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The structure of benzene

The electron

config of

carbon

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All the chemistry they knew suggested that any substancewith 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.

.

The molecular formula is C6H

6suggesting that the molecule

contained a large number of double bonds.

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

Calculate the molecular formula

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In 1865 after a dream about a snake biting its own tale, Kekul

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.

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This does not fit with Kekuls idea of alternating double and

single bonds.

Another problem was the energy of hydrogenation (addition ofhydrogen).

The hydrogenation of cyclohexane is well known.

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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 thanotherwise expected, or if it contained 3 ordinary C=C bonds.

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-360kJ/mol (3

X 120)

-208kJ/mol

E

progress

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This is a better all round model than the Kekul

structure which shows 2 extremes of the same

thing. The circle in the middle shows thedelocalisation of the aromatic system.

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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 are found

someway between that of an alkane and an alkene.

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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.

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Benzene is a flat

molecule, with all atoms

in the same plane,( bondangle 120)

When the benzene ring is

attached to an aliphaticskeleton, it is called the

phenyl group. The formula

of a phenyl group id C6H5.

Any compoundwhere the ratio of

C:H is about 1:1 is

likely to contain a

benzene ring.

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There are several methods of displaying the formula of

benzene, the standard A2 method is:

.

This represents the delocalised electrons, and isprobably the best way of representing the delocalisation

during mechanisms

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Exam points

Benzene is a flat molecule with 6 carbons bonded in aPlanar 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.

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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.

Also unless drawing

mechanisms, never include

the hydrogens attached

directly to the benzene ring.This is bad chemistry.

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Naming benzene molecules

1.2-dimethyl

benzene

1,3-

dimethylbenzene

1,4-dimethyl benzene

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Methyl 3-nitrobenzoate2,4,6-trichlorophenol

benzene-1,4-dicarboxylic

acid

acid

phenolphenylethanone

phenyl ethanoate

phenyamine

chlorobenzene

phenylethene

nitrobenzene

Benzoic acid

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Fused ring systems