The story of benzene
Write the electron configuration of Carbon. How many electrons are there in
its outer shell? How many covalent bonds should carbon form?
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
%
% divided by RAM
Divide by smallest
Ratio
C
H
92
100-92 = 8
92 / 12 = 7.67
8/1=8
7.67 / 7.67 = 1
8 / 7.67 = 1.04
1
1
Molecular formula
Known RMM / RMM of
empirical formula
78 / (12 + 1) = 6
The empirical formula is CH and the molecular formula is C6H6 suggesting that the
molecule contained a large number of double bonds.
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?
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
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
Theory versus experiment
Since three double bonds are present in benzene, then the comparable reaction
should liberate 3 times the energy of cyclohexene
= -(3 × 119) = - 357 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 (360-208) = 149 kJ mol-1 more stable than otherwise expected, or if it
contained 3 ordinary C=C bonds.
E
Benzene
(predicted value)
Difference = 149kJ/mol
Benzene (actual value)
Structure ???
-357kJ/mol (3
X –120)
Cyclohexane
-208kJ/mol
progress
Problem 3: Bond lengths of benzene
Bond Lengths
/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
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
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.
www.newton.ex.ac.uk
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?
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.
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.
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
Questions
1. Describe the 3 problems that scientists faced when trying to explain the
structure and bonding in benzene.
2. 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)
3. How does this current knowledge Q2 better explain the problems you
mentioned in Q1?
4. “We now fully understand benzene” Do you agree or disagree with this claim?
Explain your answer.
5. What is the role of theory and experimentation in the advance of scientific
knowledge? Use the story of benzene to support your answer.