Organic reaction mechanisms at A2 for OCR Chemistry A (F324)

A summary of the two mechanisms - electrophilic substitution and nucleophilic addition - that you need to know in detail for OCR A2 Chemistry A.

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Organic Mechanisms at A2 Level ­ a summary
A mechanism shows the steps by which the reactants turn into the products. A
mechanism will always show
bond breaking in the reactants
any intermediates
bond making in the products
Bond breaking ­ a covalent bond can break in two ways:
1. Heterolytic fission
If both electrons in a covalent bond between X and Y move on to Y, this
produces IONS:
X:Y X+ + :Y
We show the movement of a pair of electrons with a curly arrow:
2. Homolytic fission (at AS only)
If atoms X and Y take one each of the electrons in the covalent bond between
them, this produces FREE RADICALS:
Free radicals have no charge but they are highly reactive because they have an
unpaired electron, shown by the dot ( ).
In these mechanisms, there are three types of particle which attack the organic
definition examples
a neutral particle with an unpaired . . .
free radical Cl Br CH3
a particle which is attracted to a region of +
electrophil Br , NO2+ or a
negative charge ­ an electron pair polarised
acceptor molecule

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OH- ,H- or
nucleophil a particle which is attracted to a region of
e positive charge ­ an electron pair donor molecule with
lone pair eg NH3
electrophilic substitution Benzene and conc nitric acid
In the reaction of benzene with
a halogen eg Br2 in the
presence of a halogen carrier eg
AlBr3, the electrophile "Br+ " is
produced by the reaction:…read more

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Br+ then proceeds to
react with benzene in an
identical fashion to that of the
NO2+ ion in the mechanism
above, forming C6H5Br and HBr
When phenol undergoes these
electrophilic substitution
reactions, it does so much more
readily because the lone pairs
of electrons in the oxygen atom
delocalise into the ring and
increase the p electron density,
thus making attack by
electrophiles much more easy.
We say that the ring has been
"activated".…read more

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Ethanal and sodium
nucleophilic addition tetrahydridoborate NaBH4
NaBH4 contains hydrogen in the (-1)
oxidation state, so we can regard the
hydride ion , H- , as the effective
reducing agent.
H- acts as a nucleophile which attacks
the polar C=O group.…read more

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These two mechanisms ­ electrophilic substitution and nucleophilic
addition are the two mechanisms you need to know for OCR Chemistry
F324 (A2). You may be required to know the three mechanisms at AS.
You need to be able to write them out in detail and apply them to
reactions involving molecules with different numbers of carbon atoms or
sometimes with different specified electrophiles and nucleophiles.…read more


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