The Benzene Problem
This new compound isolated by Michael Faraday in 1825 presented a huge problem. The molecular formula of C6H6 suggested that the molecule contained a large number of double bonds.
Chemists new about alkanes and alkenes, and this new molecule fitted none of the patterns.
- Chemists new that is had to contain lots of double or even triple bonds.
- All the chemistry they new suggested that any substance with a double or triple bond would be very reactive and react readily with HBr in the dark.
- Benzene did not, it was surprisingly unreactive.
In 1865 after a dream about a snake biting its own tale, Kekulé suggested the following structure for benzene.
Techniques were developed to measure
the bond length in benzene, and the results where quite surprising. Bond Lengths /nm C-C cyclohexane 0.154 C=C cyclohexane 0.134 C-C in benzene 0.140 The bond length of C-C bonds in
benzene was someway between that of an alkane and an alkene. 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 × 119) = - 357 kJ mol-1
But the actual value for benzene was found to be different.
So benzene is (357 – 207) = 150 kJ mol-1 more stable than otherwise expected, or if it contained 3 ordinary C=C bonds.
The diagram shows the difference – the stabilisation energy of benzene.
Delocalisation
The bonding in benzene is a very special case, instead of alternating double and single bonds, this p- bonded system contains overlapping p-orbitals. The system has become delocalised. It is this delocalisation that gives benzene its stability.
The delocalisation model
Definition: Delocalised electron systems in organic molecules involve p bonds with all the carbon (or other) atoms in the system in a single plane and in which overlap of p orbitals extends over a significant number of carbon atoms.
This delocalisation leads to stronger dipole induced interactions and therefore stronger attractions between molecules.
Benzene is the most important molecule containing a delocalised system. There are others however. Pyridine, and hexa-1,3,5-triene both contain delocalised systems.
Pyridine (a heterocyclic aromatic compound)
Evidence for delocalisation in benzene
- Carbon-Carbon bond lengths are equal in the delocalised system.
- Hydrogenation energies are lower than expected.
- Delocalised systems are highly saturated, but their reactions are of substitution rather than addition..
There are several methods of displaying the formula of benzene, the standard A2 method is:
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.
Benzene is a flat molecule, with all atoms in the same plane, unlike other cyclic compounds whose atoms are not all in the same plane.
This represents the delocalised electrons, and is probably the best way of representing the delocalisation during mechanisms.
- A molecule will always be aromatic if there is a p electron system containing (4n+2)p electrons.
- 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 C:H is about 1:1 is likely to contain a benzene ring.
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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.
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Also unless drawing mechanisms, never include the hydrogens attached directly to the benzene ring. This is bad chemistry.
Quick questions:
1 Draw the displayed formula for:
| a) Methylbenzene | b) Benzene |
| c) pent-2-ene | d) cyclopentane |
Click the link below for answers.
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