The story of benzene

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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?
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
Molecular formula Known RMM / RMM of empirical
formula 78 / (12 1) 6
92
100-92 8
92 / 12 7.67
8 / 1 8
7.67 / 7.67 1
8 / 7.67 1.04
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1
The empirical formula is CH and the molecular
formula is C6H6 suggesting that the molecule
contained a large number of double bonds.
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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?
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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
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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
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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) -
357 kJ mol-1
Experimentally determined value for the
hydrogenation of benzene ?H (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 CC bonds.
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E
Benzene (predicted value)
Difference 149kJ/mol
-357kJ/mol (3 X 120)
Benzene (actual value) Structure ???
Cyclohexane
-208kJ/mol
progress
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Problem 3 Bond lengths of benzene

• 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

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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
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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
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All the bonds in benzene have the same length...

• 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?

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The double-single bonds cant 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.
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• 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.
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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 doesnt show delocalized electrons and is time
  consuming to draw
• Organic chemists tend to use B or C if they want
  to draw mechanisms

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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.