Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more. N.B. does NOT include 18C (Synthesis).
ORGANIC CHEMISTRY III
18A: ARENES – BENZENE
1. understand that the bonding in benzene has been represented using
the Kekulé and the delocalised model, the latter in terms of overlap
of p-orbitals to form π-bonds
The structure of benzene:
At A Level we may represent the structure of benzene as:
The structure of benzene can be represented using the Kekule and the
delocalised models.
o The Kekule structure (left) suggests that benzene contains C=C double
bonds, formed from a sigma and a pi bond.
o The delocalised model (right) more accurately shows that the bonding
in benzene is delocalised – the p orbitals overlap to form pi bonds.
In the delocalised model, 6 electrons in the 2p orbitals of the 6 carbon atoms
overlap.
o These electrons are delocalised around the benzene ring.
o This is a conjugated system – p orbitals overlap across multiple atoms.
This delocalised pi-system results in the lowering of the energy of the
molecule.
o This makes benzene less reactive than a typical alkene.
o Resonance energy – the amount of energy by which the molecule is
lowered.
Benzene is an arene – i.e. it is an aromatic molecule.
o Aromatic molecules are cyclic and planar with conjugated atoms.
2. understand that evidence for the delocalised model of the bonding in
benzene is provided by data from enthalpy changes of
hydrogenation and carbon-carbon bond lengths
Enthalpy changes of hydrogenation:
The hydrogenation of cyclohexene releases 120 kJ
mol-1 energy.
We would expect the complete hydrogenation of
benzene to release 360 kJ mol-1 energy, as it has
three times as many C=C bonds as cyclohexene.
, However, it actually only releases 208 kJ mol -1.
o The reaction is less exothermic than expected.
More energy is needed to break the bonds in benzene than we would expect
from the Kekule structure.
o Benzene is around 150 kJ mol-1 more stable than we would expect.
o This is its resonance energy – the amount of energy by which the
molecule is lowered.
This shows that the actual benzene structure is thermodynamically more
stable than expected, as it has a lower enthalpy content.
Carbon-carbon bond lengths:
Bond Bond length/pm
C-C 154
C=C 134
Carbon-carbon in 140
benzene
The Kekule structure would suggest that benzene had sides of alternating
length, with some carbon-carbon bonds (C=C) shorter than others (C-C).
However, the carbon-carbon bonds are all equal in length. These bonds are
longer than C=C double bonds but shorter than C-C single bonds.
We can deduce that the carbon-carbon bonds in benzene are identical in
strength, and are stronger than C-C bonds but weaker than C=C.
Reactivity:
Benzene is also much less reactive than a typical alkene and does not
participate in the expected reactions of an alkene.
It does not reaction with a mixture of KMnO 4 and concentrated H2SO4.
It does not cause bromine water to decolourise.
3. understand why benzene is resistant to bromination, compared with
alkenes, in terms of delocalisation of π-bonds in benzene and the
localised electron density of the π-bond in alkenes
Bromination of benzene:
Alkenes react readily with bromine water under standard conditions with no
catalyst.
o This is an electrophilic addition reaction, with the alkene acting as a
nucleophile.
Benzene does not react with bromine under the same conditions.
o It only reacts upon adding a FeBr 3 catalyst.
o No decolourisation of bromine occurs.
Benzene does not react readily in addition reactions because this would break
the stable delocalised pi system.
Benzene is also a poorer nucleophile than alkenes.
o The overlapping pi orbitals in benzene means that the delocalised
electrons are spread over the entire structure.
, o However, the electrons in alkenes’ C=C pi bonds are localised, making
it more electron dense and therefore more nucleophilic.
4. understand the reactions of benzene with:
i) oxygen in air (combustion with a smoky flame)
The combustion of benzene:
Benzene burns in oxygen with a smoky, yellow flame.
o C6H6 + 15/2 O2 6 CO2 + 3 H2O
o C6H6 + 9/2 O2 6 CO + 3 H2O
o C6H6 + 3/2 O2 6 C + 3 H2O
This is because it has a high carbon to hydrogen ratio; it is more likely for
incomplete combustion to take place, producing carbon (soot).
ii) bromine, in the presence of a catalyst
Overview of the halogenation of benzene – bromination or chlorination:
Benzene reacts with Br2 or Cl2 in an electrophilic substitution reaction.
A Lewis acid catalyst is used.
o Lewis acid – electron pair acceptor.
o For bromination we use FeBr3; for chlorination this is AlCl3.
o The catalyst can be generated in situ by adding iron or aluminium
metal.
The overall reaction that occurs is:
o C6H6 + Br2 C6H5Br + HBr
HBr (g) and HCl (g) are toxic so we carry out this experiment in the fume
cupboard.
Mechanism for bromination (chlorination is analogous):
We form the catalyst in situ.
o 2 Fe + 3 Br2 2 FeBr3.
Generation of the electrophile:
o Br2 + FeBr3 Br+ + FeBr4-
o Benzene is unreactive so will only react with a very strong electrophile
like Br+.
o This is the A Level version, which is technically incorrect.
The benzene attacks the Br+ ion, forming a cation intermediate.
Then the conjugated pi system deforms as the H-C bond breaks.
Regeneration of the catalyst:
o FeBr4- + H+ FeBr3 + HBr (g)
We can use ammonia to confirm the successful halogenation of benzene.
o HCl (g) + NH3 (g) NH4Cl (s)
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