Structure of benzene

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Benzene (C6H6)• Hexane has a molecular formula of

C6H14.• Cyclohexane is C6H12.

• Hexene has a formula of C6H12.

• Cyclohexene is C6H10.• So obviously benzene must be a highly

unsaturated molecule involving more than one multiple bond.

Kekule’s solution

Single bond

Double bond

A ring linked by alternate bonds.

Resonance structure of benzene

Bonds are not fixed, but continually switch.

But Kekule’s model does not fit the facts;

1) Benzene’s reactions are completely different from the alkenes.

Alkenes, eg; cyclohexene, readily decolourise bromine.

But benzene will not unless conditions are very vigorous.

2) Bond angles

• If benzene consisted of alternate double and single bonds the angles in a benzene ring would be a mixture of 109.5 and 120.

• In fact they are all 120O.

• Producing a planar (flat) structure.

3) Energetics• Chlorine readily reacts with

cyclohexene• ΔH= -183.7 kJ/mol.• Chlorine will react with benzene, but

only if it is bubbled through boiling benzene in the presence of uv light.

• If benzene had three double bonds;• ΔH= 3 x -183.7 kJ/mol = -551 kJ/mol.• In fact it is only -399.1 kJ/mol.• So benzene is more stable than

predicted.

theoretical value

real value

Energy diagram

Enthalpy(kJ mol-1)

-240-208

-152 kJmol-1

delocalisation energy

3) Evidence from bond lengths

In cyclohexane all C-C bonds are;

0.154 nm

In cyclohexene the C-C bonds are 0.154 nm, but the C=C bond is;

0.133 nm

0.133nm

0.154nm

If benzene had the Kekule structure it should have…

3 C-C bonds of 0.154nm

3 C=C bonds of 0.133nm

But in benzene all 6 C/C bonds have the same length….0.139nm

Intermediate between C-C and C=C bonds.

Each C atom forms two single bonds with other C atoms.

And one bond to a H atom.

C - HForming a flat, hexagonal ring.

NB All bond angles are 120o.

The fourth electron is left in p orbital.

Above and

Below the plane.

Electrons in the p orbitals overlap to form a delocalised ring…… Above

And below the plane of the C-C ring

So benzene is written as;

Bonding in benzene

6 electrons in a delocalised bond

above and below the plane of the atoms

Physical properties of benzene.

Benzene is a colourless liquid at rtp.

BP = 353 K (hexane 354 K)

MP = 279 K (hexane 178 K)

The difference in MPs is due to the fact that flat benzene rings stack together well and are harder to seperate.

Combustion

They give smoky flames, as the high C:H ratii means that unburnt carbon is present.

Arenes are flammable;C6H6 +7.5O2 → 6CO2 + 3H2O

Structure of benzene

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