so let's take a look at radical chlorination. this is when an
alkane can be chlorinated via a radical mechanism so
remember before we discussed that one of the
types of covalent bonds that will readily undergo
homolytic bond cleavage is a halogen-halogen bond so
given a certain amount or a
photon of UV light that is going to promote homolysis
this Cl2 molecule can dissociate into two
chlorine radicals so this is the initiation step because we have
one covalent species going to two radical species
now what can happen here is a chlorine radical
is a pretty unstable species
many radicals are so
that's what can be so deleterious when radicals interact with our DNA and
other biological systems so a radical can find an alkane
and is gonna propagate the existence of other radicals because of its own
instability and so what happens here let's say it finds
a molecule of methane or some other hydrocarbon it can
propagate the existence of an alkyl radical
by this process so here's a carbon-hydrogen covalent bond, so these
electrons will go here
and a hydrogen radical will now make a covalent bond with chlorine giving us HCl
and leaving us with in this case a methyl radical which can be
very unstable and so that means that
now that we have a methyl radical swimming around if it finds
another molecule of chlorine it is gonna propagate further
and go ahead and form a carbon-chlorine bond
leaving a new chlorine radical and so
this is our chlorinated product and
in this case was generated in a propagation step it is just as well that
a carbon radical could have found a
a chlorine radical and then the chloromethane could have been generated in the
termination step so
there's a lot of ways for this to go but in this case we do it as a propagation
step
and then any two radical species that you could find
in this system whether it was two chlorine radicals
two methyl radicals, a chlorine and a methyl radical we just said
if any of those were two collide then they would form a covalent
structure in a termination step so basically here to make this let's just
remind ourselves we're forming chlorine radicals
the chlorine radical is gonna propagate some alkyl radical
and an alkyl radical will want to bind to anything it can find so it runs
into
a chlorine molecule which is probably much more prevalent
in the system than chlorine radicals being very unstable
then it will go ahead and form the monochlorinated product
now the monochlorinated product can then go ahead and react further
if this structure comes into contact with another chlorine radical
that chlorine radical can strip away another one of those three protons
leaving another radical that can get another chlorine so that we could have a
dichlorinated
or trichlorinated or even tetra chlorinated so we could
have tetrachloromethane and so
that's how halogenation works for
alkanes, it's gonna have to be a radical mechanism
with initiation propagation and termination as we see
now another thing we gotta understand about these radical mechanisms
remember when we were looking at let's say an sn1 reaction
where we had the sp2 hybridized trigonal planar
carbocation intermediate we had to be aware of the possibility for a racemic
mixture
because but when you have a planar situation like this and recalling
that a carbon radical is also planar and sp2 hybridized
that means that let's say we have this carbon radical let's say we have three
different groups here
let's say they're just three different alkyl group so that means we have the
potential
to form a stereocenter if that carbon binds to
or forms a bond to a new group and so
let's say we have a chlorine molecule, let's say the chlorine molecule is gonna
approach from the top
and then we're gonna generate a carbon-chlorine bond, the other chlorine radical
will go away
we have a new carbon-chlroine bond so there that is here
but then let's say, similarly it could approach from the bottom
and then the chlorine would now be on the bottom here, well these three different
groups dictate that we have formed a new stereocenter
so we have to be aware of the possibility of forming a racemic
mixture of enantiomers
when we're looking at radical mechanisms so
that describes the halogenation. now the other thing that we have to
understand about halogenation
is that a free radical halogenation can potentially be
a very regioselective process and so
what we want to understand here is that a bromination
ends up looking to be very regioselective because let's say
we're going to do a bromination on isobutane
and there are four possible locations
for a bromine atom to end up and as
as it turns out we get strictly bromination on this
location here so we get the tertiary
system. now chlorination ends up not being so
regioselective we actually get a mixture of the tertiary and the primary
chloroalkane so we want to try to understand why that might be
because we are gonna be able to predict product mixtures
so the key here lies in two things. number one
we understand the difference in the stability between the bromine radical
and the chlorine radical
well just as alkyl radicals follow
alkyl cation stability
also halogen radicals follow
halogen anion stability trend so because bromine
is a larger atom than chlorine it can better accommodate
the deficiency, it's a larger atom so that's more diffuse
around a larger volume and so because the bromine radical
can better accommodate the instability or in other words it's not as
unstable as the chlorine radical is it can wait
and follow the lowest energy pathway and that means that it is stable enough to
follow
a pathway to generate the lowest energy intermediate
and so that means that a bromine radical
is more likely to go ahead and extract this implied hydrogen here
generating the more stable tertiary alkyl radical there
that is the more favorable pathway that is a lower energy intermediate
than the other possibility so bromine isn't really gonna go that way because of
its enhanced stability
due to its size, and so that means if that were
much more
frequently producing the tertiary alkyl radical intermediate
that means that when a bromine is added its gonna
add in that position because that's where the radical is and so that's why
we're gonna get almost exclusively
the tertiary bromoalkane. now with chlorine
because the chlorine radical is much less stable
it's gonna go for whatever hydrogen it can get
it wants to satisfy the instability of being a radical
and so no matter where it collides with this molecule
it's gonna extract whatever hydrogen it can get
even if it ends up generating the less stable primary alkyl
radical intermediate and so that is why we see a lot less
regioselectivity with chlorination because of the size.
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