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Organic Mechanisms at AS 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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nucleophil a particle which is attracted to a region of OH- or molecule
e positive charge ­ an electron pair donor with lone pair

Methane + chlorine
free radical substitution in uv light


uv light causes homolytic fission of
Cl2 forming Cl free radicals:

Cl2 2Cl…

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This is an example of a chain
reaction. Each Cl free radical
can lead to the reaction of
thousands of methane

Further substitution can occur
to form CH2Cl2 , CHCl3 and
The Cl free radical is just a
chlorine atom (2.8.7) ­ highly
reactive due…

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electrophilic addition Ethene + bromine




The Br2 is polarised on
approach to the p bond in

The -Br + acts as an
electrophile attacking the p

The intermediate positive ion
(a carbocation) is unstable and
rapidly combines with the
bromide ion, Br-.

Heterolytic fission occurs…

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Reaction occurs for any alkene
with any halogen

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chloromethane + sodium
nucleophilic substitution hydroxide

OH- ions are attracted to the
positively polarised carbon atom in


Electronegative Cl atom causes
polarisation of C+­Cl- bond

Hydroxide ions are
nucleophiles ­ attracted to C
in haloalkane and OH- is an
electron pair donor

Na+ ions are…

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bromomethane which is faster
than chloromethane.

Heterolytic fission involved.

These three mechanisms ­ free radical substitution, electrophilic
addition and nucleophilic substitution are the three mechanisms you
need to know for OCR Chemistry F322.

You need to be able to write them out in detail and apply them to
reactions involving…


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