Organic Mechanisms at AS for OCR F322

This resource explains the language of mechanisms and details the three mechanisms you need to know for OCR AS Chemistry A

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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:
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
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
molecules:
definition examples
a neutral particle with an unpaired . . .
free radical Cl Br CH3
electron
a particle which is attracted to a region of +
electrophil Br or polarised
negative charge ­ an electron pair
e molecule
acceptor

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OH- or molecule
e positive charge ­ an electron pair donor with lone pair
Methane + chlorine
free radical substitution in uv light
Equation
Mechanism
uv light causes homolytic fission of
.
Cl2 forming Cl free radicals:
.
Cl2 2Cl
Initiation
.
Cl + CH4 HCl +
.
CH3 Propagation
.
CH3 + Cl2 CH3Cl +
.
Cl Propagation
. .
Cl + Cl Cl2
Termination
. .
Cl + CH3 CH3Cl
Termination
. .…read more

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Notes
This is an example of a chain
.
reaction. Each Cl free radical
can lead to the reaction of
thousands of methane
molecules
Further substitution can occur
to form CH2Cl2 , CHCl3 and
CCl4
.
The Cl free radical is just a
chlorine atom (2.8.7) ­ highly
reactive due to unpaired
electron
Formation of ethane C2H6 is
good evidence for this
mechanism.…read more

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Ethene + bromine
Equation
Mechanism
Notes
The Br2 is polarised on
approach to the p bond in
ethene
The -Br + acts as an
electrophile attacking the p
bond
The intermediate positive ion
(a carbocation) is unstable and
rapidly combines with the
bromide ion, Br-.…read more

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Reaction occurs for any alkene
with any halogen…read more

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Equation
Mechanism
OH- ions are attracted to the
positively polarised carbon atom in
chloromethane
Notes
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 spectators ­ not
involved in reaction
Cl atoms easily lost as Cl- ions ,
hence haloalkanes useful for
making other organic
compounds
Bond energies C-Cl stronger
than C-Br , which is stronger
than C-I .…read more

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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.…read more

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