Keto-enol tautomerization (by Sal) | Alpha Carbon Chemistry | Organic chemistry | Khan Academy

Keto-enol tautomerization (by Sal) | Alpha Carbon Chemistry | Organic chemistry | Khan Academy

July 20, 2019 46 By William Morgan


Let’s explore another mechanism
that we can have
with the ketone.
And actually, an aldehyde can
undergo a very similar or
actually the same type
of reaction.
So let’s say that I had a ketone
that looked like this.
Let me draw my carbonyl group,
just like that, and then it is
bonded to a carbon
that is bonded to
two other CH3 groups.
And just to make it clear,
there’s three hydrogens off of
this carbon there implicitly.
But I’m going to draw the fourth
bond here, which is to
a hydrogen, because this
hydrogen is going to be
important for this reaction.
Now, we know that the
oxygen has two
lone pairs of electrons.
Let me draw it up here.
And let’s just imagine it’s
floating around in some water,
and we know that in water
there is some
concentration of hydronium.
And let’s say that one of the
hydroniums is right over here.
Hydronium is just positively
charged, so
this is right here.
Let me do it in a
different color.
This is what water looks like.
And if water gives away an
electron to a proton,
it looks like this.
It is hydronium, and then
it only has one
lone pair of electrons.
It gave away one of the other
electrons in its other lone
pair to a proton.
So you can imagine a reality,
where it’s like, hey, I could
grab that proton from this
hydronium, and then this will
turn back into water, and in
that situation, the mechanism
would look like this.
Let me do it in a
different color.
This blue electron gets given
to this proton, if they just
bump into each other just right,
and then the hydrogen’s
electron gets taken
back by what will
become a water molecule.
So if that happens, what do our
molecules now look like?
So now, what was a ketone looks
a little bit different
than a ketone.
It looks like this.
I changed it to a slightly
lighter color of green, so it
looks like that.
We have our lone pair over here,
but we no longer have
this lone pair.
At this end, we still have this
magenta electron, but now
it is in a covalent bond with
the blue electron, which was
now given to the hydrogen
proton.
Let me scroll up a little bit.
It was given to this hydrogen
proton up here.

And then this hydronium
molecule, it took back an
electron, and now it is
just neutral water.

It took back that magenta
electron, so now it has two
lone pairs again, so it
is just neutral water.
Since this oxygen up here in the
carbonyl group gave away
an electron, it now has
a positive charge.

But this is actually resonance
stabilized.
You could maybe see that this
would be in resonance, or
another resonance form of this
would be– if this guy’s
positive, so he wants to gain an
electron, so maybe he takes
an electron from this carbon,
the carbon in the carbonyl
group right over there.
So if you takes that electron,
then the other resonance form
would look like this.
Let me doing it in
the same colors.
You have now only a single bond
with this oxygen up here.
This carbon down here is still
bonded to the same carbons,
and then this carbon over here,
we could call this an
alpha carbon.
This is an alpha carbon
to the carbonyl group.
It still has a hydrogen on
it right over there.
And this oxygen, since it gained
this magenta electron,
now it has two lone pairs.
It has this pair over there,
and then it gained this
electron and this electron, so
it has another lone pair.
And, of course, it has the
bond to the hydrogen.
Since it gained an electron,
it is now neutral.
This carbon lost an electron,
so now it is positive.
So now this carbon right over
here is positive, and these
two are two different resonance
forms, so they help
stabilize each other.
And the reality is actually
someplace in between.
I could actually draw it in
brackets to show that these
are two resonance structures.
Now, you can imagine, just as
likely– and actually, I
shouldn’t just draw this as a
one-way arrow, because this
guy could take a hydrogen from
this hydronium, or a water
could take a hydrogen from this
guy, so this actually
could go in both directions.
So let me make that clear.
This could go in both
directions.
You could say that they’re in
equilibrium with each other.
You’re just as likely to go in
that direction as you really,
for the most part, are to go
on the other direction.
But you can now imagine, this
has now turned from a carbonyl
group, this has now an OH group,
this has now turned
into an alcohol, although we
have this carbocation here,
that this does not like
being positive.
And so you could imagine where
this electron right here on
this hydrogen nucleus might want
to go really bad to this
carbocation, and it just needs
something to nab the proton
off for it to go there.
And the perfect candidate
for that would
just be a water molecule.
We have this water floating
around, so let me draw another
water molecule, just
like this.
It has two lone pairs.
It can act as a weak base.
It can give one of
its electrons to
this hydrogen proton.
If it does that at the exact
same time, bumps into it in
the exact same way, this
electron can then go to the
carbocation.
And if that happened, you could
go in either direction.
This reaction is just as
likely to happen as the
reverse reaction, so we could
put this in equilibrium.

But if that were to happen, then
what started off as our
ketone now looks like this.
We have a bond to an OH group
just like this, and over
here– actually, let me
draw the rest of it.
We had our molecule that looked
like that, but now,
this electron gets giving back
to this carbocation.
We now have a double bond here
between what was a carbonyl
carbon and our alpha carbon.
So now we have this double
bond right over here.

That hydrogen has been
taken by the water,
and now that is hydronium.
So let me draw the water
or the hydronium.
So that water, it had that one
lone pair, and then the other
lone pair got broken up, because
it gave one of the
electrons to this hydrogen right
over here, and it went
back to being hydronium.
So what happened here?
We started with a ketone, and
they sometimes will call this
the keto form of the molecule,
and then we ended up with
something called
the enol form.

An enol comes from the fact that
it is an alkene that is
also an alcohol.
You could even call
it an alkenol.
It has a double bond, and on one
of the carbons that has a
double bond, it has
an OH group.
And the whole reason I show you
this mechanism is, one,
just to show you a mechanism
that could happen with an
aldehyde or a ketone.
This was a ketone, but if this
was a hydrogen right here,
this would have been occurring
with an aldehyde.
But even more, this is a pretty
common mechanism that
you’ll see in organic chemistry
classes, and
actually has a lot of functions
in biology, in general.
And these two molecules, this
ketone and this enol form,
these are called tautomers.

And the keto form is actually
the much more stable form.
In a solution, you won’t see
much of the enol form, but
these can occur.
It can spontaneously through
equilibrium get to
the actual enol form.
And so you could imagine, these
are tautomers, so this
mechanism is actually called a
tautomerization, and these are
the keto and enol forms
of the tautomers.