The_structure_of_benzene

The structure of benzene

The electron config of carbon

Objectives• Review the evidence for the delocalised

model of benzene.• Compare the delocalised and Kekule structure

of benzene in terms of p-orbital overlap• Explain the relative resistance to bromination

of benzene, compared with alkenes, in terms of the delocalised electron density of the π bonds in benzene compared with the localised electron density of the C=C bond in alkenes.

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.

Benzene did not, it was surprisingly unreactive

This new hydrocarbon isolated by Michael Faraday in 1825 presented a huge problem.

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The molecular formula is C6H6 suggesting that the molecule contained a large number of double bonds.

The % of carbon was 92% Carbon (C= 12) . Its relative molecular mass was 78.

Calculate the molecular formula

In 1865 after a dream about a snake biting its own tale, Kekulé suggested the following structure for benzene.

1. Unreactivity of 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.

Problems with the Kekulé structure

2. Enthalpy of hydrogenation of benzene.

Another problem was the enthalpy of hydrogenation (addition of hydrogen).

The hydrogenation of cyclohexane is well known.

Write equations for, and calculate the enthalpy change for the hydrogenation of:

a)Cyclohexa-1,3-diene;b)Cyclohexa-1,3,5-triene (i.e. Kekulé benzene).

∆H = -120kJmol-1

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.

-360kJ/mol (3 X –120)

-208kJ/mol

E

Progress of reaction

Bond Lengths /nmC-C 0.154

C=C 0.134

3. Bond Lengths

The table below shows the bond lengths for C-C and C=C:

Draw a the structure of benzene to scale using these values:

In benzene however, the bonds are all of the same length and of intermediate length between C-C and C=C (0.140nm).

In an attempt to save Kekulé’s model of benzene scientists suggested that the molecule existed in two forms which ‘flip’. They were called ‘resonance

hybrids‘.

The overall form is intermediate between the two:

In fact they do not exist

‘resonance hybrids’ of benzene

Evidence for delocalisation in benzene

Hydrogenation energies are lower than expected;

Highly saturated, but their reactions are of substitution rather than addition..

Carbon-Carbon bond lengths are equal in the delocalised system.

In order to account for:

A better explanation than the ‘resonance hybrid’ was required.

The modern picture of the structure of benzene involves‘delocalization’ of electrons in a ring system.

There are several methods of displaying the formula of benzene, the standard A2 method is:

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The circle represents the delocalised electrons, and is probably the best way of representing the delocalisation during mechanisms.

© Pearson Education Ltd 2009This document may have been altered from the original

A localised π-bond: as in an alkene.

Delocalised π electrons in benzene

Exam points•Benzene is a flat cyclic molecule with 6 carbons bonded in a planar hexagonal ring

•Each carbon is covalently joined to two other carbons and one hydrogen. A total of three sigma 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. The electron is in a p-orbital and the p –orbital overlaps with the other p-orbitals forming a plane of electron density above and below the carbon atoms. ( pi bond)

•All bond lengths are the same as the pi electrons are spread across the 6 carbons and are delocalised

•The bond angle around each carbon is 120o

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.

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 is C6H5.

Any compound where the ratio of C:H is about 1:1 is likely to contain a benzene ring.

Fused ring systems