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A2 Chemistry
Unit 1…read more

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The Structure of Benzene
Hydrogenation of Benzene:
Kekulé's Equilibrium Model Cyclohexene enthalpy change of hydrogenation =
of Benzene -120kJmol .
Kekulé's structure failed to explain benzene's Therefore benzene must have an enthalpy change
low chemical reactivity. of hydrogenation should be -360kJmol .
(3x cyclohexene, 3x C=C)
If C=C bonds were present, benzene should
react similarly to alkenes. The enthalpy change is actually -208kJmol .
Each C=C bond would be expected to The real structure of benzene is more stable than
decolourise bromine water. Kekulé's structure.
Benzene does NOT take part in electrophilic This energy is known as the resonance energy of
addition reactions as expected from the C=C benzene.
C-C single bonds and C=C bonds have
different bond lengths. Kathleen
Lonsdale found that all of the carbon
bonds were the same length -0.139
nm. (between the lengths of C-C and
C=C).…read more

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The Delocalised Model of Benzene
The Delocalised Model has the following features:
o Cyclic hydrocarbon ­ 6 C molecules and 6 H molecules.
o Arranged in a planar hexagonal ring where each C is bonded to 2 other C atoms and
1 H atoms.
o The shape is a trigonal planar with a bond angle of 120°.
o Each C atom has 4 outer shell electrons. 3 of these e¯ bond to 2 other C atoms and 1
H atom. The bonds in this plane are called sigma bonds. The 4th outer shell e in a
2p orbital above and below the plane of the carbon atoms.
o The e in the p orbital overlap creating a ring of electron density above and below
the plane of carbons.
o The pi-bonds spread over all 6 carbons and the ring is said to be delocalised.
Under normal conditions, benzene does not: Instead, Benzene takes part in substitution
reactions ­ a hydrogen (H) is replaced with
o Decolourise bromine water another group. The organic product retains
o React with strong acids such as HCl the delocalised structure.
o React with halogens such as bromine, chlorine or iodine.
Addition reactions will disrupt the delocalisation of the ring
structure.…read more

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Reactions of Benzene
Nitration of Benzene
Benzene's high electron density Formation of NO:
attracts electrophiles. HNO + HSO NO + HSO + HO
To preserve the ring's stability, The H reacts with the HSO to reform HSO.
benzene takes part in ELECTROPHILIC
SUBSTITUION reactions. This is acting as a catalyst.
Conditions: conc. HNO, conc. HSO, 50°C…read more

Slide 5

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Reactions of Benzene
Halogenation of Benzene
Benzene will react with halogens in the
presence of a HALOGEN CARRIER.
Halogen Carriers Include:
Iron Metal
Bromobenzene is used Chlorobenzene Formation of Br (or Cl):
in the preparation of is used as a Br + FeBr Br + FeBr
pharmaceuticals. solvent and in
pesticides. Regeneration of Br (or Cl):
H + FeBr FeBr + HBr…read more

Slide 6

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Reactivity of Alkenes and Benzene
Cyclohexene and Bromine Water: Benzene and Bromine:
The pi-bond is localised ­ this gives Benzene has delocalised electrons
cyclohexene HIGH ELECTRON DENSITY. spread over a ring structure. Alkenes
have localised electrons.
The pi-bond repels the electrons in the
Br-Br bond inducing a dipole. The Br Benzene has LOWER ELECTRON
molecule becomes polar. DENSITY and CANNOT POLARISE Br.
The electrons in the double bond Benzene is therefore resistant to
attract to the Br+ causing the double reactions with non-polar halogens.
bond to break. This forms a positive
carbocation. A halogen carrier is needed to generate
a more powerful electrophile.
The Br-Br bond breaks via heterolytic
fission forming Br. The greater charge on Br can attract
the pi-electrons from benzene so the
The Br is attracted to the intermediate reaction can take place.
carbocation forming a covalent bond.
Bromine +Benzene = orange
Bromine + Benzene + Iron fillings =
decolourised and white fumes of
hydrogen bromide gas.…read more

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