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Ring-opening Sn2 reaction of expoxides

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Ring-opening Sn2 reaction of expoxides

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I told you, in the last video, that
epoxides are very reactive.
That this little triangle right here, this equilateral
triangle, is highly strained, highly unstable.
So it makes it want to do something, get out of this
configuration.
What I want to do, in this video, is
to show you a reaction.
What we have, right here, and we saw this in the last video.
I've drawn a couple of more hydrogens.
And I've actually drawn a little bit of the three
dimensional nature of the bonds right here.
Because it'll matter.
This is cyclo-.
And remember, you can just pretend like this triangle
here, if it was replaced with a double bond, this would be
cyclohexene.
But because, instead of a double bond right here, we
have these bonds to this oxygen, this
is cyclohexene oxide.
Let's think about what would happen if we had some
cyclohexene oxide mixed in with water.
And we have some type of maybe acid catalyst in there.
So it will allow some extra hydrogen
protons to float around.
So we have water.
But since we actually put some acid catalyst there-- and it
could really be any acid catalyst, anything that would
increase the hydrogen proton concentration-- we're more
likely to have some hydronium ions floating around.
And we know what hydronium ions look like.
They look like water with an extra proton, with an extra
hydrogen on them.
So hydronium ions look like this.
This is water.
Let me do it in a different color since I already used the
blue for the cyclohexene oxide.
So this is water right here.
What happens to get hydronium is the oxygen in the water
takes a proton from someplace else, from some acid, the acid
catalyst, in this case.
It would essentially give an electron to it.
If this electron gets given to some hydrogen proton, the
ending result will look like this.
You have a bond formed with another hydrogen.
And now, you have a positive charge.
You have a positive charge on the oxygen because it gave
away an electron.
This is hydronium.
We've seen it many times.
And this is what results if you put an acid in water.
It will increase the hydronium concentration.
You'll see more and more of this.
So what's likely to happen if we have cyclohexene oxide with
water as a solvent, but we also have a good amount of
hydronium floating around?
The more acid we put in, the more hydronium we'll have.
Well, a possible reaction is, well let's see, this oxygen is
just as likely, or it may want to get a hydrogen proton just
as much as a hydronium.
So if they bump into each other in the exact right way,
this oxygen, right here, could give an electron to that
hydrogen right there.
And then the original, this orange hydronium, or this
orange oxygen right here, can take its electron back.
If that happens, what do we end up with?
Well, let's see.
So if that happened, now we have-- Let me draw this
original molecule.
It will no longer be cyclohexene oxide exactly.
Because it just got a hydrogen proton.
It's an intermediary now.
Let me try my best to draw it though.
All right.
We have that.
Then, we have the oxygen popping out of the page.
It had these two extra electron pairs, but now it
gave one of its electrons to this proton.
So now, it gave one of its electrons to this hydrogen
right here.
I call it a proton, because a hydrogen without an electron
is just a proton.
It doesn't have another neutron inside of the nucleus.
So you have the hydrogen over there.
Now, since this oxygen gave away an electron, it now has a
positive charge.
Let me draw these two purple hydrogens that
are behind the page.
So that's one hydrogen there.
One hydrogen there.
Obviously, there's other hydrogens on these carbons.
But if I drew it, it would take a while.
It would make a whole diagram messy.
But it's always assumed that a neutral carbon
will have four bonds.
And of course, this thing right here is now just water.
This thing took its electron back.
I'll draw it here.
This thing took its electron back.
And now, it's water.
It could be this one.
But there's obviously tons of water around.
So I could even draw other water molecules.
It doesn't have to just be that one.
Now, what is a likely to happen?
And this is the fun part of this reaction.
Because it's actually something we've seen before.
It's a reaction that we've seen many, many times already.
But it's just not obvious when you see it in this form.
When you look at this molecule, right here, what's
going on here?
Well, I already told you that I have this highly, highly
strained bond here.
It's like this equilateral triangle.
The bond angles are closer than they want to be.
The electrons want to get away from each other.
If you try to do this with a chemistry model, it would
actually strain the plastic or the wood of the chemistry
model to actually make it.
On top of that, this oxygen with this extra hydrogen now,
it is actually a good leaving group.
And so now, it's probably triggering some things in your
brain about the type of reaction that might occur.
And think about this carbon right here.
Think about this carbon.
Actually, these two carbons are the same.
So I'll just pick on this one, the bottom one, for fun.
Think about this carbon right here.
It is bonded.
It's a secondary carbon.
So it would not be super great for an Sn2 reaction.
But you can have an Sn2 reaction with a secondary
carbon, especially when the leaving group wants to leave
bad enough.
So if one of these water molecules, it didn't have to
be this first one right here, if it just bumps into this
carbon in the exact way, it can actually act as a
nucleophile.
Water isn't, traditionally, a very strong nucleophile.
But it can be a nucleophile, especially if the leaving
group is ready to leave and this is a
really strained bond.
So what you could imagine is that this
water gives an electron.
It loves this carbon nucleus right there.
It gives it to that carbon.
And since that carbon is getting an electron from this
water over here, then it can release an electron back to
this oxygen, making it neutral.
So it can release this back to this oxygen right here.
What will we end up with then?
Now, something interesting has just happened.
And this is typical of all Sn2 reactions.
Remember, it's Sn2.
We're substituting with a nucleophile.
Water is a weak nucleophile.
But, in this case, it'll stick better than this thing that's
all strained.
And we call it 2, Sn2, because both of the reactants are
involved in the rate determining step.
This is the rate determining step right here.
So what do we end up with?
Let me just draw the hexane ring, just like that.
Now, all of a sudden, this oxygen up here, what
will it look like?
Well, this bond gets broken.
It takes back an electron.
So you only have this bond to the oxygen, this top bond to
the oxygen.
That's the oxygen right there.
It took this electron back.
So now, it has this electron and the electron that was
bonded to the carbon.
So it has a pair of electrons.
And then, it's bonded to that hydrogen that it took from the
hydronium in the first step.
And then it's bonded to this hydrogen over there.
So it really is just an OH group now, bonded right that.
It's popping out of the page.
You still have this hydrogen that's going behind the page.
But since we had a Sn2 reaction, you can imagine this
came from the back.
And this let go from the front.
So what will now happen is, this hydrogen that was kind of
behind the page, now will pop forward.
Because this guy went from even behind that hydrogen.
So this hydrogen, over here, will now pop forward.
So this hydrogen, now, has popped forward.
I'll circle it just so you see it's the same hydrogen.
But it has popped forward, since this guy, this
nucleophile attack, happened from the back.
And now, he is attached.
He has attached down here.
This water has attached.
Well, it's not water anymore.
So you had the oxygen.
Two hydrogens.
And then it had two pairs.
But now, one of the pair turns into a bond.
Because it gives this electron there.
So these two guys are bonded.
This electron is given to the carbon.
And the bond is behind the page now.
Just like that.
And then the final step is-- now, since this guy gave an
electron, he has a positive charge here.
Just to make it nice and clean, you can imagine another
water molecule comes and takes one of those hydrogens away
from this guy.
All these hydrogens are just getting passed around between
these water molecules.
So you can imagine this water molecule
could take that hydrogen.
This oxygen takes the electron back, and
then becomes neutral.
And then our final product-- we are now done--
will look like this.
So we had cyclohexene oxide, a reactive epoxide, in water
with some acid around.
So it kind of catalyzed the leaving group and all that.
We just saw that in this.
But what does the resulting molecule look like?
So popping out of the page up here, we have an OH.
That's this right here.
I'm just simplifying a little bit.
Going behind the page, we have a hydrogen right over there.
And then, popping out of the page we have a hydrogen.
So popping out of the page, right
here, we have a hydrogen.
And then, behind the page, we have another OH group.
So we've made a diol.
And, in particular, if we wanted to name this thing
right here, we have our cyclohexane ring.
That's our main ring.
We have cyclohexane.
And then, we have OH groups on the-- We
can just start numbering.
We want to start numbering where the groups are.
So we call this the 1, 2, 3, 4, 5, 6 carbon.
We have OH groups at the 1 and the 2 carbons.
So we have two of them.
So it's a diol.
So we call this, 1 comma 2-diol, because
we have two OH groups.
Diol.
The -ol is for alcohol when you have these OH groups.
We have two of them.
And they are on opposite sides.
This alcohol, or this OH, is popping out of the page.
This OH is behind the page.
So if we really wanted to be specific about even the
stereochemistry here, we would call this trans.
They're on opposite sides.
One is in front.
One is behind.
So this is trans.
This is trans-cyclohexane-1,2-diol.
So, hopefully, you enjoyed that.
What the fun thing about this is, one, to see that epoxides
are reactive.
You just even see a new reaction.
But the really fun thing to see is that these Sn2
reactions keep popping up.
In fact, the ones we learned, Sn2, Sn1, E1, E2, these just
keep, over and over, popping up in chemistry in places
that, maybe it wasn't obvious at the first glance.
But then, when you actually think about it, it's actually
perfect for that type of reaction.

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Ring-opening Sn2 reaction of expoxides

Ring-opening Sn2 reaction of expoxides