How are arenes different from aliphatic compounds?
The arenes differ from aliphatic compounds such as alkanes and alkenes, in that they have one or more rings of carbon atoms in which the bonding electrons are delocalised.
What are arenes?
compounds containing benzene
Why are compounds containing benzene also called aromatic compounds?
Compounds that contain benzene have a characteristic smell, hence the name aromatic compounds is sometimes used
Draw and write the molecular and structural formula of benzene.
How can benzene be represented skeletally?
The bonding in benzene is not straightforward and this is reflected by the fact that its skeletal formula can be shown in two different ways:
- Kekule structure
- Delocalised structure
Draw the Kekule structure of benzene.
Draw the Delocalised structure of benzene.
Between the Kekule model and Delocalised model, which is more accurate?
Benzene was originally given the Kekule structure, i.e. a ring of six carbon atoms containing alternate single and double bonds
However, we now think the delocalised structure is more accurate.
Compare the Kekule and delocalised model of benzene:
bond length
Double and single bonds do not have the same length; in double bonds, the extra overlap of π-orbitals brings the atoms closer together and the bond is shorter.
Therefore, the double bonds in the Kekule structure would be expected to be shorter than all the single bonds.
However, analysis of benzene molecule shows that this is not the case; the bonds all have a length of 0.139nm, intermediate between single and double.
Compare the Kekule and delocalised model of benzene:
Resistance to reaction.
If the Kekule structure is correct, then benzene should readily decolourise bromine as this is a typical reaction of alkenes.
All alkenes readily undergo addition reactions and bromine readily adds across a double bond.
However, benzene does not readily decolourise bromine and does not readily undergo addition reactions.
Compare the Kekule and delocalised model of benzene:
Enthalpy change of hydrogenation.
Enthalpy change of hydrogenation for benzene is less than the value expected for the Kekule structure.
This suggests that the actual structure of benzene is more stable than the Kekule structure.
Explain the delocalised model of benzene and why it is better than the Kekule structure.
In the delocalised model, all the π-electrons are “spread out” over the benzene ring - this explains why all the bonds are the same length.
Delocalisation leads to a more stable structure.
This explains why:
a. benzene is resistant to addition reactions. Addition would lead to a loss of delocalisation and to a loss of the very stable benzene ring.
b. the enthalpy change of hydrogenation is lower.
The enthalpy of hydrogenation of benzene is significantly less than three times the enthalpy of hydrogenation of a typical alkene.
As a result of all this evidence, it was proposed that the structure did not in fact contain alternate single and double bonds but contained delocalised electrons in six overlapping p-orbitals. The delocalised electrons can be represented as a circle within the hexagonal ring.
Tell me about the bonds in benzene.
The electrons are completely delocalised.
The carbon atoms each form three covalent bonds making the angle between the atoms 120 degrees and the molecule planar.
How are the delocalised bonds in benzene formed?
In alkenes two adjacent p-orbitals overlap to form a π-bond.
However, in benzene, the six p-orbitals overlap to form a delocalised system of π bonds that extends over all six carbon atoms.
Give me a summary of benzene’s stucture.
- Benzene is a planar molecule (i.e. all 12 atoms are in the same plane), and all six carbon lengths are the same.
- Each carbon atom forms three sigma bonds, two to neighbouring carbon atoms and one to a hydrogen atom.
- Each carbon atom also has 1 electron in a p orbital above and below the plane of the molecule.
- These orbitals overlap to form a delocalised π-bond system. This is, in effect, a continuous π cloud of electrons above and below the plane of the molecule.
Tell me about the combustion of benzene.
Benzene burns in oxygen with a smoky flame due to its high carbon content.
This is a key different between aromatic and aliphatic compounds (less smoky flame)
2C6H6(l) + 15O2(g) —> 12CO2(g) + 6H2O(g)
Does benzene undergo addition reactions?
Benzene does not readily undergo addition reactions.
It tends to undergo electrophilic substitution.
Why does benzene undergo electrophilic substitution?
The delocalised electrons are an electron-rich area, and are susceptible to attack by electrophiles.
List the three key examples where benzene undergoes electrophilic addition.
1) Nitration of benzene
2) Halogenation of benzene
3) Friedel-Crafts alkylation
Write the equation for nitration of benzene.
C6H6 + HNO3 —-> C6H5NO2 + H2O
List the conditions of nitration of benzene.
Benzene reacts with a:
50:50 mixture of concentrated nitric acid and concentrated sulphuric acid under reflux at 50 degrees Celsius.
to form nitrobenzene
What happens if nitration of benzene occurs at a temperature greater than 50 degrees?
If the temperature is above this, further substitution may occur.
Draw the mechanism for the nitration of benzene.
Tell me the conditions of halogenation of benzene.
Benzene is refluxed with a halogen in the presence of a catalyst (called a halogen carrier)
(For chlorination, the halogen carrier catalyst will be AlCl3, FeCl3 or Fe and for the bromination the catalysts will be AlBr3, FeBr3 or Fe).
