so the other reaction wanna show you is the
anti-markovnikov hydrohalogenation. so remember when we were looking at
addition reactions
we were looking at hydrohalogenation and we're understanding that that was a
regiospecific reaction, regiospecific meaning that the halogen would
always go to the more substituted
of the two carbons participating in the pi bond because of the fact that it
would
only go by the more stable carbocation intermediate. remember this pi bond
would get the proton
leave the secondary carbocation and that's where the bromide would go
and we'd get the markovnikov product
in a regiospecific manner but then we did look at the mechanism but we also
noted the fact
that in the presence of peroxides this would shift
to anti-markovnikov regiospecificity, in other words we would get
the halogen on the less substituted
of the two carbons participating in the pi bond. we didn't look at why
well here's why it's a free radical reaction and so
the peroxides obviously play a very key role in this
so let's take a look the initiation step, it's
gonna be provided by the peroxides and so remember when we talked about
oxygen-oxygen covalent bonds as ones that are
able to to undergo homolytic bond cleavage
that's the initiation step, so we have this oxygen-oxygen covalent bond
undergoing homolysis and we get two hydroxide
radicals so it's those hydroxyl radicals that are gonna propagate the formation
of other radical so let's say that a hydroxyl radical finds
one of these molecules of HBr, well what's gonna happen is this radical and
then one
electron from this covalent bond are going to form a new one and now we've
got a water molecule
and then that's going to propagate the existence of a bromine
radical, then from there this is the key step
because this is where the regiospecificity is involved, the bromine
radical
is gonna interact with this pi bond that is necessary for the addition
reaction
what's gonna happen is it's going to form a new carbon-bromine bond
but it's gonna do it in a regiospecific manner
because of the stability of the alkyl radical that it will generate
so in other words one of these pi electrons is gonna go to form a covalent bond
with this bromine radical and it is only gonna happen here
because if it does so then the other pi electron
will go to this carbon, the secondary carbon
leaving the more stable of the two possible alkyl radical intermediates
if the bromine radical interacted at this position
and this pi electron went to form the new carbon-bromine bond there
then that would mean that the other pi electron would be left on the primary
carbon
and we'd then have a primary alkyl
radical intermediate and that is the less stable of the two
so we will not produce that so in a regiospecific manner
the bromine will add to the less
substituted of the two carbons participating in the pi bond
that leaves an alkyl radical intermediate
which will then go ahead and undergo another step
with HBr and it will just extract a proton there
leaving another bromine radical there. recall once again that these are all
single handed
electron pushing arrows denoting the movement of a singular electron
and so we've gotten to our product here
this is our anti-markovnikov
hydrohalogenation product, regiospecific
to be the less substituted alkyl bromide of the two possibilities
and we've managed to do it all with propagation steps, there could be
termination steps
choose anything, this bromine radical, hydroxy radical
anything
if two radical species meet they'll terminate but none of them were needed
in the mechanism generating the products we didn't draw them so once again
hydroxy radical
initiates, propagates existence of the bromine radical and then the bromine
radical goes ahead and
ads in regiospecific manner
specifically in an anti-markonikov manner
so that it generates the more stable of the possible
alkyl radical intermediates so that will shed some light on the mystery we
understood the mechanism before
for the markovnikov hydrohalogenation
now we now the mechanism for the
anti-markonikov variety as well.
thanks for watching, guys. subscribe to my channel for more tutorials
and as always feel free to email me with questions