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The structure of benzene

The electron

config ofcarbon

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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 araday in !"#$

presented a huge problem.

.

The molecular formula is %&H

& 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

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'n !"&$ after a dream about a snake biting its own tale, (ekul) 

suggested the following structure for benzene.

This did not e*plain 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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o if three double bonds are present / as in benzene, then the

comparable reaction should liberate 0 times that of cyclohe*ane.10 2 !#3- 4 1 0&3 k5 mol1!

But the actual value for benzene was found to be different. 1#3"k5

mol1! 

o benzene is 0&31#3"- 4 !$# k5 mol1! more stable thanotherwise e*pected, or if it contained 0 ordinary %4% 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 (ekul)

structure which shows # e*tremes of the same

thing. The circle in the middle shows the

delocalisation of the aromatic system.

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on! "en#ths $nm

%1%

cyclohe*ane

3.!$6

%4%

cyclohe*ane

3.!06

%1% in benzene 3.!63

The bond length of %1% bonds in benzene are found

someway between that of an alkane and an alkene.

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vi!ence for !elocalisation in ben&ene

Hydrogenation energies are lower than e*pected

7elocalised systems are highly saturated, but their

reactions are of substitution rather than addition..

%arbon1%arbon bond lengths are e8ual in the delocalised system.

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Benzene is a flat

molecule, with all atoms

in the same plane, bond

angle !#3-

9hen the benzene ring is

attached to an aliphatic

skeleton, it is called the

phenyl group. The formula

of a phenyl group id %&H

$.

 Any compoundwhere the ratio of

%:H is about !:! is

likely to contain a

benzene ring.

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There are several methods of displaying the formula of

benzene, the standard A# method is:

 

.

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

during mechanisms

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;*am points

<Benzene is a flat molecule with & carbons bonded in a=lanar ring

<;ach carbon is covalently >oined 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 si* electrons are

delocalised around the ring system, giving stability

< All bond lengths are the same.

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'ever !raw ben&ene as a

simle hea#on. This woul!

be a molecule of c*cloheane

 + this has no !elocalise!electrons, an! is not flat li-e

ben&ene.

lso unless !rawin#

mechanisms, never inclu!e

the h*!ro#ens attache!

!irectl* to the ben&ene rin#.This is ba! chemistr*.

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?aming benzene molecules

!.#1dimethyl

benzene

!,01

dimethylbenzene

!,61dimethyl benzene

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Methyl 01nitrobenzoate

2,4,6-trichlorophenol 

benzene-1,4-dicarboxylic

acid 

2-hydroxybenzoic

acid 

 phenol  phenylethanone

 phenyl ethanoate

phenyaminechlorobenzene

phenylethene

nitrobenzene

Benzoic acid

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used ring systems