Arenes - OCR

Revision notes on arenes (Chains, Rings and Spec. module) for A2 chemistry.

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Arenes
Benzene an its compounds
Benzene has s Molecular formula of C6H6
Kekule's Structure of Benzene
In theory this is correct but has been disproven by many tests
Bromine test can check for the presence of C=C
o When Br2 was added the solution wasn't de-colourised
CycloHexa-1,3,5-triene = C6H6 Benzene = C6H6
The enthalpy for the hydrogenation of Cyclohexene -120 KJ mol­1,
so the expected enthalpy for Cyclohexa-1,3,5-triene would be -360 KJ mol­1
but the enthalpy for benzene is -208 KJ mol­1
o The enthalpy is less than expected
X-ray defraction can check the distance between 2 nuclei
o Distance between C­C is 154 nm [2e­] (Single Bond - long)
o Distance between C=C is 134 nm [4e­] (Double Bond - short)
o But in Benzene C­/­C are 144 nm [3e­] (Single Bond and Bond ­ in between)
The electrons in the p-orbitals can overlap sideways with adjacent p-orbitals
o All the 6 p-electrons are free to move around the ring (above and below the plane)
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The electrons are said to be `delocalised'
Normally benzene is drawn as
The delocalised ring makes benzene stable
o not very reactive
Carcinogenic - Induces cancer
Benzene only undergoes an `Electrophilic substitution' reaction
Electrophile ­ An electron pair acceptor which forms a covalent bond
Common electrophiles include H+, Na+
The 4 reactions of Benzene include:
Nitration
Halogenation
Alkylation
Acylation
Nitration of Benzene
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Halogenation of Benzene
Alkylation of Benzene
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Acylation of Benzene
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