Leah here from leah4sci.com and in this video I’ll take you through the reaction and mechanism
for Oxymercuration Demercuration.
Oxymercuration Demercuration is an alkene addition reaction where the pi bond reacts
with mercuric acetate and water to get mercury and an alcohol group attached to the carbon
followed by a reduction using sodium borohydride to kick out the mercury.
This reaction is regeioselective because it follows Markovnikov's rule where the OH adds
to the more substituted carbon but unlike other Markovnikov addition reactions, there
is no carbocation rearrangement because mercury acts like a carbocation baby sitter which
is something we’ll talk about in the mechanism.
So let’s take a look at the mechanism.
We have a mercury ion bound to two acetate group.
OAC is simply an oxygen single bound to a carbon which is double bound to another carbon
and a CH3.
Acetate is the conjugate of acetic acid and has a negative charge.
here we have two of them down to the mercury ion.
To start this reaction, we have the nucleophilic pi electrons reaching out for the electrophile
or the mercury atom.
Markovnikov cannot have three bonds and so one of the acetate groups will be kicked out
along with its lone pair of electrons to float around as acetylene solution.
To show the product we redraw the carbon skeleton but now we have a question of, do we add the
mercury to the primary carbon or to the secondary carbon?
This is a Markovnikov addition reaction because we have fleeting carbocation forming and Markovnikov
rule says you form the more stable intermediate, bring the carbocation on a more substituted
carbon.
In this case the primary carbon will form a very unstable carbocation so it’s safer
to add the mercury acetate there.
Notice the bond between mercury and carbon is the red electrons that used to be the pi
bond on the starting alkene.
The secondary Alkene would form a more stable carbocation but it’s a very leading positive
charge.
mercury has a lone pair of electrons and as soon as that carbocation forms Mercury immediately
attacks before it even has a chance to undergo a carbocation rearrangement.
Always think of Mercury carbocation babysitter, meaning that it hold on to that carbocation
and doesn’t let it go anyway.
So event hough we have a tertiary carbon nearby which will form the more stable carbocation,
it’s not happening in this reaction.
Instead we have mercury bound to two carbon atoms.
The first red bond use to be the pi electrons and the second green bonds is the electrons
on mercury attacking.
Since the carbon is no longer positive, that positive charge transferred onto the mercury
giving us a mercurium intermediates.
Even though mercury has the positive charge, it’s pulling slightly on the electrons between
itself and carbon making both carbon atom feel that slight positive charge.
the primary carbon is not as capable of holding a charge and is therefore very slightly positive.
The secondary carbon would be much more capable of holding that positive charge and therefore,
even though they’re both partially positive this one is considered even more partially
positive which means when we have a water molecule as a nucleophile coming in for the
attack in the next step, the water will be attracted to the more substituted secondary
carbon and attack that carbon atom.
This causes the bonds between the secondary carbon and mercury to break and the electrons
collapse onto the mercury atom.
The next intermediate, we now have a red bond between the primary carbon and Mercury OAC
and we have a blue bond between the secondary carbon and oxygen which is holding to hydrogen
atoms.
Since Oxygen use one of its lone electron pairs to attack the carbon it only has one
pair remaining giving it a formal charge of plus one.
I keep referring to charges on the atoms but if you’re not comfortable quickly finding
the formal charges, go back to my orgo basics video series and learn my trick for finding
the formal charge.
you can find this on my website at leah4sci.com/orgobasics.
We have the Axonium or positive Oxygen attached to carbon.
This is almost our product because the final product would be an alcohol.
For the next step, you can show a water molecule or acetate in solution deprotonating your
Oxonium..
We’ll show oxygen reaching for hydrogen with one of its electrons pairs to grab that
hydrogen atom but since hydrogen can only form one bond, the electrons binding hydrogen
and oxygen will collapse onto the Oxygen atom breaking the Oxygen from Hydrogen.
What we form is the carbon skeleton with the HgOAC still attached.
But now we have oxygen with just one hydrogen, it’s initial blue lone pair and the new
purple lone pair restoring the neutral charge.
this is the alcohol product.
We also form acetic acid and solution because the acetate is now carrying that actual extra
proton.
Let’s move that out of the way.
The next step is where it gets tricky.
Most professors will allow you to just show what happens without requiring an explanation.
In fact, one of my students told me her professor called the last step magic.
Now take my word for it, it’s not magic, but the mechanism slightly involved with I
think a radical intermediate which we’re not gonna show.
What we will show is the simple summary of the step.
For this step, we have a reduction reaction where sodium borohydride is going to replace
the HgOAC of the molecule.
This is called reduction because the reactive atom in this molecule is the hydride or the
H minus and when you add hydrogen to a molecule that is considered a reduction reaction.
And so here we have our final product with the green hydrogen sitting on the primary
carbon where mercury used to be and the alcohol sitting on the secondary more substituted
carbon in keeping with Markovnikov’s rule.
Be sure to join me in my next video where I take you through a similar mechanism except
we’ll use alcohol instead of water as our solvent to get a completely different product.
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