Sn1 and Sn2 epoxide opening discussion

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Sn1 and Sn2 epoxide opening discussion

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I want to follow up the last video on the Sn2 ring opening
of the cyclohexene oxide.
Because the way I explained it, it might have been a
little non-intuitive.
Because when we first learned about Sn2 and Sn1 reactions,
we said, well if you have an aprotic solvent with an OK
leaving group, but a really good nucleophile, you'll have
an Sn2 reaction.
But if you have a protic solvent with an excellent
leaving group and a weak nucelophile, then that would
lead you to an Sn1 reaction.
All of the ingredients that I showed you in the last video
actually were more in this direction.
This was actually an acid catalyzed reaction, so it was
definitely protic.
And then this ring is strained right here.
This epoxide ring.
And we already protonated this oxygen, so it was a good
leaving group.
And clearly, the water, the H2O right here, is a weak
nucleophile.
So if we just went off of the checklist that we learned
about when we just explored the differences between Sn1
and Sn2 reactions, we probably would have predicted an Sn1
reaction here.
We probably would have said the first step would have been
that this bond just broke on its own, and this guy just
left, forming a carbocation.
And only then would the water, the weak nucleophile, would
have attacked the carbon.
In that case, it could have attacked from either side.
Because all of a sudden it would lose its handedness,
this carbon right here.
But everything I've read-- and I've actually never performed
this reaction in the chemistry lab.
But everything I've read says that this is the
reaction that occurs.
That you actually have this, I guess you could call it
stereospecificity.
The trans version is actually what predominates.
And actually what is going on is an Sn2.
So my best explanation is that this is really more of, you
could call it an Sn2 like reaction.
Because it really meets the checklist more for an Sn1 in
terms of what the reactions are.
But because the end product is actually selective, you
actually end up with the trans, as opposed to both the
cis and the trans.
We're assuming that an Sn2 type
reaction must have occurred.
But I really wanted to make this clear.
Because it really kind of goes against the grain of our
checklist of predicting whether an Sn1 or an Sn2
reaction would occur.
The reason why, at least in my mind, I think that the Sn1
reaction maybe didn't occur is, in order for it to occur,
this carbocation needs to form.
And that carbocation is secondary.
It's not tertiary.
So it's maybe just not quite stable enough.
Now Sn1 reactions can occur with epoxides.
I'll show you two examples.
So if I have-- let me draw two examples here.
So this is ethane.
If I just did a double bond here, it would be ethene.
But if I have these bonded to an oxygen,
this is ethylene oxide.
And once again, if this was an alkene, it would be called
either ethene-- that's actually the official name,
but some people call it ethylene.
And so the common name for this is ethylene.
Actually I spelled it wrong.
It's ethylene, not with the-- ethylene oxide.
I've even seen on the web a couple of times, people call
this ethene oxide.
Which, in some ways, would be more consistent.
Let's say on one hand, we have ethene oxide, and let's say on
the other hand, we have something
that looks like this.
Let me try my best to draw it.
We have something that looks like this.
And so this part, if we just look at the main chain.
You could either view this as a propane.
Or having three carbons.
1, 2, 3.
And then a methyl breaking off of it.
But this y shape, where you have four carbons here, is
usually called isobutyl.
So you could call this isobutylene oxide.
So let's call this isobutylene oxide.
That's what you're more likely to see.
Isobutylene oxide.
And remember, they just pretend like
this is a double bond.
That's where we get the isobutylene oxide.
But then to make it clear that it's really not a double bond,
that there's really an oxygen in there, it's really not an
alkene, you put the oxide there.
Now let's think about the different reactions that might
occur if we have a nucleophile in either situation.
So if we have a nucleophile over here for the ethylene
oxide, either of these carbons are primary carbons.
So if you have a carbon right here.
Let me draw all of its bonds.
It's bonded to one other carbon.
It's bonded to a hydrogen, and another hydrogen right there.
So it's a primary carbon.
It would not be a stable carbocation.
So in this case, you would definitely predict that an Sn2
reaction would have to occur with this epoxide.
That the nucleophile would attack back here.
And then this electron would be taken back by the oxygen.
And that would open the ring.
So this would be an Sn2 situation.
And in my mind, this is pretty clear.
But in this situation, you do have a primary
carbon right here.
It's only bonded to one other carbon.
But over here, you have a tertiary carbon.
It's bonded to 1, 2, 3 carbons.
So this would be a stable carbocation.
So in this circumstance, you would actually expect that
this epoxide ring is so strained, that the first step
is that the oxygen would just take the electron.
The oxygen would just take the electron.
And then you're left with something
that looks like this.
Let me do it in the same colors, so it's clear.
You would be left with something
that looks like this.
And now, since it took the electron, just like that.
Or actually, maybe let me take a step back.
It has to be very good leaving group.
It wouldn't necessarily just take the electron right there.
The first step in order to make this a good leaving group
would be if you maybe had a water around, or something
that it could take, or some type of acid, that it could
take a hydrogen proton from.
So the first step is it would take a hydrogen proton.
Let me do this a little bit neater.
It would take a hydrogen proton.
So it would give an electron to a hydrogen proton, so that
it becomes a good leaving group.
So now our molecule looks like this.
Do it in the same colors.
Our molecule will look like that.
That's our carbon.
And now this is an oxygen bonded to a hydrogen.
And now this has a positive charge.
This now will just be a water.
I won't draw it anymore.
And now this is an excellent leaving group.
Tertiary carbon.
This is classic Sn1.
Classic Sn1 reaction.
So this guy can essentially leave. And then we are left
with down here-- let me draw it over here.
We're left with something that will look like this.
We have our carbon bonded, not to
hydrogens, to other carbons.
So these are carbons over here.
And then this carbon is bonded to this OH.
We now have an alcohol.
And this is now a carbocation.
And now this is ready to be attacked by even a weak
nucleophile.
Even a weak nucleophile could attack this.
So when you have a tertiary carbon in your epoxide ring,
look for an Sn1 reaction.
But if you have anything less, as far as I can tell, based on
the reactions that I've looked at in multiple books-- if you
have secondary or primary carbons, it seems like Sn2 is
going to be more likely to break your epoxide ring.