hey it's professor Dave, let's talk about oxymercuration-demercuration.
so let's take a look at another kind of addition reaction we can do. this one is actually
very similar to dihalogenation or even more so the halohydrin formation and
what we're gonna do is this so we have this compound, the tip-off is we've got
mercury in there so oxymercuration demercuration we're referring to
mercury so this compound is mercury with two acetate groups on there, Ac if you
don't know what Ac is that's this acetyl group here so it's just this carbonyl
and a methyl group so OAc, there's two acetate groups on that mercury atom this
is the solvent system we're gonna run this in and it's two steps so the first
step involves the and we're gonna look at the mechanism in a second here but
this is the first step and then the second step is a reduction and what
these conditions do if you're gonna make like a flashcard to sort of memorize
what these what this does if you just see the reagent conditions it's gonna do
a markovnikov hydration so we remember markovnikov hydration we looked at acid
catalyzed markovnikov hydration so by a different mechanism we would do
something similar and so it's markovnikov hydration. remember hydration
means that we are putting an H and an OH on there an H on one carbon and OH
on another, H plus OH is water and it happens with markovnikov
regiospecificity. and so that means that the OH is going to end up on the
more substituted carbon of the ones that were in the double bond. so we've got
these two carbons in the double bond, this one is more substituted because of
that methyl group so that's where the OH ends up, so if we just see these
reagent conditions we know we're doing oxymercuration demercuration and we
know that that is a markovnikov hydration so we're gonna put the OH
on the more substituted carbon. so that's just a general overview of what this
reaction does. so now let's look at the mechanism, and this is where we're gonna
see that it's actually very similar like we said to dihalogenation or even more
so halohydrin formation. and so we've got this
situation here we have this alkene, remember we're looking at this edge on
like this and we by now we know that pi bonds can behave nucleophilically that's
nothing new so that can go ahead and interact with this electron deficient
mercury atom and let's kick off one of those acetate groups, and so that is
going to give us this mercurinium ion intermediate and so this may look
familiar if you imagine this is just a bromine atom we would have our bromonium
intermediate just like in dihalogenation or halohydrin formation.
and so now this is where this is where the hydration is gonna happen because
now we can have water pop this open just like with halohydrin formation so we
can kick that open right we're gonna pop open the mercurinium ion intermediate
and we're gonna have this situation here so we put water on the molecule and then
we can have water another molecule of water or we can have the acetate ion
that we kicked off over here just something in solution is going to go
ahead and get that proton and neutralize that oxygen atom and we get to here so
we've popped that OH on there and so then the second step remember back
here the second step is a reduction and sodium borohydride is going to be the
step that is going to kick this off and get us the proton there so that's the
product we've got we we have H and we have OH so that's why this is a
hydration and so this is gonna be our product. that's the mechanism and so just
a couple of key points here number one in terms of the regiochemistry that's
gonna all happen here this is very similar to halohydrin formation let's
say we had this situation right let's say these were that was a methyl over
there so this would have been the more substituted just like with those other
mechanisms we learned this mercurinium ion intermediate this sigma bond is
going to end up basically coming apart a little bit more than this other one in
an attempt to slightly neutralize that positive mercury atom so that's again
like a bromonium ion intermediate where one of those
bonds is going to you know be able to come apart a little more easily
because if water can come here this is gonna be the more partially positive
carbon due to that electronic effect and so that's where the regiochemistry comes
from. so that is one aspect to be aware of, another thing that we want to be
aware of is that there is a variation of this called alkoxymercuration demercuration
and all that we need to be aware of for that is if we run this
instead of in water if we run this in alcohol so we have ROH instead of H2O
that is what is going to make this alkoxymercuration demercuration and
that will simply give us OR instead of OH so that would be again it would
still everything would be the same mechanistically etc still be markovnikov
but we'd have an alkoxymercuration demercuration because we've done this
in alcohol instead and we would now have a R so that is a method of ether
synthesis. and so that is what we need to know mechanistically and in terms of the
regiochemistry involved with oxymercuration demercuration.
so just to put this together let's take a look at an example for a substrate so
we're looking at this and remember that if we see this say on a test we see
structure like this and we see these conditions we know that we want to think
oxymercuration demercuration, and we have memorized even if we don't want to
remember the mechanism we haven't memorized that these conditions are
going to yield a markovnikov hydration and so a markovnikov hydration what that
means is we're gonna look at the two carbons participating in the pi bond and
we're going to we're going to notice that this is the more substituted carbon
of the two in the pi bond so we know that's where we're gonna put the
hydroxyl group, so we take a look over here but we are gonna get a product
mixture because the thing is we do have regiospecificity but we do not have
stereospecificity. so we're generating a chiral center here when we put that
hydroxyl group on there we are gonna get a racemic mixture we're gonna get both
versions here so we will have that and so this is a useful technique because
it's another way of doing a markovnikov hydration and it is a way of doing it
without carbocation rearrangement. so if we did acid catalyzed hydration say in
this example we might expect to or we would expect to get a carbocation
intermediate with the carbocation here we could get a methanide shift. there
could be carbocation rearrangement. if we have a situation where we do not want
carbocation rearrangement this would be a better technique to employ
because we can see that we could get this desired product maybe not in stereo
specific fashion but we are going to get the regiochemistry and the structural
isomer that we want so that's your oxymercuration demercuration and
alkoxymercuration demercuration.