Tip:
Highlight text to annotate it
X
so now we have
Markovnikov's addition and then the anti-Markovnikov addition
two things to keep up with but as I said before
these are it's nice to be able to have the option of
directing that bromine in one direction or the other one limitation to this
anti-Markovnikov is that it only works with HBr
if you're using the chloride other halogens
you always get Markovnikov addition and so with HBr we have to
be told clearly, are there peroxides
or not and that would be stated so that you would know
which possibility you're looking at. in the absence of words like peroxides or an
indication
of ultraviolet light we assume it is the normal reaction.
Well, Markovnikov's addition can be understood in terms of carbocations
forming
anti-Markovnikov conditions
can be understood in terms of radicals and their stability
and we've seen radical reactions back in chapter four as well
radicals typically don't have a positive or negative charge they are
neutral
but they feature unpaired electrons and so they are
unstable in the same way that carbocations are unstable
but they do exist for some period of time and they're the key to figuring out
what products that we make
so with this anti-Markovnikov addition
what we get are radicals of bromine atoms
and that bromine atom can add across the double bond
again one of the bonds from the double bond
makes a covalent bond to bromine and that's going to leave an unpaired electron on
the carbon next door
and so here are the two possible radicals
here is one of the two possible radicals we can make
and we expect this one to be more stable because it's a secondary radical
if the bromine had ended up on the second carbon
and the radical, the unpaired electron, on first, it would be less stable
so just as we tend to make the most stable carbocations
we also tend to see with reactions that involve
radicals they want to make the most stable carbon radicals
and so this is the key difference between this mechanism and what we saw
before
in the regular Markovnikov we had
hydrogen and a positive charge that were the new features of the molecule
here the bromine-- halogen is coming in first and then in a subsequent step
a hydrogen makes a bond where that radical is
and so we get a stable molecule but the bromine
ends up in just the opposite orientation of where it would be if there were no
radicals at all and in fact in real life
one has to be meticulous to keep sources of radicals
out of the reaction flask so that
that we don't accidentally get an anti-Markovnikov addition if we don't want one
with HBr again it can go either way
so oftentimes if we want the normal
addition we do these reactions in the dark so there's no stray
ultraviolet light and we make sure that we don't accidentally have even a speck
of peroxides or anything that can create radicals
but again it's only the bromide that has this variation
chlorides always are normal Markovnikov. iodide's--
always the normal Markovnikov.
well let's talk a little bit about hydration that's adding the elements to make water
across a double bond
and you can see here the recipe is an acid like sulfuric acid and water
itself
so these are fairly concentrated solutions us of sulfuric acid
we do need an acid to make hydration work but we don't want to use like HCl or HBr
because as we've just seen we'll
get the halogen in our product but here the intention is to make an alcohol
so we've got a solvent that is
got a lot of water in it but we again need some source of hydrogen ions
H2SO4 is great at being an acid
but it does not have any tendency for the sulfate to behave like a nucleophile
at least not at these concentrations and so
as it says here this follows the Markovnikov rule and we get
the -OH group where we would expect if we follow the rule that the incoming
hydrogen goes to the carbon
already has the most hydrogens, and again for the ring compound you can mentally
add in hydrogens when you doing problems like this
to be sure where the hydrogens are in the first place