A2 OCR Chemistry - Unit One - Modules 1-3

OCR Chemistry A2 Notes for all of the topics in Module One :)

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Aromatic Chemistry
The simplest aromatic compound (arene) is BENZENE (C6H6). A benzene
molecule is usually represented as a hexagon surrounding a circle.
An AROMATIC COMPOUND is any compound with a benzene ring
An ARENE is an aromatic hydrocarbon containing one or more benzene rings
Arenes occur naturally in crude oil and coal, and can be produced in refineries from crude oil.
The word aromatic is derived from the Latin `aroma' meaning fragrance. Early scientists
noted that organic materials from resins or plant material gave off pleasant smells. Chemists
analysed these sweet-smelling materials and found that they contain benzene rings. However,
not all compounds containing benzene rings are sweet-smelling (e.g. aspirin)
Naming Compounds Based on Benzene
For a group to be attached to a benzene ring, a hydrogen must be replaced.
If there is more than one group attached then we number to give the lowest value around the
ring. We start numbering at the alkyl group / major functional group:
Benzene is a starting material for the synthesis of many other aromatic materials
including ethylbenzene, phenol and styrene. Smaller quantities of benzene can be used to make
a wide range of materials such as detergents, explosives, pharmaceuticals and dyestuffs.

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Benzene is a colourless liquid with a sweet odour. It is highly flammable and its natural sources
include volcanoes and forest fires. Benzene is a component of crude oil, petrol and cigarette
The major problem with benzene is that it is a carcinogen (cancer causing) molecule.
Structure of Benzene
In 1825, Faraday isolated benzene as a by-product in the manufacture of gas for street lighting
from the distillation of whale oil and he determined that the empirical formula was CH.…read more

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C-C Bond Length in Benzene
The Kékulé structure of alternating single and double bonds was represented as a
symmetrical molecule. But C-C single bonds and C=C double bonds have different lengths.
X-Ray diffraction threw some light on the structure. It was found that all the C-C bond lengths in
Benzene were equal in length.
C­C 0.153nm Cyclohexane
C­C 0.139nm Benzene
C=C 0.…read more

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The real benzene structure is 152 kJ
mol-1 more stable than the Kékulé
This energy is known as the
delocalisation energy or resonance
energy of benzene.
Kékulé's structure was disproved:
Bond Length: Double bonds are shorter than single bonds, yet all bond lengths are the same in
benzene. The length of Benzenes C-C bonds are in between the length of single and double bonds.…read more

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This means that each C-C has 3 electrons. 2 in the bond and 1 from the delocalised system.
This makes it slightly less electron rich than alkenes which means that they are not as good at
polarising electrophiles.
The bonding in benzene cannot therefore be described as 3 C=C and 3 C­C bonds.…read more

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Benzene & Its Reactions
Benzene in toxic and carcinogenic so Methylbenzoate (or Methoxybenzene), a derivative of
benzene is used:
Methoxybenzene is slightly more reactive than benzene. The conditions involving benzene will be more severe.
SUBSTITUTION is a type of reaction where an atom or group of atoms is replaced with a
different atom or group of atoms.…read more

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Nitration of Benzene
The nitration of benzene is the first step in a historically important chain of reactions. These
lead to the formation of dyes, pharmaceuticals and explosives.
The reaction for the nitration of benzene is:
C6H6 + HNO3 C6H5NO2 + H2O
Reagents: concentrated nitric acid and concentrated sulphuric acid (catalyst)
Conditions: reflux at 50°C (too much heat = more than one nitro group can be added)
(Round-bottomed flask containing cooled nitric acid and sulphuric acid.…read more

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Halogenation of Benzene
The reaction for the Halogenation of benzene is:
C6H6 + X2 C6H5X + HX
Benzene will not react with bromine on its own. It requires the help of a type of catalyst called a
halogen carrier.…read more

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Function of Halogen Carrier (catalyst):
Benzene is more stable than alkenes which means that Cl2 is not a strong enough
electrophile.…read more

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Further Substitution of Benzene
It is possible to substitute more than one functional group. But the functional group already
on the ring affects:
- How easily it can be done
- Where the next substituent goes
Methyl benzene used to be called Toluene. It has a methyl group attached directly to the
benzene ring.…read more



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can you email it to me plz : [email protected]


very useful thanks for sharing! :)

Izzy Mason

sooooooooo useful :)

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