1. The story of benzene

2. A new hydrocarbon isolated by Michael Faraday in 1825
Carbon = 92% Carbon (Atomic mass = 12)
Hydrogen = ? % (Atomic mass = 1)
Relative molecular mass = 78.
Calculate the empirical formula then the molecular formula C H %
% divided by RAM
Divide by smallest
Ratio
Molecular formula
Known RMM / RMM of empirical formula
78 / (12 + 1) = 6 92
= 8
92 / 12 = 7.67
8 / 1 = 8
7.67 / 7.67 = 1
8 / 7.67 = 1.04 1 1
The empirical formula is CH and the molecular formula is C6H6 suggesting that the molecule contained a large number of double bonds.

3. 2 minute challenge: Draw as many possible structures for a hydrocarbon with the formula C6H6 Make sure the structures satisfy the requirements of 4 bonds per carbon and 1 bond per hydrogen!
In 1865 after a dream about a snake biting its own tale, Kekulé suggested the following structure for benzene.
Does this structure meet all the requirements of benzene?

4. Problem 1: Lack of reactivity of benzene
Chemists at the time were convinced that benzene (like other alkenes) should react
with bromine in the dark at room temperature.
Bromine water (brown) + alkene → Bromoalkane solution (colourless)
Observation: This did NOT happen with benzene.
Conclusion: Benzene is not a normal alkene

5. Predict the enthalpy of hydrogenation of benzene
Problem 2: Thermodynamic stability of benzene
Enthalpy of hydrogenation (addition of hydrogen) to cyclohexene
was found to be -119kJmol-1
Predict the enthalpy of hydrogenation of benzene

6. Theory verses experiment
Since three double bonds are present in benzene, then the comparable reaction should liberate 3 times the energy of cyclohexene
= -(3 × 119) = kJ mol-1
Experimentally determined value for the hydrogenation of benzene
ΔH (hydrogenation) = -208kJ mol-1
Is Benzene more or less stable than expected?
Benzene is ( ) = 149 kJ mol-1 more stable than otherwise expected, or if it contained 3 ordinary C=C bonds.

7. E -357kJ/mol (3 X –120) -208kJ/mol progress Benzene (predicted value)
Difference = 149kJ/mol
-357kJ/mol (3 X –120)
Benzene (actual value)
Structure ???
Cyclohexane
-208kJ/mol
progress

8. Problem 3: Bond lengths of benzene
/nm
C-C cyclohexane
0.154
C=C cyclohexane
0.134
Compare the length of a single bond to a double bond
Draw what benzene would look like if the different length double and single bonds are alternating to form a 6 member ring.
Clue: Its not a perfect hexagon

9. What benzene would look like if it had fixed alternating double-single bonds…
Different bond lengths in benzene would cause distortion
Q. Who is this celebrity?
A. Simon Cowell

10. Direct evidence
1981 an atomic surface probing technique was developed called Scanning Tunneling Microscopy (STM). The first published STM image showed benzene with an undistorted hexagonal shape.

11. All the bonds in benzene have the same length...
Bond Lengths
/nm
C-C cyclohexane
0.154
C=C cyclohexane
0.134
C-C in benzene
0.140
How does the carbon-carbon bond length of benzene compare to double and single carbon-carbon bonds?
1. All bonds are of equal length
2. The bond length is between a double bond and a single bond.
What do these two facts suggest about the structure of benzene?

12. The double-single bonds can’t be fixed in position!
Resonance suggests the two structures rapidly alternate between the two forms.
The resonance explanation suggests that Benzene is in such rapid equilibrium between the two forms; we detect a ‘blurred’ combination of the two forms.
The electrons from the double bonds are therefore drawn as a circle shared equally between the carbon atoms.

13. The spreading of electrons STABILIZES the molecule.
Current theory:
Some text books still use the word “resonance” to describe the structure of benzene.
There is NO evidence to support two rapidly changing forms of benzene.
Modern organic chemists use the word “conjugation” to describe how the electrons are delocalized (spread) across the whole molecule…
Valence bond theory explains the bonding in benzene as a series of unhybridized p-orbitals which overlap forming a cloud of electron density above and below the molecule.
The spreading of electrons STABILIZES the molecule.

14.         .
Drawing benzene
Which drawing most accurately represents the structure and bonding in Benzene? Explain
Are any of the pictures completely incorrect? (i.e. not Benzene) if so which and why?
Which representation takes longest to draw? Which is quickest to draw?
In your opinion which picture is best overall? Why?
D is incorrect (cyclohexane not benzene!)
A doesn’t show delocalized electrons and is time consuming to draw
Organic chemists tend to use B or C if they want to draw mechanisms

15. Benzene notes

16. Naming Benzene compounds

17. Benzene Common Groups Group Name -OH Hydroxy- -F Fluro- -Cl Chloro-
-Br
Bromo- -I
Iodo-
-NO2
Nitro-
-CH3
Methyl-

18. Naming Benzene Compounds
Aliphatic
Aromatic
2-chloro-butane
chlorobenzene

19. Naming Benzene Compounds
Aliphatic
Aromatic
2-methyl-butane
methylbenzene

20. Naming Benzene Compounds
Aliphatic
Aromatic
Ethanoic Acid
Benzoic acid

21. Naming Benzene Compounds
Aliphatic
Aromatic
Methylethanoate
methylbenzoate

22. Naming Benzene Compounds
Aliphatic
Aromatic
propanal
2-phenyl-propanal

24. Complete the naming worksheet

25. Questions
Describe the 3 problems that scientists faced when trying to explain the structure and bonding in Benzene.
What is the current understanding of the structure and bonding of Benzene? (answer as if you were talking to a chemist who had never heard of Benzene)
How does this current knowledge Q2 better explain the problems you mentioned in Q1?
“We now fully understand benzene” Do you agree or disagree with this claim? Explain your answer.
What is the role of theory and experimentation in the advance of scientific knowledge? Use the story of benzene to support your answer.

26. Prep: Finish the exam Qs set last lesson Read pages 10-13 for next lesson