The biggest difference between primary alcohols and secondary and tertiary alcohols is that
secondary and tertiary, I told you guys, can make more stable carbocations.
So you already might be figuring that the mechanism is going to include a carbocation
for these structures and that's actually right.
So let's go ahead and get started.
This mechanism that I'm going to show you right now only applies to secondary and tertiary
alcohols.
And these are going to follow the E1 dehydration mechanism, meaning that if you remember back
to E1, E1 is actually going to be a two-step reaction and it's going to involve a carbocation
intermediate.
So let's go ahead and get started.
In my first step, I'm starting off with an acid, again, notice that once again I'm using
H3O+ which is just my generic acid in water.
That could have been any strong acid to begin with.
So now I've got alcohol.
I've got my acid.
Your first step of any acid catalyzed mechanism is to protonate.
So do you see anything that I could protonate easily?
Yes, you do.
There is an alcohol that has very nucleophilic lone pairs.
So I can take those lone pairs, one of them, and I can go and use it to make a new bond
to that very electrophilic H or very acidic H. What that's going to do is it's not going
to give me a leaving group that instead of being terrible, alcohol sucks, it's actually
going to be really awesome because it's just going to be water.
And remember that water is a good leaving group because it's neutral after it leaves
and what's better than to be neutral.
That means it really wants to get off there.
Now I've got my water.
So here's the second step, the second step, I just said this was E1, by the way, that
was the first step is protonation, but now we're actually on to the E1 stage.
At this point, we need to do the first step and because this is E1, maybe you guys can
predict what that first step will be.
And if you don't know, that's fine.
Maybe you didn't watch that video or you haven't learned it yet.
But it's just going to be that the water leaves all by itself because it's so stable it doesn't
really need anything to kick it out.
For the E2 reaction, I need to have a concerted three arrows.
But in this case, I have such a great leaving group, it's just going to take off by itself
and what I'm going to wind up getting is a carbocation.
Notice that, in this case, that's a secondary carbocation, which is pretty stable.
Now that I have a carbocation, there's one thing you guys should be watchful for.
Do you guys remember anything tricky about carbocations?
Maybe you don't remember and that's fine.
But just so you guys know, carbocation like to do one thing, they like to shift.
Carbocations love to shift to more stable positions.
That means that at the end they're actually going to generate constitutional isomers.
So that's something really important to know.
In this case, would this carbocation shift?
No, because it's secondary.
It doesn't matter, but – and there's no where else that it can shift that it would
become more stable.
But if I had a methyl group around or something like that, this would be able to shift and
become more stable carbocation.
So now that we are mindful of that, we're onto the last step.
The last step is beta-elimination just like normal E1.
So what can I do to eliminate it?
I can use that conjugate base of my acid, which is just water, pretty good nucleophile.
It's okay nucleophile.
And I'm going to go ahead and say that my carbocation is the alpha-carbon, meaning that
any carbon that's attached to it would be beta.
So this is beta and this is beta.
My question is do any of these beta-carbons have at least one hydrogen on them?
And actually they do, they both have at least one hydrogen.
Is there a difference between which one I use?
Could I use the top one or the bottom one and get the same thing?
Yeah, it doesn't matter because both of them would lead to the same product.
So let's just go ahead and use the bottom one.
So what that means is I would take my nucleophile, grab an H and make a double bond.
So what that's going to make is a new double bond, so this is the pi bond.
This is my elimination product.
Also what we call dehydration product because dehydration is just the name for an elimination
with water or with alcohol.
And we would get the leaving group originally which was water, remember that water left
in that first step.
And then we would also get H3O+.
What's the significance of H3O+?
That that is my catalyst once again.
Remember that we said that it was important to get the same catalyst that you started
with because that's the whole definition of a catalyst.
So I hope that makes sense guys.
It's pretty related to the other elimination reactions that we have to learn in orgo one.
If you're struggling with these mechanisms, that means you have to go back or go forward
or whatever direction you need to go in your curriculum to learn more about elimination.
But hopefully, this wasn't so bad.
So let's go ahead and do some practice problems based on everything we just learned.