Reactions of Arenes

Substitution or Addition?

Questions

Quiz

An Arene is the name given to a benzene ring that has had a hydrogen substituted for another group - UNLESS an oxygen is directly connected to the ring. This would be a phenol.

E.g.

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methylbenzene on the left, phenol on the right

Despite first appearances, benzene is relatively unreactive. Reactions involving benzene need quite extreme conditions, usually involving catalysts or ultra-violet light.

However like all volatile organic compounds, benzene burns in excess oxygen.

            2C6H6(l) + 15O2(g) → 12CO2(g) + 6H2O(l)

With less oxygen however benzene produces carbon in the form of soot.

            2C6H6(l) + 3O2(g) → 12C(s) + 6H2O(l)

Addition reaction of benzene are very difficult to achieve, mainly due to the stabilisation effect of the delocalised electrons.

  • The majority of reactions involving benzene end up with the delocalised ring being left intact.
  • The main reaction mechanism with benzene reactions is substitution.
  • Due to the large numbers of delocalised p electrons above and below the plane, most reactions occur through an electrophilic mechanism.

There are two types of reaction, either electrophilic substitution or addition.

Substitution or Addition

1.                  Using a strong electrophile like the NO2+ (nitro) group results in the substitution of hydrogen in the ring.

2.                  Using UV light, it is possible to break open the delocalised ring of electrons, to give an addition product. This is also possible with a transition metal catalyst and extreme temperatures.

Notice that in the addition product, the aromaticity has been broken up.

Questions

1.         Identify the following molecules as alkane, alkene, arene or        cycloalkane

a) CH3CH2CH2CH2CH3  b) C6H5CH3    c) CH3CH=CHCH2CH3 
d) CH3CH(CH3)CH2CH3 e) C6H12  

2.         Benzene is classified as aromatic because of the delocalised system of electrons around the benzene ring.

            a)         What is meant by a delocalised system of electrons?

            b)         Why is knowledge of bond lengths important in the discussion of the structure of benzene?

c)         What thermochemical data supports the idea of a stable ring structure for benzene with delocalised electrons?

3          Describe the two types of reactions that benzene is normally capable of carrying out. Give an example of each.

4.         Draw a diagram to represent the delocalised structure of benzene. Include the p-orbitals.

5.         What evidence is present to suggest delocalisation is present in benzene? There are at least 3 points.

6.         Why should benzene never be drawn as a hexagon?

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