Addition of Water (Acid-Catalyzed) Mechanism

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Addition of Water (Acid-Catalyzed) Mechanism

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In a typical organic chemistry problem set or exam, they'll
give you some type of reaction.
In this case, we have-- what is this molecule?
We have one, two, three, four, five carbons.
It is an alkene.
It starts on the number two carbon if we start numbering
from here, and the number two carbon also
has a methyl group.
So this is 2-methyl-pent-2-ene, usually
naming it-- we don't have to name it when we worry about
mechanisms. But they'll give you a molecule like
2-methyl-pent-2-ene.
They'll draw it out like this and they'll draw a reaction.
They're saying in the presence of water and catalyzed by
hydronium ions, we end up with this thing over here.
And then they'll give you a bunch of white space below it,
and they say how did this happen?
Give us the mechanism.
And that's what we're going to do in this video.
So let's start with this molecule right there, the
2-methyl-pent-2-ene.
So let me draw it right here.
So we have double-bond carbon.
I have a hydrogen.
I have two carbons right over here, and then I have a
carbon, H3C, and then-- and we put H3 before the C just to
make it clear the carbon is what's bonded
to the other carbon.
That's why we don't write CH3, but that wouldn't be
completely incorrect, and then you have HC3 over here.
And then we're going to have some
hydronium molecules around.
So let me draw a hydronium molecule.
So it is an oxygen in the middle and then
we have three hydrogens.
One, two, and three.
And oxygen also will have one unbonded pair of electrons,
and it has a positive charge here.
And the reason why this has a positive charge, if you think
about regular water, regular water looks like this: six
valence electrons.
Let me do the valence electrons in another color.
One, two, three, four, five, and six.
The other half of these bonds comes
from hydrogen's electrons.
Now, in the case of a hydronium molecule you have a
proton sitting out here.
It has absolutely no electrons, and to form the
bond, one of oxygen's unpaired valence electrons go to the
hydrogen, and that's what forms the bonds.
You could imagine that that electron is right over there.
That's why it has a positive charge.
Now, oxygen wants that electron back.
It doesn't like having this positive charge, and we've
seen in the last videos that sometimes the carbons, one of
the carbons in the double bond, is pretty predisposed to
at least temporarily giving up an electron.
And we saw that the one that's most likely to give up an
electron is the one that's bonded to the most carbons.
And in this case, this guy right here is bonded to the
most carbons.
He's bonded to two carbons.
This guy's only bonded to one.
He is a secondary, or if he loses his electron, it will be
a secondary carbocation as opposed to if this guy loses
his electron, it will be a primary carbocation.
This one will be more stable.
So what we can do is, we'll say, look, this electron
that's at this end of this bond right here, it goes to
the hydrogen right over there.
And then the electron that this hydrogen was taking from
the oxygen will go back to the oxygen, and it all kind of
happens simultaneously.
This electron is attracted, I guess you could say, to the
partial positive charge, because maybe this electron
spends more time over there.
And then this electron goes back to the
oxygen to make water.
So what happens after this end of the reaction or this part
of the reaction happens?
So after that happens, we will-- let me clear out this
water, actually.
That's not essential to our mechanism, to
our reaction mechanism.
So after that happens, what will everything look like?
Well, we'll have our basic molecule.
So we have a H3C, we have a carbon, we have another H3C.
It is now a single bond to the other carbon, which is bonded
to a hydrogen and two other carbons, CH2-CH3.
Now, this guy has lost his electron.
That electron went to the hydrogen.
So this guy now is a positive carbocation.
Carbocation by definition is positive so maybe I'm being a
little redundant there.
So it's positive.
And then this bond-- let me do this in orange.
This bond is now between this carbon and the hydrogen.
The electron went to that hydrogen to form a bond.
So this bond is now with this hydrogen over here and then
this thing that was a hydronium molecule will now be
just water.
So I'll draw it like this.
It got its electron back.
Let me draw its two electron pairs, just like that.
And then it has two hydrogens.
It is H2O.
And you can imagine that maybe that was one of the electrons
that was on oxygen end, and then this was the electron
that it had given essentially to the hydrogen, but it's now
giving back, and now they're both back with the oxygen.
That's the one it always had on its end, and this is the
one that it just got back from the hydrogen.
Now, what do you think will happen next?
Well, when you can just look at the final products of this
reaction you say, well, at some point, I have to have an
OH attached right over here, and that's usually a good way
to figure out what's going to happen next.
So somehow, it looks like this might attach to
that positive charge.
It was attached to a positive charge before.
It was attracted to that hydrogen proton, so why can't
it do it with the carbocation?
And then once it does that, maybe we lose
one of these hydrogens.
And actually this reaction-- I shouldn't draw these strict
arrows here, because every step of this reaction is
really-- there's some form of equilibrium.
It doesn't only go in one direction.
Things can go in both directions here.
So let me clear this.
And let me show that these aren't just one-way reactions.
They can go in both directions.
But if we are trying to show a mechanism, we're assuming that
we're going in the direction that we're
drawing this out in.
So in the next step, let's making that oxygen attack that
carbocation.
Or another way, I guess a better way, the carbocation is
attacking the oxygen.
It's going to take back an electron.
So let's do that.
Let's make it-- lets draw it taking back electron or an
electron being given back to this carbocation.
So then we'll have, let's say, that orange electron right
there-- let me do it in a different color.
Let's say we have this.
That orange one is attracted to the
carbocation and forms a bond.
Now, what's going to happen?
Let me draw it as an equilibrium reaction.
But we're going to go in this direction just to see how we
can get that final end product.
We have H3C.
We have the carbon that was a carbocation.
We have H3C.
Now, we have the single bond to a hydrogen to two carbons
right over here.
We have this business, that bond that was formed in the
early part of our mechanism.
Let me draw that.
And then we have that blue hydrogen there that helps us
keep track where things came from.
And now this has attached.
We have gotten that orange electron to the carbocation.
It will form a bond.
It's going to form a bond.
And what's that bond going to look like?
Let me draw this oxygen over here like that.
It has that electron pair still all to itself.
It has the two hydrogens.
It had that green electron, but now it's not just paired
up with this orange electron.
That orange electron went.
It left the oxygen and went to that carbocation.
So now the orange electron is right over there and
it formed this bond.
Now carbon has four valence electrons.
This carbon has one, two, three, four valence electons.
This oxygen just lost an electron again and so it now
has a positive charge again.
Now, what do you think will happen next?
We're very close to the final product.
The thing we have here looks almost identical to the thing
that we have to end up at, except we have a whole
hydronium group, I guess you could say, or
actually a whole water.
We have H2O attached here.
They only have an OH.
And this oxygen right here, it's a positive charge.
It wants to get one of its electrons back.
It could take it back from here or it could take it back
from one of these hydrogens.
And if it takes it back from one of these hydrogens, then
we will be at our final end product.
We will have a neutrally charged OH group right there.
So how would that happen?
Well, we're in an aqueous solution.
There is water all around us.
There are molecules of water, so let me draw one of those
molecules of water that's just floating around, and maybe one
of its electrons right here.
Let me color code it.
That makes it a little bit more fun.
Maybe one of it's electrons are attracted to the partial
positive end of this molecule or this hydrogen.
So I'll do it in green.
It goes to this hydrogen.
This hydrogen no longer needs this electron on this end of
the bond, and so it goes back to the oxygen like that.
And then what is going to happen?
What do we have left at that end, or, in general, after
that part of the reaction happens?
We have all of this stuff over here.
Maybe I should just copy-- well, let me just redraw it.
So we have H3C, a carbon-carbon single bond.
I'm going to draw everything in yellow that we started off
with in yellow.
CH2-CH3, H3C, and then we have that first hydrogen that got
attached on this end.
We figured out it was going to be this end more likely
because of Markovnikov's rule.
One way to remember is this carbocation was more stable.
The other way is the side that has more hydrogens will get
more hydrogens.
The side that has more functional groups will get
more functional groups.
But now we have all of this business.
We have this magenta bond here, and we have an oxygen in
the middle.
It is still bonded to that hydrogen over there.
This pair of electrons, I'm just going to do it on the
side, just so we can draw it how we can
conventionally draw it.
And now we had-- and let me make it very
clear with the colors.
We had this electron on one end of a bond, so it's still
there on that end of the bond.
And then we have this electron that was with the hydrogen,
but now it's gone back to the oxygen.
So now we have two pairs again.
We have six valence electrons around the oxygen: one, two,
three, four, five, six.
It is neutral.
And now one of those water molecules has become a
hydronium molecule.
So now that water looks like this.
That thing that was a water molecule looks like this.
It has its lone pair.
And then it has one end, that electron right there, but this
blue electron went and joined the hydrogen.
Let me draw it little closer to the hydrogen right there.
And then it has that bond.
So when all is said and done, we were able to get to the
molecule that the mechanism, or that the reaction
described, and it was actually the most
reasonable path to follow.
We followed Markovnikov's rule.
We figured out that this was the most stable carbocation,
and then we just added the water.
This is an addition reaction of water in the presence of an
acid, in the presence of hydronium.
Hopefully, you found that vaguely useful.