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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:


Page 2

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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

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and the 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…

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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.


The reaction is carried out in…

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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…


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