The structure of benzene
The electron
config of
carbon
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.
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.
This does not fit with Kekulé’s idea of alternating double and
single bonds.
Another problem was the energy of hydrogenation (addition of
hydrogen).
The hydrogenation of cyclohexane is well known.
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 C=C bonds.
E
-360kJ/mol (3
X –120)
-208kJ/mol
progress
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.
The bond length of C-C bonds in benzene are found
someway between that of an alkane and an alkene.
Bond Lengths
/nm
C-C
cyclohexane
0.154
C=C
cyclohexane
0.134
C-C in benzene
0.140
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.
Benzene is a flat
molecule, with all atoms
in the same plane,( bond
angle 120)
Any compound
where the ratio of
C:H is about 1:1 is
likely to contain a
benzene ring.
When the benzene ring is
attached to an aliphatic
skeleton, it is called the
phenyl group. The formula
of a phenyl group id C6H5.
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
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.
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.
Naming benzene molecules
1.2-dimethyl
benzene
1,4-dimethyl benzene
1,3dimethylbenzene
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
Fused ring systems