Next we'll look at how different parts of the reaction will affect which mechanism we follow.
So, so far we've talked about the two mechanisms but we've never figured out whether a specific reaction will follow the SN1 mechanism or the SN2 mechanism.
So we'll look at a few different things. We'll look at the leaving group.
We'll look at the nucleophile.
We'll look at the alkyl halide.
And we'll look at the solvent.
And we'll see how each one of these has an effect on the reaction.
So first off, let's just look at the leaving group.
Okay, so for now, we'll we'll just assume that it's an SN2 reaction
to draw our products.
So as I change from bromide to iodide,
what changes in my reaction?
Well, really the only difference is here, bromide versus iodide.
And so what we care about with the leaving group is how stable our leaving groups are.
And of course the stability of the leaving group is related to the pKa of their conjugate acids. So HBr has a pKa of about minus 9.
HI has a pKa of about minus 10.
What does that mean? That means that I minus is about 10 times more stable on its own than Br minus.
And if this one is about 10 times more stable on its own, then it should be a better leaving group.
Okay, so whatever is more stable on its own is a better leaving group.
So if you want to think of it this way, a better leaving group
is the conjugate base of a stronger acid.
So that would make this reaction faster.
With iodide as my leaving group, then the first reaction with bromide is my leaving group.
Okay. So that compares between those two, but those are both SN2 mechanisms. What happens in an SN1 mechanism?
Well, this the rate determining step of an SN1 mechanism. And we can already see where the difference lies.
In the rate determining step, I'll either form a bromide or an iodide.
Which one of those is more stable?
The iodide is more stable. So which one of these reactions should be faster?
The one with the iodide should be faster.
So if you see this, it doesn't matter whether you have an SN2 or an SN1 type reaction mechanism; this one is SN2, this one is an SN1, better leaving groups make both mechanisms faster.
The downside of that is they do not help us choose between the two mechanisms
or choose the more likely mechanism.
So that's the leaving group effect.
What about the alkyl halide? So let's look at an SN2 first.
I'll look at a primary alkyl halide versus a tertiary alkyl halide.
I'll keep using the same reaction.
Uh, you'll find out actually in the next chapter that there's a competing reaction with this, which is an elimination. But for now we'll keep using the same reaction.
If I'm going to do an SN2, that means that the hydroxide has to come in as the chloride leaves. Hydroxide comes in
as the chloride leaves.
Now, there's a difference between these two and that is that there's very little sterics in the primary alkyl halide
because this hydroxide is coming in passed two hydrogens, which are very small atoms.
Conversely, in tertiary alkyl halide, this hydroxide is coming in, there are three methyl groups out here. And of course each one of these has hydrogens on it.
So steric issues are a much more important with the tertiary alkyl halide.
So we have steric problems doing an SN2 on tertiary alkyl halide.
What about an SN1 reaction?
Well, in the SN1, the chloride leaves first in both cases leaving a carbocation behind,
which then has the hydroxide add to make the final product. But now when this hydroxide adds, these are both planar,
which means the top and the bottom are both open. So there are no steric issues really in an SN1 reaction.
It's much easier to bring our hydroxide into the planar intermediate than it was back here to come in and have to displace the halogen off of the SB 3 hybridized carbon that already has four groups surround it.
These carbons only have three groups surround them. So the difference here
is actually that we make a primary carbocation in this case, versus a tertiary carbocation.
And carbocation stability
says that tertiary is better than a secondary carbocation, which is more stable than a primary carbocation, which is more stable than a methyl carbocation.
So if I look at these two reactions now, this one is going to be favored
'cause it makes a more stable carbocation.
While up here, the first one was favored because of very small sterics. So in a primary alkyl halide, it wants to do an SN2 type reaction,
whereas a tertiary alkyl halide
wants to do an SN1 type reaction.
Why again? The primary wants to do an SN2 because of less sterics.
And a tertiary wants to do an SN1 because it has a more stable carbocation.
So hopefully the next question that you have is, what about secondary?
And for secondary, it depends.
And I'm going to leave it at that. And we'll come back to it after we look at some other effects.
Không có nhận xét nào:
Đăng nhận xét