Solvent Effects on Sn1 and Sn2 Reactions Solvent Effects on Sn1 and Sn2 Reactions
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- 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.
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