Steric Hindrance

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Steric Hindrance

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So I've drawn out these four scenarios here.
What we want to talk about is what type of reaction is
likely to happen in each of these scenarios, and if there
is some type of reaction, how fast is it likely to occur.
So let's look at this first scenario up here.
I have an iodide anion, which we saw on the last video is a
very strong nucleophile.
So this right here is a very, very strong nucleophile.
And actually, we have the iodide anion in every one of
these scenarios.
In every one of these scenarios, we have this very
strong nucleophile, and let's see the other reagent that it
might react with.
Here I have a methyl carbon that's attached to a fluorine,
or if we actually name this actual molecule,
this would be what?
Fluoro.
This would be fluoromethane, because we only have one
carbon there.
But when I say it's a methyl carbon, that means that this
carbon is only attached to other hydrogens.
It's CH3.
It's not attached to any other carbons.
Let me label all of them right now.
So in this scenario right here, I have a methyl.
I have a methyl carbon.
It's a methyl carbon, not attached to any other carbons.
It is attached to this fluorine, and if we kind of
pattern match it based on other things, maybe fluorine
is our leaving group.
But what's left over, the substrate, we have this methyl
carbon in the middle of it.
Over here, this would be a primary carbon.
It is only attached to one other carbon, so this is a
primary carbon.
It's attached to one other carbon.
This one right here is secondary.
It's attached to two other carbons, the secondary carbon.
And then finally, this is a tertiary carbon.
It's attached to one, two, three, other carbons.
So that right there is tertiary.
So we've already identified the
difference between the scenarios.
All of them have this very strong nucleophile, and then
the reagent, the thing that it might react with, the carbon
in the middle, has different levels of connectivity to
other carbons.
This has no connectivity.
This is connected to one other carbon, two other carbons,
three other carbons.
So what's likely to happen here?
Well, strong nucleophile, and I'm not going to go too much
into the solution just yet.
I'll do a future video on that, probably the next one.
But we have a strong nucleophile.
Let's just say this is a good solution
for doing Sn2 reactions.
And so there's no reason why an Sn2
reaction can't occur here.
Strong nucleophile, it can give its extra-- let's say
it's giving that electron right there, it could give its
extra electron to the carbon, to this carbon,
to this methyl carbon.
And right as it's doing it, this fluorine could take away
the electron from carbon, could take away that electron,
so that electron could go to fluorine.
What I've just drawn here is an Sn2 reaction.
Once again, S stands for such substitution, N stands for
nucleophile, and then the two means that both of the
reagents are active in-- this really is only one step of the
process, but in the slowest part of the process.
And this is the slowest part of the process.
This is actually the whole process right here.
So we get an Sn1 reaction.
We get Sn1 reaction going on, so that we have a substitution
with a-- sorry, sorry, Sn2 reaction.
I want to be very careful here.
Sn2 reaction.
We have substitution with a nucleophile, and both
reactants are present during the rate-determining step.
And there's only one step here, so both
of these are involved.
Now let's think about what happens in this scenario over
here on the right.
What's going to happen over here?
Well, once again, strong nucleophile.
It's not too different.
Now this is a primary carbon.
We have little CH3 group over here, but this thing should
still be able to get its way into that carbon.
It still should be able to give its electron.
It still should be able to give its electron to that
carbon, and then this electron that the carbon had had could
now go to the fluorine to make a fluoride anion.
And I haven't drawn the final step, but we've seen Sn2
reactions before.
But once again, this is an Sn2 reaction.
Now, we said not only what would happen, but we also said
how fast might it happen?
Or how can we compare them?
I'll tell you right now, that this one on the left is going
to happen very fast relative to this one, which will only
happen fast. I'm not giving any absolute numbers, but
we're just comparing them relatively.
And the reason why this is very fast and this is only
fast is because this one has this CH3 group in the way.
So this is a big part of a molecule.
This is big.
This right here is big, especially relative to just a
hydrogen atom.
So this is big.
This allows less directions with which-- remember, all of
these chemical reactions in organic chemistry and
chemistry in general, you always draw them as these nice
organized things.
But these are really just these atoms and molecules
bumping into each other in just the right way with just
the right energies.
And if you have this big thing blocking one of the entry
paths onto the carbon, you're not going to have this
reaction happen as quickly.
There's going to be-- sometimes, it may be the
iodide ion was coming from the direction, and it gets
deflected by this over here.
In this case, it wouldn't have, because that hydrogen
isn't big enough to deflect it.
So this is going to happen just not as fast as this one
right here when we're dealing with the methyl carbon.
And I think you see where we're going here.
So what happens when we have two of these carbons right
over here when we're dealing with a secondary carbon, a
carbon attached to two?
If the solution is right, and we're going to talk about the
types of solutions that favor Sn2 reactions, we will have an
Sn2 reaction still.
We'll still have an Sn2 reaction, but it will be
slower than these guys up here.
So we still are going to have an Sn2 reaction,
but it will be slower.
So far, this is the fastest, this is the next fastest, and
this is slower than either of those two.
And it's the same exact logic.
This only had one CH3 blocking the entry into
this primary carbon.
This has two CH3's.
It has this one over here, it has this one over here, and it
has this one up here.
So it lowers the likelihood that an iodide anion will be
able to bump into this carbon and in the
exact the right direction.
So that's why it's going to occur slower.
This iodide anion has to come in just the right direction
for it to occur.
That's less likely, so overall the reaction is slower.
Now, finally, what's going to happen here?
You might be tempted to say this is going to be really
slow, but the reality is that this tertiary carbon is
blocked really in every direction.
It's really in every direction.
It's blocked there, blocked there, and blocked over there.
So this iodide anion is actually not even going to be
able to get to it.
So I'll write no Sn1.
And we'll see in the next video that maybe there's
another type of reaction that we've learned-- sorry, I
should say Sn2.
No Sn2.
I'm sorry if part of my brain makes me think that I might
have said Sn1 earlier.
No, everything in this video is Sn2.
Everything in this video, we have the nucleophile attacking
the other reagent.
They're both present, so it's Sn2, although my brain
sometimes wants to say Sn1.
These are all Sn2 reactions, and you're not
going to have an Sn2.
And actually, if your solution is right, you actually might
have an Sn1 reaction here, and we'll talk about that later.
But you have no Sn2 here, because the nucleophile cannot
get to the carbon.
It's blocked I guess you could say by these methyl groups.
Now, the term for this blocking, I guess you could
say, of the point that the thing needs to attack or kind
of give its electron, or move into is
called steric hindrance.
You know what it means to hinder something.
You're making it hard for it to happen.
And steric just means three dimensional, or the shape, the
three-dimensional shape of things: hindrance.
So by the virtue of the three-dimensional shape of
this or the way that this is made up, it's blocking the
reaction from happening.
It's hindering the reaction.
So the big takeaway from here is you're only going to have
an Sn2 reaction.
You're going to have a very fast Sn2 reaction if you're
dealing with a methyl carbon, a little bit slower if you're
dealing with a primary carbon.
If you have a secondary carbon that's blocked by two other
carbons, it might happen if the solution is just right,
but it's going to happen very slowly.
And if it's completely blocked, then you're going to
have no reaction.
And the one last thing, you might be saying, well, hey,
how can I not come from the other side?
You can't come from the other side because the point that
you want to give the electron that's taking away is kind of
being blocked by the fluorine in this situation.
That Sn2 reaction will only happen on the opposite side
from the leaving group, just as a reminder.
This is the nucleophile.
This is the leaving group.
In Sn2 reactions, the nucleophile goes from one end,
and then the leaving group leaves from the other.
It cannot happen on the same side, which actually makes a
lot of sense if you think about it.
That's as opposed to the Sn1 reaction, where it can happen
on the same side.
And what we'll see is if the solution is right, this type--
having a mixture of this and this would actually favor Sn1.
I'll do a whole video on that, but you could
think about what happens.
If the solution is right, the fluorine can take this
electron first, then this will become a carbocation.
And this would actually be a very stable carbocation.
The very same thing that was causing steric hindrance will
actually make this a stable carbocation, and then that
will make it favorable for a nucleophile, and really even a
weak nucleophile.
to attack it, or to bond to it, or
to give it its electron.