The structure of benzene

1
The structure of benzene
The electron config of carbon
2
This new hydrocarbon isolated by Michael Faraday
in 1825 presented a huge problem. .
The of carbon was 92 Carbon (C 12) . Its
relative molecular mass was 78. Calculate the
molecular formula
The molecular formula is C6H6 suggesting that
the molecule contained a large number of double
bonds.
Benzene did not, it was surprisingly unreactive
All the chemistry they knew suggested that any
substance with a double or triple bond would be
very reactive and react readily with HBr in the
dark.
3
In 1865 after a dream about a snake biting its
own tale, Kekulé suggested the following
structure for benzene.
This did not explain why the structure was so
unreactive, the chemists of the time were
convinced that it should react with bromine in
the dark at room temperature. This does not
happen with benzene.
4
This does not fit with Kekulés idea of
alternating double and single bonds. Another
problem was the energy of hydrogenation (addition
of hydrogen). The hydrogenation of cyclohexane is
well known.
5
So if three double bonds are present as in
benzene, then the comparable reaction should
liberate 3 times that of cyclohexane. -(3 120)
- 360 kJ mol-1
But the actual value for benzene was found to be
different. -208kJ mol-1 So benzene is
(360-208) 152 kJ mol-1 more stable than
otherwise expected, or if it contained 3 ordinary
CC bonds.
6
E
-360kJ/mol (3 X 120)
-208kJ/mol
progress
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This is a better all round model than the Kekulé
structure which shows 2 extremes of the same
thing. The circle in the middle shows the
delocalisation of the aromatic system.
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The bond length of C-C bonds in benzene are found
someway between that of an alkane and an alkene.
9
Evidence for delocalisation in benzene
Hydrogenation energies are lower than expected
Delocalised systems are highly saturated, but
their reactions are of substitution rather than
addition..
Carbon-Carbon bond lengths are equal in the
delocalised system.
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Benzene is a flat molecule, with all atoms in the
same plane,( bond angle 120)  
When the benzene ring is attached to an aliphatic
skeleton, it is called the phenyl group. The
formula of a phenyl group id C6H5.
Any compound where the ratio of CH is about 11
is likely to contain a benzene ring.
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There are several methods of displaying the
formula of benzene, the standard A2 method is
.
This represents the delocalised electrons, and is
probably the best way of representing the
delocalisation during mechanisms
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Exam points

• Benzene is a flat molecule with 6 carbons bonded
  in a Planar ring
• Each carbon is covalently joined to two other
  carbons and one hydrogen. A total of three
  covalent bonds
• The remaining outer electron of each carbon is
  shared with the other carbons in the ring. The
  six electrons are delocalised around the ring
  system, giving stability
• All bond lengths are the same.

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Never draw benzene as a simple hexagon. This
would be a molecule of cyclohexane this has no
delocalised electrons, and is not flat like
benzene.
Also unless drawing mechanisms, never include the
hydrogens attached directly to the benzene ring.
This is bad chemistry.
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Naming benzene molecules
1.2-dimethyl benzene
1,4-dimethyl benzene
1,3-dimethylbenzene
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2-hydroxybenzoic acid
benzene-1,4-dicarboxylic acid
phenyamine
chlorobenzene
nitrobenzene
Methyl 3-nitrobenzoate 2,4,6-trichlorophenol
phenyl ethanoate
phenylethanone
phenol
Benzoic acid
phenylethene
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Fused ring systems