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Free radical halogenation is the result of chlorine or bromine
added to an alkane in the presence of uv light (hv). The reaction
begins with an initiation step, the separation of the
halide into two radicals (atoms with a single unpaired electron)
by the addition of uv light. Note the use of a single headed
arrow when representing the movement of a single electron.
step, the formation of the chlorine radicals, is immediately
followed by the propogation steps--steps directly involved
in the formation of the product. As an example, isobutane
will be used. The first step is the abstraction of the tertiary
hydrogen atom (note that these are not protons, but actual
hydrogen atoms since they each have one electron), forming
the tertiary radical.
attached to more highly substituted carbons (ie. carbons with
the most other carbons attached to them, like tertiary carbons)
are kinetically more reactive because the radical they form
is stabilized by neighboring alkyl groups that have the ability
to donate part of their electron density inductively through
the sigma framework to the electron-deficient radical carbon.
the tertiary radical is stabilized by electron donation
from neighboring alkyl groups.
of note about free radical processes is that the intermediates
are so highly reactive and short lived that usually there
is a mixture of products. This is the major downfall of radical
reactions, and why they have been overlooked in industry for
many years as mixtures of products are undesired, although
now these radical reactions are regaining popularity with
new methods to control single product formation. In our example,
for instance, there would certainly be some formation of the
primary radical and, ultimately, isobutyl chloride, although
it would be a minor product (assisted by the fact that statistically
there are nine primary hydrogens and only 1 tertiary hydrogen).
Free radical chlorination is less selective than bromination,
so bromination more selectively adds bromine to the more highly
The tertiary radical then reacts with one of the chlorine
radicals formed in the initiation step to form the product.
Notice that the chlorine radical is regenerated, so this reaction
can, in theory, go on forever as long as there are reagents.
This is called a chain reaction.
Side reactions that can stop the chain reaction are called
reacts exactly the same way as chlorine; however, it is far
more selective. If propane, for example, was the substrate,
2-bromopropane would be the dominant product, and there would
be only a small amount of 1-bromopropane. Chlorine is not
quite as selective, and there would be a greater amount of
the chlorination of the primary carbon.
can't the other halogens such as fluorine or iodine be used?
Iodine reacts endothermically and too slowly to be of much
good, while fluorine is at the other pole--it reacts too violently
and too quickly to be selective, and can, if uncontrolled,
break carbon-carbon bonds. To understand why this is so, derive
the DH's for the 4 reactions (flourination
is highly exothermic, iodonation is endothermic).