Aldol reaction - Alpha Carbon Chemistry - Organic Chemistry, Chemistry Video Lecture - JEE | Best Video for JEE

In this video, I want to
introduce you to a mechanism
called the aldol reaction.
And it's easily one of the most
important mechanisms and
reactions in all of organic
chemistry because it's a
powerful way to actually create
carbon-carbon bonds.
And it'll actually be a little
bit of a review of what we saw
with enol and the enolate ions
and the keto-enol tautomerism.
I have trouble saying that.
Anyway, let's start with
a couple of aldehydes.
And just for convenience I'll
make them identical.
So let's say that this is one
of aldehydes, it just has a
carbon chain there.
That's what the r is, it
could be of any length.
Who knows what's there.
And then I have another
carbon here.
And then this is bound to the
carbonyl group and we're going
to make it an aldehyde, although
you could do this
reaction with a ketone
as well.
And just to make things clear,
this carbon right here-- which
is going to be involved in a lot
of the business here-- let
me draw it's hydrogens.
Normally we don't have to
draw its hydrogens.
And just as a bit of review, the
carbon that is next to the
carbonyl carbon is called
an alpha carbon.
If this was a ketone, this would
have also been an alpha
carbon if this was a carbon.
And we're going to see in this
reaction, besides just
exploring the reaction, is
that these hydrogens are
actually much more acidic then
traditional hydrogens attached
to carbons on the rest
of the chain.
And it comes from the fact that
this proton can be given
to something else, the electron
can go to that
carbon, and then it'll be
resonance stabilized.
And we're going to see
that in a second.
Now I said I would draw two
molecules of that, because we
need two molecules.
We're actually going to
be, to some degree,
joining the two molecules.
So let me draw another aldehyde
right over here.
And I'm going to draw it
symmetric to this, because
it'll make it, I think, a little
bit easier to visualize
the two molecules.
Actually, let me just draw
it the same way.
But I'll draw it in
a different color.
So you have the r group
and then you have the
oxygen right there.
And I won't draw all of the
hydrogens on this guy, but
this and this are the
exact same molecule.
It's just the hydrogens
are implicit here.
Now, the aldol reaction I'll
show you will be in a basic
environment.
So you could imagine that it'll
be catalyzed by a base.
And so, imagine we have some
hydroxide laying around.
Some of the hydroxide anion.
Let me do that in a
different color.
So let's say we have some
hydroxide anion floating
around-- negative charge,
just like that.
I just told you that these
hydrogens are much more acidic
then hydrogens anywhere else
on a carbon chain-- these
alpha hydrogens.
So you could imagine a situation
where an electron
from the hydroxide is given to
one of these hydrogen protons
and then the electron that was
associated with that hydrogen
is now given back to
this alpha carbon.
And so if that were to happen,
the next step in our reaction
would look like this.
And I'll draw it
in equilibrium.
Actually, let me draw
it this way.
So the products of that step
would be in equilibrium, with,
you have your carbon chain or
the rest of your molecule
right there.
And that's just to show that
it could be anything.
It's attached here to the alpha
carbon, which is now
going to be negative-- I'll show
that in a second-- which
is attached to the carbonyl
group, which is
attached to a hydrogen.
And actually, I'll
stop drawing that
hydrogen for now, too.
Just, we know it's there.
But I'll keep drawing this
hydrogen right over here.
The other hydrogen was taken
away and this alpha carbon now
has a negative charge
because it got the
electron from that proton.
And of course, we have
the hydroxide.
It grabbed this hydrogen
and it is now water.
Now the reason why this was
acidic to begin with is
because this is resonance
stabilized.
And I'll show you that
it's resonance
stabilized right now.
This alpha carbon right here can
give its electron to the
carbonyl carbon.
And if the carbonyl carbon gets
an electron, it can give
an electron to this
oxygen up here.
It'll break the double bond.
So this configuration
is resonance
stabilized with this.
So I could draw it like this.
You have your r and then
you have a single
bond to this oxygen.
It now gained an electron.
It is now negative.
And you now have a double
bond, just like that.
And I could draw this hydrogen
if I like, or I don't have to.
It's implicitly over
there now.
And you might be familiar
with this.
This is the Enolate anion.
This right here is
the enolate ion.
If we had a hydrogen right here,
it would be enol, and we
would say hey, this
is the keto form,
this is the enol form.
We've seen this before.
Now, what's interesting about
the enolate ion is it can act
as a nucleophile.
It can do a nucleophilic attack
on the other aldehyde's
carbonyl group.
But it does it in kind of
a non-conventional way.
And I'll show you how it
does it right now.
So it does the attack
like this.
So let me draw this
guy over here.
So you have the carbonyl group
and then you have its alpha
carbon and then you have an
r group right over there.
There's actually a hydrogen
right over here, as well.
I just flipped it over.
This and this are the
same molecule.
And let me make it clear-- these
two guys right here are
residence forms. And, once
again, this is the reason why
it's easier to take this
hydrogen than other hydrogens
on a traditional carbon chain.
Easier to take an alpha hydrogen
to a carbonyl group
because you have this
resonance structure.
But this enolate ion, especially
this configuration
of it, you can imagine it doing
something like this.
You can imagine this oxygen
giving back the electron to
the carbonyl carbon-- to
this carbon right here.
And when that happens, then this
guy is going to be giving
up an electron.
And that electron that he gives
up-- let me do it in a
new color-- this electron that
he gives up could go and do a
nucleophilic attack on
this carbonyl group.
And so if that carbonyl carbon
gets-- let me do this in a new
color-- if this carbonyl carbon
gets an electron, then
it could give away an electron
to that oxygen right up there.
So the next step after
this, we would have
something like this.
And once again, I'll show it as
happening in equilibrium.
So from here we go right over
there, and what we have is a
situation-- let me draw this guy
on the left first. So we
have a double bond to
this oxygen now.
Actually, let me draw
the second.
So this is this oxygen.
We now have a double bond.
And let me do it in this same
purple color right over here.
And then we have the rest
of what was an aldehyde.
Where you have-- let me do it in
that same color-- and then
you have your r group
right over there.
But now this electron
gets in an attack
on this other aldehyde.
So this guy right here, this
alpha carbon is that same
alpha carbon we've been dealing
with, is now bonded to
this carbonyl carbon.
So it is now bound
to this carbonyl
carbon right over here.
And so it will look like this.
Let me draw it with
the right colors.
Get the orange out.
So that carbonyl carbon,
it now has a single
bond to this oxygen.
This electron was taken
back by it.
So this oxygen now has
a negative charge.
And it is bound to
its alpha carbon.
And then that is bonded to
another group, probably a
carbon chain or something that
contains a carbon chain or
another function or group.
Whatever you want to call it.
And then the final step.
This anion can get rid of its
negative charge by essentially
grabbing a hydrogen maybe
from this water
that was formed before.
Obviously, not going to be the
same molecule, but it could
grab it from this in
a previous step.
This water molecule that was
formed in a previous step.
And of course, this is all
in a basic environment.
So it can give an electron to
this hydrogen, and then the
hydrogen proton would lose an
electron to the hydroxide and
the hydroxide will become
negative again.
And so what will be
the final product?
The final product will be-- and
I'm just going to try my
best to redraw this thing
right over here.
You have this part of the
molecule, so you have this
carbonyl group right
over here.
It is attached to this radical
group right over there.
So that is this part.
And I can even do
the same colors.
This bond right over here is
this bond right over here.
And then this carbon is attached
to a carbon that's
attached to a hydroxyl
group now.
So it'll look like this.
And let me draw it.
So this oxygen is now this
oxygen, and it just captured
this hydrogen.
So it is now a hydroxyl group.
It's now an -OH group.
And then, finally, this
guy is bound to what
was an alpha carbon.
It's not anymore.
What was an alpha carbon,
which is then bound to a
radical group.
And if we want, we can remember
that there was
always, from the get-go,
there was always a
hydrogen over here.
So why is this called
the aldol reaction
and why does it matter?
Well, it's called the aldol
reaction because what we
formed is both an aldehyde--
notice this is an aldehyde--
and it's an alcohol.
So that's where the word
aldol comes from.
But the more important thing
about this-- and I don't want
to mislead you-- could have also
done this with a ketone
You could have had a methyl
group or a ethyl group.
You could have had a big
carbon chain here.
It still would've worked.
So the aldol reaction doesn't
only form things that are
aldehydes and alcohols.
It could have formed something
that's both a
ketone and an alcohol.
But that's why it's called
the aldol reaction.
But the more important thing
about the aldol reaction is,
one, it shows you how the
enolate ion can be a
nucleophile.
It shows you why the alpha
hydrogens are more acidic than
hydrogens on other parts
of carbon chains.
But the most useful aspect of
it is it's a useful way to
actually join two carbon
chains together.
Notice, we were able to join
this alpha carbon right here
to this carbonyl carbon over
here to form this aldol.
Or sometimes this will be
called-- because this is still
an alpha carbon right here, this
is an alpha carbon, this
is a beta carbon-- and so
sometimes this will be
referred to as a beta hydroxy.
And we've probably used things
from the pharmacy that has
this word in it.
This is also called
a beta hydroxy.
This is alpha, this is beta.
It has a hydroxyl group
on the beta carbon.
Beta hydroxy aldehyde.
Anyway, hopefully you found
that entertaining.