Sn1 Reactions Introduction to Sn1 reactions
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- Let's think about what might happen if we had a solution of
- this molecule and water and how they might react.
- And just as a bit of review on naming, let's
- just name this molecule.
- The longest chain, let's see, we have one, two, three
- carbons, so it is going to be a prop-.
- It's an alkane, All single bonds, so
- it's going to be propane.
- And then this is our longest chain right
- here, one, two, three.
- We have this methyl group off of the number two carbon, and
- we also have the bromo group off of the number two carbon.
- So it's going to be 2-bromo.
- let me write this here.
- It's going to be 2-bromo-2-methyl.
- let me write that a little bit neater.
- I'll need more space to the left.
- This is going to be 2-bromo-2-methyl-- for this
- methyl group off the number two carbon-- propane.
- So let's think about how it might react with just water.
- Now, in this example, water is going to be our nucleophile.
- It has some electrons already over there, but even more,
- it's very electronegative, which we've
- seen multiple times.
- So it's hogging electrons from the hydrogen.
- It's not as strong of a nucleophile as the hydroxide
- anion that we saw in the Sn2 reaction, but it is a
- nucleophile.
- This is actually a weak nucleophile.
- This right here is a weak nucleophile.
- It wouldn't mind maybe being attracted to a nucleus because
- it has extra negative charge.
- It has a partial negative charge on this side of the
- oxygen, because it's hogging hydrogen's electrons.
- You have partial positive charges on these sides.
- But it's not as strong as a hydroxide, which has an actual
- negative charge, not just a partial negative charge, and
- really wants to offload electrons, or is
- attracted to a nucleus.
- So this is a weak nucleophile.
- And we're going to talk more about-- we're going to see one
- type of reaction in this video, and we're going to talk
- more in several videos about when this type of reaction
- versus an Sn2 type of reaction might occur.
- So let's look at this right here.
- We have a bromine right over here.
- We know that bromine is very electronegative, and that it
- is somewhat stable when it has a negative charge.
- At least it'll have eight valence electrons, which makes
- it feel relatively good, although charge is never a
- super stable situation.
- So maybe, and I'm not going to say that the reaction will go
- super fast in this direction, but maybe bromine will take
- away the electron that it has from carbon.
- It was already hogging it a little bit.
- It is already more electronegative.
- Let me draw all of bromine's valence electrons.
- So it has that one that's on that side with the carbon.
- Let me draw the carbon electron on the other
- side of that bond.
- And then it has six other valence electrons.
- So one, two, three, four, five, six, seven
- total valence electrons.
- So maybe we can imagine a situation where bromine takes
- this carbon's electron altogether.
- So let's draw that out.
- So it literally will take this electron altogether.
- And once again, I said this isn't going to be the super
- fast direction, but it could happen.
- And so this reaction won't necessarily go super fast.
- It'll kind of be in equilibrium.
- So if that were to happen, we'd be in equilibrium with
- this situation.
- So let me draw my original.
- So I have my carbon.
- I have that CH3 group behind it.
- I have the CH3 group in front of it.
- I have the CH3 above it.
- But now the bromine has left.
- It's no longer bonded.
- And it has its original seven valence electrons: one, two,
- three, four, five, six, seven.
- This one right here was originally bonded with the
- electron from carbon, but now bromine has taken that orange
- electron altogether.
- It has a negative charge.
- The carbon has lost that orange electron and so it has
- a positive charge.
- Now, it could be interesting if oxygen comes into the mix.
- Sorry, if water comes into the mix.
- So let me draw the water molecule right here.
- It is a weak nucleophile, but this guy
- really wants an electron.
- He has a positive charge.
- It is a tertiary carbocation, so it's reasonably stable.
- That's actually one of the reasons why
- this can even happen.
- If this was a primary or if it was connected to no other
- carbons, then it would be very hard for this thing to just
- turn into a carbocation.
- But because it's tertiary, it's somewhat stable, but it
- doesn't like to have that positive charge.
- It really wants an electron.
- So you could now imagine that this oxygen-- oh, sorry, this
- water-- let me be careful.
- This water will give one of its electrons.
- So maybe that electron right there will be
- given to the carbon.
- You could view it that this is the nucleophile and it's being
- attracted to carbon's positive nucleus.
- So then what will happen?
- And once you're in this state, this reaction actually will
- occur much faster than this one right here.
- This is actually a reasonably stable situation.
- That's why I drew the equilibrium.
- But now this will actually be a faster reaction.
- So I'll just draw an arrow here.
- And then it will look like this.
- So we have our original carbon, the original molecule.
- That's the thing that's behind it, CH3.
- We have another CH3 in front of it.
- And now, this water is attached to it.
- So let me draw the oxygen and the two hydrogens.
- Oxygen had these two electrons.
- It had another electron here.
- That's that one right over there.
- Let me do it in this color.
- So this has this electron here.
- But its pair, the thing that it was in a pair with, is now
- given to the carbon.
- And they will be bonded.
- These two guys are still paired up.
- Now, since oxygen-- this water had a neutral charge.
- But now it has given away one of its electrons, so now the
- water itself will have a positive charge, or I guess
- this water group, or you can say it's kind of an
- oxonium group now.
- It has a positive charge.
- And so the last thing that might happen is maybe another
- water molecule, or actually it could even be the bromine,
- comes along and grabs one of the hydrogen protons so that
- this electron can be taken back by this oxygen.
- Let me show you what I'm talking about.
- So maybe you could imagine another water molecule.
- There's gazillions of them.
- Let me draw another water molecule here.
- We have another water molecule right here.
- And this would happen simultaneously.
- This might give one of his electrons to, say, that
- hydrogen right there.
- And if that happens simultaneously, this electron
- that this hydrogen had could be given back to this original
- oxygen right here.
- So that oxygen will take back that electron.
- And when that reaction happens, we will be left
- with-- let me draw our original structure.
- So we have the CH3 behind us.
- We have the CH3 in front of us.
- We have the CH3 above this carbon that's doing all the
- reacting, and then it is bonded to an oxygen, which is
- bonded to a hydrogen, to only this hydrogen, because it took
- back the electron from that hydrogen right
- there, just like that.
- You also have the bromine with its negative charge, and it
- has eight valence electrons now.
- And then, of course, you have an extra oxonium.
- So this oxygen right here gave this
- electron to this hydrogen.
- So now it will be bonded to that hydrogen.
- And if you wanted to draw all of the valence electrons on
- this oxygen right here, it would look like this.
- So you have these two, so one, two.
- You have the one with that three that's
- bonded with that carbon.
- Let me do that in a different color.
- So you have this one is the same thing as
- that valence electron.
- It has the one bonded with the hydrogen.
- Let me make the bond clear.
- It has a bond here with the hydrogen.
- So that's this hydrogen bond right here, this
- bond with the hydrogen.
- I don't want to call it a hydrogen bond.
- You get the idea.
- It has that valence electron right there on
- that end of the bond.
- And then it had this valence electron.
- It had this one right here at this end of
- this bond with hydrogen.
- That was just like that.
- And then it took the other electron from the hydrogen,
- and so it's right there.
- So once again, it still has six valence electrons.
- It is now a neutral oxygen: one, two,
- three, four, five, six.
- So we were able to react this 2-bromo-2-methylpropane with
- water, which is a weak nucleophile.
- We'll talk more about strong and weak nucleophiles.
- And we got to-- what is this thing right over here?
- Our longest chain is one, two, three.
- It's still prop-.
- We're going to have prop- as a prefix.
- And I actually haven't taught you this.
- But since we have an OH group, this is an alcohol.
- And the way you name alcohols is the suffix is -anol.
- So we would call this, this is prop-, and sometimes they'll
- just say propanol, but since we want to specify which
- carbon we're on, this is the one, two, three carbon.
- This is propan-2-ol.
- And, of course, we also have a methyl
- group on the two carbon.
- So it's 2-methylpropan-2-ol.
- Now, this whole reaction that I just showed you is called an
- Sn1 reaction.
- And you might have a gut sense of why it's
- Sn1 instead of Sn2.
- So let me write it down.
- It's Sn1 reaction.
- The S still stands for substitution.
- The n still stands for nucleophilic, right?
- We had a weak nucleophile in the water molecule, so it
- substituted.
- Now, the 1 stands for the fact that the slowest-- the
- rate-determining step in this mechanism, the one that is
- going to happen the slowest. That only
- involves one of the reactants.
- This right here was the rate-determining step, the
- bromine taking the electron away from the carbon.
- It did not involve this water at all.
- It did not involve both the reactants like we saw in the
- Sn2 reaction.
- Since it only involves this one reactant,
- it is an Sn1 reaction.
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