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AS Chemistry Unit 2
Alkanes
Photochlorination of Methane
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The three stages of the photochlorination of methane are
initiation
,
propagation
and
termination.
The initiation stage involves
homolytic fission
of the
Cl-Cl
bond to produce two
chlorine free radicals.
In the initiation stage, the
Cl-Cl
bond is
weaker
than the
C-H
bond in
methane
which means that it
breaks first.
The conditions for the initiation stage are
UV light
and
room temperature.
Why does the Cl-Cl bond break first in the initiation stage?
It is
weaker
than the
C-H bond
in
methane
and so it
breaks first.
Why is
UV
light
used in the initiation stage?
UV light is used because the
mechanism
requires the
formation
of
free
radicals.
The propagation stage is where a
free radical
reacts and another is produced (
chain reaction
).
The propagation stage has
four
possible stages that each contains
two
steps, (one
hydrogen
atom is
substituted
by a
chlorine
atom in each stage).
The termination stage is where two
free radicals
react together to produce a
molecule
and are
removed.
Where have the two free radicals, used in the termination stage, been produced?
In the
propagation
steps.