structure of benzene

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Structure of benzene

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Structure of benzene. Benzene (C 6 H 6 ). Hexane has a molecular formula of C 6 H 14 . Cyclohexane is C 6 H 12. Hexene has a formula of C 6 H 12 . Cyclohexene is C 6 H 10 . So obviously benzene must be a highly unsaturated molecule involving more than one multiple bond. - PowerPoint PPT Presentation

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Page 1: Structure of benzene

Structure of benzene

Page 2: Structure of benzene

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.

Page 3: Structure of benzene

Kekule’s solution

Single bond

Single bond

Single bond

Double bond

Double bond

Double bond

A ring linked by alternate bonds.

Page 4: Structure of benzene

Resonance structure of benzene

Bonds are not fixed, but continually switch.

Page 5: Structure of benzene

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.

Page 6: Structure of benzene

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.

Page 7: Structure of benzene

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.

Page 8: Structure of benzene

theoretical value

real value

Energy diagram

Enthalpy(kJ mol-1)

-120

+ H2

+ 2H2

-240-208

+ 3H2

-152 kJmol-1

delocalisation energy

+ H2

-120

+ 3H2

-360

Page 9: Structure of benzene

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

Page 10: Structure of benzene

0.133nm

0.133nm

0.133nm

0.154nm

0.154nm

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.

Page 11: Structure of benzene

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

C - C

C - C

And one bond to a H atom.

C - HForming a flat, hexagonal ring.

NB All bond angles are 120o.

Page 12: Structure of benzene

The fourth electron is left in p orbital.

Above and

Below the plane.

Page 13: Structure of benzene

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;

Page 14: Structure of benzene

Bonding in benzene

C C

C C

C C H

HH

H

HH

6 electrons in a delocalised bond

above and below the plane of the atoms

Page 15: Structure of benzene

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.

Page 16: Structure of benzene

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