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. thanks for watching, guys. subscribe to my
channel for more tutorials and as always feel free to email me with questions