Comparing E2 E1 Sn2 Sn1 Reactions

Back

⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.

Comparing E2 E1 Sn2 Sn1 Reactions

相關課程

0 / 750

What I want to do in this video is to try to figure out
what type of reaction or reactions might occur if we
have-- what is this?
One, two, three, four, five It's in a cycle.
This is bromocyclopentane.
If we have some bromocyclopentane dissolved
and our solvent is dimethylformamide.
Sometimes you'll see that just written as DMF.
And I've actually drawn the formula for it here, so we can
think about what type of a solvent it is.
And also, in our solution, we have the methoxide ion.
So we also have the methoxide ion right here.
So let's think about what type of reaction might occur.
And just to narrow things down, we'll think about it in
the context of the last four types of reactions
we've looked at.
So this might be an Sn2 reaction, an Sn1 reaction, an
E2 reaction, or an E1 reaction.
We're going to look at all the clues and figure out what's
likely to occur, and then actually draw the mechanism
for it occurring.
Now, the first thing since they gave us the solvent and
other things that are in the solvent, let's think about how
those might affect the reaction.
So if we look at this solvent right here, whenever you look
at any of these reactions, when you look at the solvent,
you just want to think about it.
Is it protic or not?
And protic means that it has hydrogens that can kind of be
released or that their electrons could be nabbed off
and these protons could just float around.
And if we look over here, we do have hydrogens, but all of
the hydrogens are bonded to carbon.
And carbon is unlikely to just steal a hydrogen's electrons
and let the hydrogen float around.
Carbon is not that electronegative.
If you had hydrogens bonded to an oxygen, that'd be a
different question.
Then you would have a protic solvent.
But in this case, all the hydrogens bonded to carbons,
not likely to get their electrons nabbed off and float
around as free protons.
So this is an aprotic solvent.
Now, we've gone over this a little bit with Sn2 and Sn1,
but the same idea applies.
In order to have an Sn2 or an E2 reaction, you have to have
either a strong nucleophile or a strong base, and the same
thing could actually be both, although they're not always
correlated.
We've seen that before.
Now, if you had a protic solvent, it would stabilize
the strong base or the strong nucleophile.
The protons would react with them.
They would take the electrons from that strong base or that
strong nucleophile.
So in order to have an Sn2 or an E2, you have to have no
protons flying around, so you need an aprotic solvent.
So this aprotic solvent will favor Sn2 or an E2 reaction.
Now, so our mind is already thinking in Sn2 or E2, let's
think about the reactants themselves.
So over here, we have the methoxide ion.
And let's think about whether it's a strong or weak.
Let's think about it first as a strong or weak nucleophile.
It's actually a pretty strong nucleophile.
It is a strong nucleophile.
So that would put us in the direction of an Sn1.
So we have two data points.
I'm sorry, for an Sn2.
We have two data points for Sn2 because remember, it has
to just kind of go in there and be active.
It's not too big of a molecule, so it's not going to
be hindered.
But it's also an extremely strong base, even stronger
than hydroxide.
So it's also an extremely strong base, which might lead
us or that does imply that we're going
to have an E2 reaction.
Now, the last thing we need to think about is the carbon
where the leaving group might leave from.
And immediately, when you look at the bromocyclopentane,
there's only one functional group attached to the chain,
and that is the bromo group right here, right there.
It is attached to this carbon.
We could call that the alpha carbon, and it
is a secondary carbon.
This carbon right here is bonded to
one, two other carbons.
This alpha carbon-- let me write it this way.
This alpha carbon is a secondary carbon, and that
kind of makes it neutral in this mix.
If it was a methyl or primary carbon, it
would favor Sn2, actually.
I mean methyl, the only thing you could have is an Sn2.
And if it was a tertiary carbon, it would favor Sn1 or
E1 because it would favor a stable carbocation.
The leaving group could just leave. And if this guy was
bonded to another carbon, it would be very stable.
But in this situation, it's a secondary carbon
bonded to two carbons.
It's a little bit neutral.
Any of these reactions might occur.
When we look at all of the other data points, they're
pointing at both Sn2 or E2.
We have a strong nucleophile/base.
We have an aprotic solvent.
It's going to be Sn2 or an E2 reaction.
So let's actually draw the reactions.
Let me do the Sn2 first. So let me do it in orange.
So if we were to have an Sn2 reaction, let
me redraw the molecule.
Let me draw the cyclopentane part.
I want to make sure-- let me draw it the same way I had it
drawn up there.
So the pentagon is facing upwards.
And then we have our bromo group right there.
So we have our methoxide ion right over here.
So CH3O minus.
Or another way we could view it is that this oxygen has
one, two, three, four, five, six, seven valence electrons
with a negative charge.
One of these electrons right over here, this can attack the
substrate right over there, that carbon.
Right when that happens, simultaneously this bromine is
going to be able to nab an electron
from that same carbon.
And then we are going to be left with-- the bromine now
becomes the bromide anion.
It had one, two, three, four, five, six,
seven valence electrons.
One, two, three, four, five, six, seven.
Now it nabbed one more electron, making it bromide.
Now it has a negative charge.
And if we were to draw the chain, it
would look like this.
Well, we could draw it on this, and I might as well draw
it on this side, just so it's attacking from the other side.
This is the chiral substrate, so we don't have to be too
particular about how we draw the connections to the carbon.
We're not actually even showing anything
popping in or out.
But we would have the methoxide ion, where now it's
bonded, so it's no longer an ion, so it's
OCH3, just like that.
It has bonded to this carbon.
Obviously, implicitly this carbon had another hydrogen
that we are not showing.
Just that quickly, that was the Sn2 reaction.
That is the mechanism.
Now let's think about what the E2 reaction is.
To do the E2 properly, to give it justice, we're going to
have to draw some of the hydrogens.
So on the E2 reaction, let me draw that in blue.
Let me draw the cyclopentane part.
Let me draw it big.
Actually, over here, it's less important to draw it too big.
So let me draw the pentagon.
The pentagon just like that.
That is the bromine, three, four, five, six, and then it
has a seventh valence electron right over here.
This is the alpha carbon.
That right there is the alpha carbon.
And then there are two beta carbons.
There are two beta carbons right over there and there.
They each have two hydrogens on them.
They each have two hydrogens.
I know it's becoming a little hard to read.
They each have two hydrogens on them.
And in an E2 reaction, the strong base will react-- let
me make it a little cleaner than that.
Let me get rid of the beta.
The beta makes it's a little dirty.
OK, so they each have two hydrogens on them.
Now in an E2 reaction, the strong base-- over here, the
methoxide ion was acting as a strong nucleophile.
In E2, it's going to act as a strong base.
It's going to nab off a hydrogen off of one of the
beta carbons.
And you might want to say, OK, which one?
Let's look at Zaitsev's rule.
It doesn't matter.
These are symmetric.
They are both bonded to two other carbons.
They both are bonded to the same number of hydrogens.
It doesn't matter.
It's actually going to be random which one, and you
actually won't be able tell the difference
because it's symmetric.
So let's just draw it like this.
Let me draw the methoxide ion.
One, two, three, four-- or anion, maybe I
should say-- five, six.
And then it has one bond to the CH3.
It has a negative charge, very, very, very strong base.
It can go over here and nab the hydrogen and leave
hydrogen's electron behind.
Maybe I'll take a color.
This electron can be given to the hydrogen so that it forms
a bond with it.
Hydrogen's electron-- let me do this in a suitably
different color.
Hydrogen's electron that is sitting right over there can
now be given to the alpha carbon.
It can now be given to the alpha carbon to
form a double bond.
And now that the alpha carbon is getting that electron, now
the bromo group can leave. It's a decent leaving group.
And that was another thing that we should think about in
our equation.
But a good leaving group actually favors all of the
reactions: Sn2, E2, Sn1, E1.
And so the carbon's getting the electron, and then the
bromine can then take this carbon's electron.
And just in one step that's what's distinctive about the
E2 and the Sn2 reactions.
All of the reactions are involved in the
rate-determining step and there really is only one step.
Just like that, after that happens, what we're left with
is the methoxide anion takes the hydrogen,
so it becomes methanol.
Let me draw that.
So it becomes methanol.
So it had one, two, three, four and then five, that's
this one right there, but then this guy goes and bonds with
the hydrogen.
This guy goes and bonds with the hydrogen, just like that,
and hydrogen leaves its electron behind.
Let me draw the cyclopentane part now.
And so the cyclopentane looked like this before, if I just
focus on the ring.
Now, this guy was bonded to a hydrogen.
He was bonded to this hydrogen over here, but now that
electron is going to be used to form a bond with this alpha
carbon right over here.
Let me draw the alpha carbon.
And the alpha carbon is right over there.
Obviously, implicitly at every one of these
edges, we have a carbon.
But now, a double bond is going to form
with that alpha carbon.
We could just draw it like that, a double bond.
Obviously, there's another carbon here.
I could write up another carbon over there.
And now this double bond will form.
And now the bromide has left.
It's taken an electron with it from that carbon now that the
carbon doesn't need it.
It was already starting to hog it because it's so
electronegative.
So that's bromine.
It takes that orange electron.
Now, it is bromide.
And we're done.
And so just to go back to the original question here, which
reaction is likely to occur or which mechanism?
It's actually both Sn2 and E2.
You would see a mix of both of these occurring because you
have all of the environmental factors that
would enable both.
And so you would have both of these mechanisms.
Here's the-- let me separate them out.
Here's the Sn2 reaction.
You would have the Sn2 reaction occurring in your
whatever, your vial, or your pot, or whatever you're making
all of this stuff occur in, and you would also have your
E2 reaction.
So you would see some of all of these, some of all of those
products and these products right over there.