A2 OCR CHEMISTRY BENZENE RECALL

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Arenes are aromatic hydrocarbons with one or more benzene rings
Aromatic is when a hydrocarbon contains a benzene ring
Kekule's idea
C-C & C=C are different bond lengths
X ray studies show that all six of the carbon to carbon bond lengths in benzene
are the same 0.139nm
Between the bond lengths of:
C=C 0.134nm
C-C 0.153nm
Enthalpy of Hydrogenation
Cyclohexene's enthalpy of hydrogenation is -120 KJ mol-1
So if the Kekule theory was correct you would expect Benzene to
have an enthalpy of hydrogenation of (-120 x 3) = -360 KJ mol-1
HOWEVER this is not the case, it's actual experimental value is
-208KJ mol-1
Less energy is transferred to the external environment because more energy needs to be
inputted to break the bonds of Benzene, it's MORE stable than Kekule's theoretical
structure.
Low reactivity
Unlike alkenes, Benzene does not readily react in addition reactions, e.g. it will not
decolourise Bromine water without UV radiation, due to it's stability
HOWEVER it will take part in electrophilic substitution reactions
An electrophile is a species that accepts a pair of electrons to form a new covalent bond

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Delocalised Model of Benzene
The shape around each carbon atom is trigonal planar
o
giving a bond angle of 120
Every carbon atom has 4 outer electrons, 3 bond with 2 carbon atoms and 1 hydrogen atom,
leaving one outer shell electron in a 2p orbital above and below the plane of carbon atoms
This electron overlaps with the other electrons in the p orbitals of carbon atoms on the
sides, giving a ring of electron density above and below the plane
Giving pi bonds…read more

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Decolourises Bromine in an addition reaction like a typical alkene would
Bromine is non polar but reacts with the C=C as the pi bond has localised electrons above and
below the two carbon atoms in the double bond giving a region of high electron density
This can then polarise Bromine to allow a reaction
WHEREAS
Benzene can't react with Bromine without a halogen carrier because the pi electrons in benzene are
delocalised and the benzene ring is extra stable so the halogen cannot be adequately…read more

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Phenol and Bromine react more easily than Benzene and Bromine
Because a lone pair of electrons in the p orbital on the oxygen atom in phenol group is drawn
to the benzene ring
More electron density results in the ring structure and is activated
This increase in electron density polarises bromine molecules which are then more attracted
to the ring structure in phenol than in benzene, with all electrophiles including bromine.…read more

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