Let's talk a little bit more about the comparison between
Sn1 and Sn2 reactions, and maybe we can also talk about
what type of conditions make one or the other more
favorable, especially the solvent.
We've talked about it a little bit, but we'll go into a
little bit more detail.
So just as a bit of a review, an Sn1 reaction, you might
have some type of a-- let me just draw it like this.
So you'll have some carbon that's
attached to three things.
I'm making these three things just arbitrary colors.
They could be other carbons, CH3.
It could be hydrogens.
It could be other things.
And then it's also attached to a leaving group.
Let me do this in a color that I haven't used yet: in pink.
And then you also have a nucleophile floating around.
That'll be our nucleophile.
It has one excess electron.
It has a negative charge.
And in an Sn1 reaction, the first step only involves one
of the reactants, one of the reagents.
That's why it's called Sn1.
So in the very first step, the nucleophile just waits
passively while the leaving group leaves.
So maybe there's an electron right there, the leaving group
already has an electron.
It swipes one, so it has to be reasonably electronegative.
It swipes one from that carbon, and so once it does
that, our situation will look like this.
You have your substrate.
It now just lost an electron.
It lost that electron, so it now has a positive charge.
Let me add that little trace of something.
Let me erase this right here so you don't think that
electron is still there.
So it now has a positive charge.
The leaving group, which was neutral, took an electron.
So it out now has a negative charge.
The leaving group now took an electron, so it now has at
least two valence electrons there.
It has a negative charge, probably a lot more.
And then in the next step, this nucleophile-- so let me
copy and paste this.
That's not what I wanted to do.
Let me copy and paste it again.
So copy and paste.
OK.
So that thing that was waiting patiently during the
rate-determining steps says, hey, there's
a carbocation there.
I'm a nucleophile.
I'm not a strong nucleophile in the case of an Sn1, but I
still like giving away electrons to nucleuses.
I love nucleases.
I see a positive carbocation there.
Actually, a carbocation is positive by definition, so let
me give my electrons to him.
And in the very last step, we have a situation where this
guy-- let me copy and paste this again.
Let me copy and let me paste.
That guy is bonded to the nucleophile.
Since the nucleophile gave the electron to this carbon, the
carbon doesn't have a charge anymore.
Since the nucleophile lost that
electron, it's now neutral.
So you have it bonded to the nucleophile, and, of course,
you have the leaving group still here
with a negative charge.
We've hinted at this a little bit, but in order for this
step to happen on its own with only this reagent happening
spontaneously to some degree, or for it to just happen.
And we won't talk about the rates.
And it won't necessarily happen fast, but in order for
this to happen at all, this has to be a stable
carbocation.
And we've seen before that in order for this to be a stable
carbocation, it has to be bonded to things that are
willing to lend it some electrons so it's not super
hungry for electrons.
And in order for that to happen, this carbocation right
here has to be tertiary.
Tertiary is ideal.
Tertiary carbocation means that these three things over
here are all carbons or carbon chains.
So ideally, it would be tertiary and secondary will
also work if at least two of these guys are carbon.
That'll also work if the solvent is favorable, if the
environment we're dealing with is a favorable environment.
If only one of these were carbon, so if this was a
primary carbocation or this was a methyl carbocation where
nothing here is a carbon, then you're unlikely
to have an Sn1 reaction.
Now, on the other side of the picture, Sn2 reaction, this is
the one where you have a super strong nucleophile, or at
least a strong nucleophile, and the whole reaction happens
in one step involving both of the reagents.
So you start here with your leaving group.
The way you can think about it, Sn1, the leaving group is
the thing that does something first. In Sn2, it all happens
at once, but the nucleophile's kind of the aggressor.
It really wants to give that electron away badly.
So in the very first step, it comes in here and gives it to
that carbon, even though that carbon didn't necessarily want
it that badly.
It would have a partial positive charge if this thing
is really electronegative and might be hogging this electron
already, but that thing that it was already hogging, now it
could say, hey, you already got an electron.
I can take it from you.
And so in literally one step, we end up with
something like this.
Actually, let me just redraw that.
So we have our leaving group has left.
It's taken the electron that the
carbon didn't need anymore.
So it now has this electron and that one and maybe more,
and it has a negative charge.
And now this carbon is attached to the blue thing.
I'll just draw the purple thing, and maybe in back, it's
attached to the orange thing.
Let me draw it so you can see what I'm talking about, so the
blue thing and then the purple thing.
And then the nucleophile had come from the other direction,
so now it's also bonded to the nucleophile.
The nucleophile is giving an electron, so it loses its
negative charge.
And essentially, the negative charge gets transferred to the
leaving group, which is able to take an electron, and then
that carbon in the middle stays neutral.
And we saw before that this can only reasonably happen
with a strong nucleophile.
That has to be a strong nucleophile, and there can't
be a lot of steric hindrance.
These things can't really block this
nucleophile from attacking.
So this will work best if this is a methyl or primary carbon.
And if the conditions are just right and if the nucleophile
is strong enough, maybe secondary,
maybe a secondary carbon.
Because once you have too many carbons or big carbon chains
over here, then it blocks the nucleophile.
It cannot attack.
If these are all hydrogens, there's a lot of space for
this nucleophile to attack.
If only one of these are carbon, still a lot of space.
Now, that's just kind of a general comparison, and it
gives you some clues.
Weak nucleophile, you're dealing with Sn1.
Strong nucleophile, Sn2.
If this is a tertiary or secondary carbon, so if it's
bonded to a lot of other carbons, Sn1
should be in your brain.
If this isn't bonded to a lot a carbons and you have a
strong nucleophile, Sn2 should be in your brain.
And now the last thing you should have to think about is
the solvent itself.
What are these reagents actually dissolved in.
So let's think about it.
So in either an Sn1 or an Sn2 reaction, we're dealing with
things that have charge.
And in general, if you're dealing with ions or things
that have charge, they're going to be more soluble in
polar solvents because the charge likes to be around
other things with charge so that they can mix and match.
So in both situations, you want to have polar solvents.
And you are familiar with polar solvents.
Examples of polar solvents are, well,
water is a polar solvent.
H2O, and it's polar because oxygen is so electronegative
it hogs electrons.
So you have a partial negative charge on the oxygen side and
partial positive charges on the hydrogen side.
Another example of a polar solvent that you may have not
seen before is something like diethyl ether.
So diethyl ether would look like this.
CH2, CH3, CH2, CH3.
And I know we havn't named ethers yet.
Let me write this.
This is diethyl-- let me make sure I get that "y" right.
This is diethyl ether.
And this is a bit of a review.
This is probably the best way to learn things, as
we learn as we go.
An ether is something that has a form.
You have some carbon chain or some-- you could call it a
radical-- some group attached to a carbon chain, I guess is
the way my brain processes it, to an oxygen which is attached
to another carbon chain.
It doesn't have to be the exact same one.
And diethyl ether is the same.
And you're literally just saying diethyl.
You have to ethyl groups attached to an ether.
Ethyl is two carbons.
So two carbons, oxygen, two more carbons.
Diethyl ether.
And this would also be a polar solvent because oxygen is more
electronegative then carbon, so you still have this partial
negative charge-- let me do this in a different color-- on
the oxygen end and you still have a partial positive charge
on your carbon end.
So this is also a polar solvent.
Both of these are polar solvents.
Now, the difference between these, and this will probably
be a new term for you, is this water and most types of
alcohol, something that has hydrogen that could be knocked
off, this is a protic sovent.
And protic just comes from proton.
And that means that if something is dissolved in
this, or if you have a solution of this, then these
hydrogens might be able to get knocked off and you might have
free protons lying around.
Because let's say this oxygen swipes this
electron from this hydrogen.
If it swipes that electron from that hydrogen, then you
have a situation where you have OH minus, right?
The oxygen now has a negative charge.
And this hydrogen, all it had to its name was that electron.
If you take that away, all it is is a proton.
So when someone draws H plus, that literally is a proton.
That literally is a proton.
And this can occur in water, that you have some protons
floating around, so water is fundamentally a
protic polar solvent.
Diethyl ether, on the other hand, it doesn't have
hydrogens where oxygens can take its hydrogens away.
The hydrogens here are all bonded to carbon, which is not
that electronegative, so this is an aprotic solvent.
Let me draw this.
Now, let's think about whether an Sn1 or Sn2 is better and--
well, I already said the solvents have to be polar.
That's because we're dealing with charged
things in both reactions.
But which one will be better?
So let me write this down.
So in both situations, we want a polar solvent, so let me
write that down.
And I'll give you a hint: one of them will be good in a
protic environment and one of them will not be good in a
protic environment.
So in an Sn2, if you had-- let's just think it through.
Let's just not memorize this.
If we had protons floating around in an Sn2 situation,
what is likely to happen?
We have this strong nucleophile.
We have this thing that wants to give away electrons.
Now, it could give it away to a carbon that might maybe have
a partial positive charge and then the leaving group could
take it away, but what wants electrons more?
A carbon that has a partial negative charge or a proton,
something that has a full positive charge?
Well, a proton would want it more.
So if you had protons floating around, the nucleophile would
just give its electrons to the proton and the Sn2 reaction
wouldn't actually occur.
The protons would take all the electrons.
So you can't have protons floating around.
You cannot have protons floating
around in an Sn2 reaction.
Then this carbon will never get the electron.
The proton will take it.
So you need an aprotic solvent.
So Sn2 would work well in diethyl ether.
If this is a strong nucleophile, it's a solution
in diethyl ether, and these were all hydrogens, then you'd
be just ready to go for an Sn2 reaction.
Now, on the other end of the spectrum, the key in an Sn1
reaction is that this carbocation be stable.
And for a carbocation to be stable-- or actually even more
important, instead of the carbocation being stable,
that's dependent on it being a tertiary or secondary carbon.
But order for the leaving group to be stable when it's
negative, it actually would be nice if there was
some protons available.
Maybe it could give electrons to the proton.
Maybe it could bond with the proton.
So in this situation, the protons will actually help
stabilize the situation.
So in an Sn1 reaction, the ideal is actually a polar
protic solvent.
So if you see a situation where you have a secondary or
tertiary carbon, it is a weak nucleophile, and you're
dealing with a polar protic solvent, the most famous of
which is water, but an alcohol would also work, then you
should be thinking Sn1.
If you see a strong nucleophile, there's not much
steric hindrance.
There's space that it can attack.
This is a methyl or a primary, maybe a secondary carbon, and
it's happening in an aprotic polar solvent like diethyl
ether, then you know you're ready for an Sn2.