CBSE Class 12 > Class 12 Videos > Syn Dihydroxylation
Syn Dihydroxylation Video Lecture - Class 12
FAQs on Syn Dihydroxylation
| 1. What is syn dihydroxylation? |  |
Ans. Syn dihydroxylation is a chemical reaction that involves the addition of two hydroxyl groups (-OH) to a double bond in a molecule. It is a highly useful transformation in organic chemistry as it allows the formation of vicinal diols, which are compounds with two hydroxyl groups attached to adjacent carbon atoms.
| 2. What are the reagents used for syn dihydroxylation? | |
Ans. The most commonly used reagents for syn dihydroxylation are potassium permanganate (KMnO4) and osmium tetroxide (OsO4). These reagents can selectively add two hydroxyl groups to a double bond, resulting in the formation of a vicinal diol.
| 3. How does syn dihydroxylation occur? | |
Ans. Syn dihydroxylation can occur through two different mechanisms: the concerted pathway and the stepwise pathway. In the concerted pathway, the double bond attacks the oxidant (KMnO4 or OsO4) simultaneously from both sides, resulting in the formation of a cyclic intermediate called an osmate ester. This intermediate is then hydrolyzed to yield the vicinal diol. In the stepwise pathway, the oxidant first forms an intermediate with one face of the double bond, and then a nucleophile attacks the other face to produce the vicinal diol.
| 4. What are the applications of syn dihydroxylation? | |
Ans. Syn dihydroxylation is widely used in organic synthesis for the preparation of various compounds. It is particularly useful in the synthesis of natural products, pharmaceuticals, and complex molecules. The presence of vicinal diols in molecules can introduce new functionalities and increase the complexity of the target compound.
| 5. Are there any limitations or challenges in syn dihydroxylation? | |
Ans. Yes, there are some limitations and challenges in syn dihydroxylation. One limitation is that the reaction requires the use of toxic and expensive reagents such as OsO4. Another challenge is the potential for over-oxidation, leading to the formation of unwanted byproducts. Additionally, the reaction conditions need to be carefully controlled to achieve high selectivity and yield. Researchers are continuously exploring new catalysts and methodologies to overcome these limitations and make syn dihydroxylation more efficient and sustainable.
Text Transcript from Video
In the last video, we
saw how to add two OH
groups on anti to each other.
In this video, we'll
see how to add them
on the same side,
or syn addition.
So we start with our
alkene and to our alkene,
we add osmium tetroxide.
So this is the OSO4 here.
And we could also add
water and tert-butanol.
And what that does, is that
forms your diol over here,
adding your two OH groups on the
same side for a syn addition.
You don't have to use
water and tert-butanol.
There are several
other ways to do it.
I've seen hydrogen peroxide
and aqueous bisulfide.
So just do whatever your
professor has for your class.
So let's look at the
mechanism and figure out
why this is a syn
addition of the OH.
So as you can see over
here, we have our alkene
and our osmium
tetroxide right up here.
And in this
mechanism, we're going
to get a concerted
six electron movement.
So six electrons are going
to move at the same time.
So these electrons in here are
going to bond to this carbon.
That's going to push the
electrons in this pi bond
off to bond to this oxygen.
And then the
electrons in this bond
are going to move into
here onto the osmium.
So let's look at what
that would give us.
So here we have our
osmium right here.
And then that's double
bonded to these oxygens.
And then down here, there used
to be two bonds to this oxygen.
Now there's only one,
because the other electrons
moved in here to form the
bond between the oxygen
and the carbon.
I'll go ahead and put a wedge
and a dash here for this guy.
And then this carbon
over here on the right
has a wedge and a dash
bonded to something else.
And then it is now bonded
to this oxygen on the right
here, like that.
And then our lone
pair of electrons
moved in here on the osmium.
So let's color coordinate so
you can follow along here.
So these electrons
in here were the ones
that moved in here
to form our bond.
And the electrons in this
bond right in here, those
move to form this bond.
And then finally, these
electrons in here.
These are the ones that are
now a lone pair of electrons
on the osmium, like that.
So we formed an osmate
ester right here.
And we can hydrolyze
our osmate ester
with the addition of water.
So if we have add some water
to it, so H2O-- or like I said,
there are several
other things that you
can add instead--
the water is going
to hydrolyze this osmate ester
and give you back your diol
up here.
It'll give you back
this, with your two OH
groups on the same side.
And the reason why those two OH
groups end up on the same side
is because of this osmate
ester intermediate.
The way those two
oxygens add-- the way
these two oxygens add here--
they add on the same side.
They add in a syn fashion.
All right, so what's the
other product we get?
After this osmate
ester is hydrolyzed,
the osmium is going to be right
here bonded to our two oxygens.
And that's going to form an OH
on either side here, like that.
So if you're looking
at oxidation states,
so if we check out the oxidation
state of osmium over here
on the left-- so this
osmium-- if you remember
from general chemistry,
doing your oxidation states,
you'll see that the osmium
is a plus 8 stage over here.
So it's plus 8 over
here on the left.
And after the osmate
ester is hydrolyzed,
we get this structure over here.
And again, from
general chemistry,
if you assign your oxidation
states to that osmium,
you're going to get a plus 6.
So we have a plus 8 to plus 6.
So a decrease or a reduction
in the oxidation number.
So osmium is reduced while those
two carbons are being oxidized,
while your alkene is
oxidized to a diol.
So that's a redox
reaction, of course.
Let's look at how you
would do this in practice,
because the problem with
osmium is it's very toxic.
It's also very expensive.
So you don't want to
use very much of it.
You want to use as little of
the osmium as you possibly can.
So how do you get around the
toxicity and also the cost?
So let's look at cyclohexene.
If we were to add osmium
tetroxide to cyclohexene,
it would be OHSO4.
We want to add only a
catalytic amount, the smallest
amount that we absolutely need.
So we're going to add a
catalytic amount of this,
just a very, very small
amount of osmium tetroxide.
And we're going to add
something called NMO,
which is
N-methylmorpholine-N-oxide.
And what this is
going to do, is it's
going to regenerate your OSO4
by oxidizing the osmium plus 6
back into osmium plus 8.
So the NMO is going to
take this one over here,
the osmium that's in
the plus 6, and it's
going to oxidize it back
into the plus 8 stage so
that we can reuse our
OSO4 in the reaction.
And that way, we can use very,
very small amounts of it.
So that's what the
presence of NMO does.
All right.
Our solvents-- we use water
and tert-butanol, just
like we did above.
So I'll just write out the
abbreviation this time,
so TBUOH.
And we're going to get
a syn addition of OHs.
So we're going to get an OHs
that add on the same side.
OH that adds on the same side.
And you might think,
"Oh, well, wouldn't we
get another product here?
I could draw two dashes
instead of two wedges.
Wouldn't that be a
different product?"
Well, in reality, these are
the exact same molecule.
So this is actually
a meso compound.
So there's a plane of
symmetry right here.
And you can superimpose these
two molecules on each other.
Therefore, they're
the same thing.
So we're only going to get
one product for this reaction
where we add our two
OHs on the same side.
Let's look at another way to
achieve syn dihydroxylation
here.
So let's look at a
different way to do it.
In this case, we're going
to use permanganate.
So we take our
alkene and then we
take permanganate, MNO4
minus, so something
like potassium permanganate.
And usually you do this reaction
in cold sodium hydroxide
solution-- a cold aqueous
solution of sodium hydroxide,
so water's in there as well.
And what you're going to get
is a syn addition of your OHs.
You're going to get your
two OH groups adding on
to the same side, just
like we did before.
And your other
product would be MNO2.
So you can follow this
reaction pretty easily,
because we know that
MNO4 minus is purple.
And the oxidation state
of manganese on the left
here is plus 7 when you
assign your oxidation states.
And over here on the
right, it goes to plus 4.
So there's a reduction.
Manganese has been reduced
while those two carbons have
been oxidized for
this redox reaction.
MNO4 minus is purple.
MNO2 is kind of a really,
really dark brown kind of color.
So when you do this reaction,
it's pretty much instantaneous.
Everything turns from purple
to brown and you get your diol.
And there's a fast way of
figuring out oxidation,
if something is an
oxidation or a reduction.
You could assign
oxidation states
like I did in the
previous video,
like I did in one of
the earlier videos.
Another way to do
it is to just count
the number of bonds to oxygen.
So over here on the left, your
alkene-- zero bonds of carbon
to oxygen.
Over here on the right,
each of your two carbons
has one bond of
carbon to oxygen.
So you can tell it's
been oxidized that way.
That's a fast way of doing it.
Why is this a syn
dihydroxylation?
Why do our 2 Hs add
on on the same side?
Well, if we draw out the
mechanism really fast,
so we have MNO4
minus-- so let's draw
our permanganate anion-- so
here is the dot structure
for our permanganate anion-- so
negative charge on this oxygen,
because it has three lone
pairs of electrons around it.
So if we can fit in those
three lone pairs of electrons,
so it's a very, very
similar mechanism to before.
We take our alkene.
We're going to get a concerted
six electron movement.
So these electrons are
going to move into here.
These electrons are going
to form a bond here,
and these electrons are
going to bond right here.
So it's just like before.
It's just like with the osmium.
So we get our manganese
and these two top oxygens
don't do anything, so let's go
ahead and leave them like that.
This oxygen here gets
bonded to a carbon, carbon,
oxygen, like that.
And then we have our wedge
and our dash, like that.
And then our lone pair of
electrons and our manganese.
So it's the exact
same thing as before.
We can hydrolyze it to
get our diol like that.
It's the exact same
mechanism, which
is why our two OHs
add syn like that.
The problem with using
permanganate is permanganate
is such a strong
oxidizing agent.
It's hard to stop it at
this first oxidation.
So if you do everything
really cold, with ice,
and then you stop your
reaction immediately,
it's not too hard to
get your diol out.
But if you heat things up or
let things go for a long time,
you won't be able to stop the
oxidation at one oxidation.
It'll keep going.
So let's go ahead
and draw what would
happen if we added some
heat to our oxidation
with permanganate.
So we add potassium
permanganate.
So KMNO4 and we add some
heat to it this time.
So what's going to happen
if you add heat to it?
It's not going to
stop at the diol.
It's going to keep oxidizing.
So instead of one bond
of carbon to oxygen,
you're actually going to get
two bonds of carbon to oxygen.
So let me redraw my
alkene here, and let
me show you a clever way
of doing alkene cleavage.
So if I just make this a
little bit longer than usual,
here's my alkene.
The result is going to cleave
this bond between my two
carbons.
So I'm going to
actually break it.
That's alkene cleavage, so
I'm going to go like that.
I'll shorten it a little
bit right here, like that.
And I'm going to put
oxygens in there.
So to draw my product
for this alkene cleavage,
I'm going to put an oxygen
here and an oxygen here.
So there are now
two bonds of carbon
to oxygen, instead of one
bond like we did up there.
So it's been oxidized.
And this could be a ketone.
This could be a carboxylic acid.
This could be CO2, and it all
depends what kind of reaction
that you're doing.
So let's do a quick example.
So let's start over here.
Let's start with this as
our starting reactant.
So put a methyl group there, a
hydrogen here, a methyl group
here, and a methyl group here.
So we add potassium
permanganate and heat.
And I'm going to go ahead
and redraw my reactant,
so we can do that little trick.
So I'm going to
redraw my reactant.
This time I'm going to make
these little bit skinnier.
And I'll make this a CH3
and make this a hydrogen
and put our two methyl
groups over here, like that.
So alkene cleavage,
so you heat it up.
You can't stop at
the first oxidation.
Permanganate is such
a strong oxidizer,
so it's going to keep
on going, and we're
going to get alkene cleavage.
We're going to break the
bond between our two carbons,
and then each of these carbons
is going to get an oxygen.
So now there are two
bonds of carbon to oxygen.
So I'll go ahead and do
that, put my oxygen in there,
like that.
And it doesn't even stop here.
So the molecule on the
left, our aldehyde here--
we haven't talked about
aldehydes in this course--
but aldehydes are easily
oxidized, especially
in the presence of
something like permanganate.
So this aldehyde right here is
going to get oxidized again.
So we have acid aldehyde.
It's going to get oxidized.
And if we oxidize an aldehyde,
two bonds of carbon to oxygen,
we would oxidize to something
that has three bonds of carbon
to oxygen.
So that's our three bonds of
carbon to oxygen, like that.
So acid aldehyde gets
oxidized to acetic acid.
So let's just emphasize that.
Over here, two bonds
of carbon to oxygen.
Over here, three bonds
of carbon to oxygen.
So it's been oxidized.
Once again, you could
assign oxidation states.
We just don't have time
in this video to do that.
So you're going to get acetic
acid as one of your products.
And then we had acetone over
here as our other product.
So alkene cleavage,
this reaction
isn't very useful, because
of these side reactions.
It's hard to stop the oxidation.
So if your goal is to add two
OH groups on the same side,
it's probably a
little bit better
to use the osmium
way of doing it,
since the permanganate
way is not
quite as predictable
in its products,
because of over oxidation.
So in the next video, we'll
look at another alkene cleavage,
ozonolysis.
saw how to add two OH
groups on anti to each other.
In this video, we'll
see how to add them
on the same side,
or syn addition.
So we start with our
alkene and to our alkene,
we add osmium tetroxide.
So this is the OSO4 here.
And we could also add
water and tert-butanol.
And what that does, is that
forms your diol over here,
adding your two OH groups on the
same side for a syn addition.
You don't have to use
water and tert-butanol.
There are several
other ways to do it.
I've seen hydrogen peroxide
and aqueous bisulfide.
So just do whatever your
professor has for your class.
So let's look at the
mechanism and figure out
why this is a syn
addition of the OH.
So as you can see over
here, we have our alkene
and our osmium
tetroxide right up here.
And in this
mechanism, we're going
to get a concerted
six electron movement.
So six electrons are going
to move at the same time.
So these electrons in here are
going to bond to this carbon.
That's going to push the
electrons in this pi bond
off to bond to this oxygen.
And then the
electrons in this bond
are going to move into
here onto the osmium.
So let's look at what
that would give us.
So here we have our
osmium right here.
And then that's double
bonded to these oxygens.
And then down here, there used
to be two bonds to this oxygen.
Now there's only one,
because the other electrons
moved in here to form the
bond between the oxygen
and the carbon.
I'll go ahead and put a wedge
and a dash here for this guy.
And then this carbon
over here on the right
has a wedge and a dash
bonded to something else.
And then it is now bonded
to this oxygen on the right
here, like that.
And then our lone
pair of electrons
moved in here on the osmium.
So let's color coordinate so
you can follow along here.
So these electrons
in here were the ones
that moved in here
to form our bond.
And the electrons in this
bond right in here, those
move to form this bond.
And then finally, these
electrons in here.
These are the ones that are
now a lone pair of electrons
on the osmium, like that.
So we formed an osmate
ester right here.
And we can hydrolyze
our osmate ester
with the addition of water.
So if we have add some water
to it, so H2O-- or like I said,
there are several
other things that you
can add instead--
the water is going
to hydrolyze this osmate ester
and give you back your diol
up here.
It'll give you back
this, with your two OH
groups on the same side.
And the reason why those two OH
groups end up on the same side
is because of this osmate
ester intermediate.
The way those two
oxygens add-- the way
these two oxygens add here--
they add on the same side.
They add in a syn fashion.
All right, so what's the
other product we get?
After this osmate
ester is hydrolyzed,
the osmium is going to be right
here bonded to our two oxygens.
And that's going to form an OH
on either side here, like that.
So if you're looking
at oxidation states,
so if we check out the oxidation
state of osmium over here
on the left-- so this
osmium-- if you remember
from general chemistry,
doing your oxidation states,
you'll see that the osmium
is a plus 8 stage over here.
So it's plus 8 over
here on the left.
And after the osmate
ester is hydrolyzed,
we get this structure over here.
And again, from
general chemistry,
if you assign your oxidation
states to that osmium,
you're going to get a plus 6.
So we have a plus 8 to plus 6.
So a decrease or a reduction
in the oxidation number.
So osmium is reduced while those
two carbons are being oxidized,
while your alkene is
oxidized to a diol.
So that's a redox
reaction, of course.
Let's look at how you
would do this in practice,
because the problem with
osmium is it's very toxic.
It's also very expensive.
So you don't want to
use very much of it.
You want to use as little of
the osmium as you possibly can.
So how do you get around the
toxicity and also the cost?
So let's look at cyclohexene.
If we were to add osmium
tetroxide to cyclohexene,
it would be OHSO4.
We want to add only a
catalytic amount, the smallest
amount that we absolutely need.
So we're going to add a
catalytic amount of this,
just a very, very small
amount of osmium tetroxide.
And we're going to add
something called NMO,
which is
N-methylmorpholine-N-oxide.
And what this is
going to do, is it's
going to regenerate your OSO4
by oxidizing the osmium plus 6
back into osmium plus 8.
So the NMO is going to
take this one over here,
the osmium that's in
the plus 6, and it's
going to oxidize it back
into the plus 8 stage so
that we can reuse our
OSO4 in the reaction.
And that way, we can use very,
very small amounts of it.
So that's what the
presence of NMO does.
All right.
Our solvents-- we use water
and tert-butanol, just
like we did above.
So I'll just write out the
abbreviation this time,
so TBUOH.
And we're going to get
a syn addition of OHs.
So we're going to get an OHs
that add on the same side.
OH that adds on the same side.
And you might think,
"Oh, well, wouldn't we
get another product here?
I could draw two dashes
instead of two wedges.
Wouldn't that be a
different product?"
Well, in reality, these are
the exact same molecule.
So this is actually
a meso compound.
So there's a plane of
symmetry right here.
And you can superimpose these
two molecules on each other.
Therefore, they're
the same thing.
So we're only going to get
one product for this reaction
where we add our two
OHs on the same side.
Let's look at another way to
achieve syn dihydroxylation
here.
So let's look at a
different way to do it.
In this case, we're going
to use permanganate.
So we take our
alkene and then we
take permanganate, MNO4
minus, so something
like potassium permanganate.
And usually you do this reaction
in cold sodium hydroxide
solution-- a cold aqueous
solution of sodium hydroxide,
so water's in there as well.
And what you're going to get
is a syn addition of your OHs.
You're going to get your
two OH groups adding on
to the same side, just
like we did before.
And your other
product would be MNO2.
So you can follow this
reaction pretty easily,
because we know that
MNO4 minus is purple.
And the oxidation state
of manganese on the left
here is plus 7 when you
assign your oxidation states.
And over here on the
right, it goes to plus 4.
So there's a reduction.
Manganese has been reduced
while those two carbons have
been oxidized for
this redox reaction.
MNO4 minus is purple.
MNO2 is kind of a really,
really dark brown kind of color.
So when you do this reaction,
it's pretty much instantaneous.
Everything turns from purple
to brown and you get your diol.
And there's a fast way of
figuring out oxidation,
if something is an
oxidation or a reduction.
You could assign
oxidation states
like I did in the
previous video,
like I did in one of
the earlier videos.
Another way to do
it is to just count
the number of bonds to oxygen.
So over here on the left, your
alkene-- zero bonds of carbon
to oxygen.
Over here on the right,
each of your two carbons
has one bond of
carbon to oxygen.
So you can tell it's
been oxidized that way.
That's a fast way of doing it.
Why is this a syn
dihydroxylation?
Why do our 2 Hs add
on on the same side?
Well, if we draw out the
mechanism really fast,
so we have MNO4
minus-- so let's draw
our permanganate anion-- so
here is the dot structure
for our permanganate anion-- so
negative charge on this oxygen,
because it has three lone
pairs of electrons around it.
So if we can fit in those
three lone pairs of electrons,
so it's a very, very
similar mechanism to before.
We take our alkene.
We're going to get a concerted
six electron movement.
So these electrons are
going to move into here.
These electrons are going
to form a bond here,
and these electrons are
going to bond right here.
So it's just like before.
It's just like with the osmium.
So we get our manganese
and these two top oxygens
don't do anything, so let's go
ahead and leave them like that.
This oxygen here gets
bonded to a carbon, carbon,
oxygen, like that.
And then we have our wedge
and our dash, like that.
And then our lone pair of
electrons and our manganese.
So it's the exact
same thing as before.
We can hydrolyze it to
get our diol like that.
It's the exact same
mechanism, which
is why our two OHs
add syn like that.
The problem with using
permanganate is permanganate
is such a strong
oxidizing agent.
It's hard to stop it at
this first oxidation.
So if you do everything
really cold, with ice,
and then you stop your
reaction immediately,
it's not too hard to
get your diol out.
But if you heat things up or
let things go for a long time,
you won't be able to stop the
oxidation at one oxidation.
It'll keep going.
So let's go ahead
and draw what would
happen if we added some
heat to our oxidation
with permanganate.
So we add potassium
permanganate.
So KMNO4 and we add some
heat to it this time.
So what's going to happen
if you add heat to it?
It's not going to
stop at the diol.
It's going to keep oxidizing.
So instead of one bond
of carbon to oxygen,
you're actually going to get
two bonds of carbon to oxygen.
So let me redraw my
alkene here, and let
me show you a clever way
of doing alkene cleavage.
So if I just make this a
little bit longer than usual,
here's my alkene.
The result is going to cleave
this bond between my two
carbons.
So I'm going to
actually break it.
That's alkene cleavage, so
I'm going to go like that.
I'll shorten it a little
bit right here, like that.
And I'm going to put
oxygens in there.
So to draw my product
for this alkene cleavage,
I'm going to put an oxygen
here and an oxygen here.
So there are now
two bonds of carbon
to oxygen, instead of one
bond like we did up there.
So it's been oxidized.
And this could be a ketone.
This could be a carboxylic acid.
This could be CO2, and it all
depends what kind of reaction
that you're doing.
So let's do a quick example.
So let's start over here.
Let's start with this as
our starting reactant.
So put a methyl group there, a
hydrogen here, a methyl group
here, and a methyl group here.
So we add potassium
permanganate and heat.
And I'm going to go ahead
and redraw my reactant,
so we can do that little trick.
So I'm going to
redraw my reactant.
This time I'm going to make
these little bit skinnier.
And I'll make this a CH3
and make this a hydrogen
and put our two methyl
groups over here, like that.
So alkene cleavage,
so you heat it up.
You can't stop at
the first oxidation.
Permanganate is such
a strong oxidizer,
so it's going to keep
on going, and we're
going to get alkene cleavage.
We're going to break the
bond between our two carbons,
and then each of these carbons
is going to get an oxygen.
So now there are two
bonds of carbon to oxygen.
So I'll go ahead and do
that, put my oxygen in there,
like that.
And it doesn't even stop here.
So the molecule on the
left, our aldehyde here--
we haven't talked about
aldehydes in this course--
but aldehydes are easily
oxidized, especially
in the presence of
something like permanganate.
So this aldehyde right here is
going to get oxidized again.
So we have acid aldehyde.
It's going to get oxidized.
And if we oxidize an aldehyde,
two bonds of carbon to oxygen,
we would oxidize to something
that has three bonds of carbon
to oxygen.
So that's our three bonds of
carbon to oxygen, like that.
So acid aldehyde gets
oxidized to acetic acid.
So let's just emphasize that.
Over here, two bonds
of carbon to oxygen.
Over here, three bonds
of carbon to oxygen.
So it's been oxidized.
Once again, you could
assign oxidation states.
We just don't have time
in this video to do that.
So you're going to get acetic
acid as one of your products.
And then we had acetone over
here as our other product.
So alkene cleavage,
this reaction
isn't very useful, because
of these side reactions.
It's hard to stop the oxidation.
So if your goal is to add two
OH groups on the same side,
it's probably a
little bit better
to use the osmium
way of doing it,
since the permanganate
way is not
quite as predictable
in its products,
because of over oxidation.
So in the next video, we'll
look at another alkene cleavage,
ozonolysis.
About this Video
Apr 20, 2026 Last updated
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