Sn1 Reactions

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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.