enthalpy change of hydrogenation of benzene to be?
-360kJ mol^-1
What is the experimental enthalpy value for the hydrogenation of benzene?
-208kJ mol^-1
Explain why a less negative enthalpy value of hydrogenation means that the delocalised electrons give benzene stability.
More energy must have been put into the break the bonds in benzene than would have been needed to break the bonds in the Kekulé structure.
Why does benzene burn with a smokey flame?
There isn’t enough oxygen in the aorta burn benzene completely. Many carbon atoms stay as carbon producing soot in the hot gas.
Give the equation for the complete combustion of benzene.
2C6H6 + 15O2 → 12CO2 + 6H2O
What does the benzene ring attract and why?
It has a high electron density so it attracts electrophiles.
Why does benzene usually undergo substitution reactions rather than addition reactions?
To preserve the delocalised ring.
How do you make nitrobenzene?
Warm benzene with concentrated nitric and sulfuric acids.
What is the role of sulfuric acid in the nitration of benzene?
It acts as a catalyst to help make the nitronium ion, NO2+ which is the electrophile.
What is the electrophile in the nitration of benzene?
NO2+
Give two equations to show how the electrophile is formed for the nitration of benzene.
HNO3 + H2SO4 → H2NO3+ + HSO4-
H2NO3+ → NO2+ + H2O
Where does the pair of electrons move in the first stage of the mechanism for the nitration of benzene?
From the benzene ring to the electrophile, NO2+.
What is the charge on the benzene ring of the intermediate product for the nitration of benzene?
+1
Where does the pair of electrons move int he second stage of the mechanism for the nitration of benzene?
From the bond C-H with hydrogen back to the ring.
Where does the lost H+ ion go after the nitration of benzene?
Back to the HSO4- to reform the catalyst, H2SO4
How do you ensure mononitration of benzene?
Keep the temperature below 55ºC.
What happens if the temperature is above 55ºC when nitrating benzene?
You will get multiple substitutions.
How do you make benzenesulfonic acid?
Warm benzene to 40ºC with fuming sulfuric acid for about 30 minutes.