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Free radical halogenation is a reaction that substitutes a
chlorine or a bromine for a hydrogen on an alkane. This reaction
is a photochemical one. That is, it occurs only when performed
in the presence of uv light (abbreviated hv).
free radical reactions are described in three steps: initiation
steps, propagation steps, and termination steps (described
below). Note the use of a single headed arrow when describing
the movement of a single electron.
begins with an initiation step, which is the separation
of the halogen (X2) into two radicals (atoms with
a single unpaired electron) by the addition of uv light. This
is called the initiation step because it initiates the reaction.
step, or 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
(C4H10) will be used in the chlorination
reaction. The first step is the abstraction of the hydrogen
atom from the tertiary carbon (a tertiary carbon is a carbon
that is attached to three other carbon atoms) Note
that these are not protons (H+ ions) that are being abstracted,
but actual hydrogen atoms since each hydrogen has one electron.
This first propogation step forms the tertiary radical.
In the last step, the tertiary radical then reacts with another
one of the chlorine molecules to form the product. Notice
that another 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.
on free radical stabilities:
attached to more highly substituted carbons (ie. carbons with
many carbons attached to them) are more reactive in free-radical
halogenation reactions because the radical they form is stabilized
by neighboring alkyl groups. These neighboring alkyl groups
have the ability to donate some of their electron density
to the electron-deficient radical carbon (a radical is short
one electron of filling the atom's valence octet). Thus the
hydrogen on the tertiary carbon here is abstracted in preference
to the 9 other hydrogen atoms attached to a primary carbon
(a carbon that is attached to only one other carbon atom)
because it forms a more stable radical.
the tertiary radical is stabilized by electron donation
from neighboring alkyl groups.
of free-radical halogenation
of note about free radical processes is that the intermediates
are so highly reactive and short lived that usually you obtain
a mixture of products, even though there is preference for
forming more highly substituted free radical intermediates.
In this example with isobutane, for instance, there would
certainly be some abstraction of hydrogens attached to the
primary carbons, leading to a different product than the above
product (can you draw it out?).
reacts exactly the same way as chlorine; however, it is far
more selective. If propane (CH3CH2CH3),
for example, was the substrate, 2-bromopropane would be the
dominant product, and there would be only a small amount of
1-bromopropane. Free radical chlorination, though, would not
be quite as selective, and there would be a greater amount
of the chlorination of the primary carbon than in the bromination
Side reactions that can stop the chain reaction are called
termination steps. These termination steps involve the destruction
of the free-radical intermediates, typically by two of them
can't the other halogens such as fluorine or iodine be used?
Iodine reacts endothermically (energetically uphill) and too
slowly to be of much good in these free radical processes,
while fluorine is at the other pole--it reacts too violently
and too quickly to be selective, and can, if uncontrolled,
even break carbon-carbon bonds. To understand why this is
so, derive the DH's for the 4 reactions
and compare them (you will find that flourination is highly
exothermic, while iodonation is endothermic; chlorination
and bromination, however, are right in the middle).