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Topic 4.6


Structure and stability of benzene
Reactions of benzene
Uses of aromatic compounds

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1. Structure

Benzene is a cyclic hydrocarbon containing six carbon atoms and six hydrogen atoms. It was
originally thought to be a ring of six carbon atoms containing alternate single and double bonds
as follows:

This molecule has the IUPAC name cyclohex1,3,5triene. It has the molecular formula C6H6

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The delocalized electrons can be represented as a circle within the hexagonal ring:

This molecule is known as benzene. It is usually represented by its skeletal formula:

The electrons are completely delocalized. The carbons atoms each form three covalent bonds
making the angle between the atoms 120o and the molecule…

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2. Stability

The delocalization of the electrons in the benzene ring has a significant effect on the stability of
the molecule. Benzene might be expected to behave like alkenes i.e. react readily with bromine
and hydrogen to undergo addition reactions, but this is not the case.

The delocalization of the…

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However the enthalpy change for this reaction is only 209 kJmol1. This is less exothermic than
expected, suggesting that the benzene molecule has much less energy than it would have if the
electrons were not delocalized:

The enthalpy of hydrogenation of benzene is significantly less than three times
the enthalpy…

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a) nitration

Benzene reacts with a 50:50 mixture of concentrated nitric and sulphuric acid, under reflux at
5055oC, to form nitrobenzene.

Step 1:Nitric acid is a weaker acid than sulphuric acid, and acts as a base as the electrophile is

H2SO4 + HNO3 H2NO3+ + HSO4
H2NO3+ H2O +…

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b) alkylation

i) using haloalkanes

Benzene reacts with chloroalkanes in the presence of anhydrous AlCl3 under reflux at 50oC to
form alkylbenzenes.

Step 1:The AlCl3 accepts a Cl from the chloroalkane to form a carbocation:

RCl + AlCl3 R+ + AlCl4

Step 2:The R+ is the electrophile and attacks the…

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If chloromethane (CH3Cl) is used, then methylbenzene (C6H5CH3) is formed.

If chloroethane (C2H5Cl) is used, then ethylbenzene (C6H5C2H5) is formed.

ii) using alkenes

Alkylbenzenes other than methylbenzene can be formed by reacting benzene with alkenes in the
presence of HCl and AlCl3, under reflux at temperatures below 50oC.

Step 1:…

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Unsymmetrical alkenes can give two possible carbocations when HCl is added. In such cases
two different products are possible.
In most cases one carbocation is produced in greater quantities than the other. Thus a major
product and a minor product will be formed.

Eg propene can react with HCl to…


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