1. The structure of Benzene
12 November 2018
The structure of Benzene
C4.2 Aromaticity

2. The Structure of Benzene
Task: A hydrocarbon X was analysed – it contained 7.7% hydrogen. Mass Spectrometry showed a molecular ion peak at 78. IR spectroscopy showed at least one narrow peak at 3000cm-1. Use this information to deduce the structure of benzene and hence suggest an IUPAC name for this structure.
Benzene has the empirical formula CH and the molecular formula is C6H6 suggesting that the molecule contains a large number of double bonds.

3. History: The Structure of Benzene
In 1825 Faraday determined that the empirical formula was CH.
It was later found that the Mr = 78.  This suggested that the molecular formula was C6H6.
The structure of benzene provided chemists with a major problem. GD

4. History: The Structure of Benzene
Kekule suggested a ring structure with alternate double bonds
This structure has the IUPAC name of cyclohex- 1,3,5-triene GD

5. History: The Structure of Benzene
Although Kekule's structure explained the ring structure and the formula C6H6, there were still some chemical and physical properties that could not be explained:
Kekule's structure failed to explain the low reactivity of benzene.
According to the Kekule structure, 3 double bonds would be as reactive (if not more reactive) as alkenes.
It is in fact quite unreactive compared to alkenes and will not decolourise bromine water GD

6. History: The Structure of Benzene
Kekule suggested that the reactivity was due to an equilibria which involved the double bonds switching positions.
He suggested that the C=C changed position before any reactions could occur: GD

7. History: The Structure of Benzene
If Kekule’s structure was correct then there should be 3 C-C bond lengths and 3 C=C bond lengths
However, x-ray crystallography found that all the C-C bond lengths were equal in length. GD

8. History: The Structure of Benzene
C – C              0.153nm                    cyclohexane
C = C              0.134nm                    cyclohexene
C – C              0.139nm                    benzene
This suggests that the C - C bond is somewhere between a single and double bond GD

9. History: The Structure of Benzene
Hydrogenation of benzene
Actual ∆H = -208 kJ mol-1 GD
Benzene

10. History: The Structure of BenzeneGD

11. The Structure of Benzene
The weaknesses of the Kekule model lead to the delocalised model for benzene.
Benzene has the following properties which need explaining:
6 carbon atoms and 6 hydrogen atoms
Arranged in a hexagonal ring
The shape around each carbon atom is trigonal planar with a bond angle of 120o
Carbon - carbon bond lengths are all the same

12. Knowledge Check
State the empirical formula of benzene (1) Explain how X-ray crystallography helped scientists to develop the delocalised model of benzene (3) GD

13. Mark Scheme
CH (1) Using x-ray diffraction, it is possible to measure the lengths of the bonds in a molecule. (1) Carbon to carbon single bonds and carbon to carbon double bonds have specified lengths. (1) The carbon to carbon bonds in benzene don’t match these lengths (1) GD

14. The Structure of Benzene
Task: Complete the cut and stick activity to create a model for benzene.
Sideways overlap of p-orbitals
p-orbitals above and below plane of benzene ring
Delocalised ring of electron density above and below the ring plane of benzene
Overall: The delocalised structure of benzene forms when p-orbitals overlap sideways forming a pi-electron cloud above and below the plane of the carbon atoms.

15. The Structure of Benzene
Drawing the structure of benzene:
–     Represents 2e’s in a bond
=     Represents 4e’s in a double bond
As neither of these is appropriate to show the distribution of electrons in benzene, the following structure is often used:-

16. Past Paper Question
Describe the structure and bonding in benzene. [4] Explain why the delocalised model of benzene accounts for the observed stability of benzene better than the Kekule model [3] GD

17. Past Paper Question Describe the structure and bonding in benzene. [4]GD

18. Past Paper Question
Explain why the delocalised model of benzene accounts for the observed stability of benzene better than the Kekule model [3]
The delocalised model has the pi-bond electron density spread out rather than having concentrated areas of electron density from separate double bonds as in the Kekule structure. (2)
Compounds containing delocalised electrons are more stable than those that do not have delocalised electrons. (1) GD