So let's go ahead and start off with my primary mechanism.
So what if I have a primary alcohol.
We know it's not that easy to dehydrate.
It doesn't make good carbocations.
So what mechanism are we going to use?
It turns out that for this mechanism, because we don't want to use a carbocation, we're
going to use the E2 mechanism.
The E2 mechanism, if you guys remember back to elimination is a concerted mechanism.
That means everything happens at once.
So where are we going to start with this/ Well, if you guys remember, anytime that we
have an acid catalyzed reaction, anytime it says acid catalyzed – this might be the
first time you've heard me say this because maybe you haven't watched the other videos
or it's cool.
That means that you're always going to get a protonation step at the very beginning.
Protonation just means I'm adding a proton to something.
Whenever you hear me say acid catalyzed, even this applies for orgo two, think there's going
to be a proton added to something.
In this case, I'm starting off with an acid.
Notice that the acid I'm using here is H3O+.
Now that's actually a conversation in and of itself because you're probably thinking,
“Hey, Johnny,what happened to the whole HA?
Why isn't it sulfuric acid?”
Well, because if you think about it, let's say I was using sulfuric acid, H2SO4 plus
water.
What's going to happen is that any time you have any acid in water that is always going
to be in equilibrium with the deprotonated form.
What that means is that I'm going to get H3O+ plus the anion so that would be basically,
HSO4-.
Does that make sense?
Basically, what I'm trying to show you is that no matter what acid you use, it could
be HI, it could be sulfuric acid, you're always going to have H3O+ as part of the solution
because you're going to get the water grabbing a proton from the strong acid and turning
into H3O+.
So here when I'm using H3O+, this is just going to be our general acid that we use for
these reactions.
But if your professors asking you to draw the mechanisms, then obviously use the specific
acid that he gave you.
Now we're going to go ahead and start.
The first step is protonation.
Do you guys see anything that's nucleophilic or that has electrons that could grab this
electrophilic H?Now keep in mind there's a strong dipole pulling away from the H so that
I have a partial positive here and a partial negative on the O.
And the answer is that yes, I have this oxygen with two lone pairs.
It's very nucleophilic.
It can go ahead and grab this H. But if it makes a bond to the H, we have to break a
bond to the rest of the water so what we're going to get at the end of this first step
is we're just going to get a water molecule like that.
And then plus we would get plus another molecule of water actually because we just removed
a proton.
So now here's the next step.
In the next step, I've got this water molecule.
Notice that I've got this down here.
And now I'm just drawing one of the H's.
That H was always there.
I'm just going to draw it right now.
It turns out that in this next step, I'm going to do beta-elimination.
Now beta-elimination remember is just based off of your alpha is wherever your leaving
group is.
In this case, alcohol was a terrible leaving group, but now I just protonated it, so it
became a great leaving group.
So I'm going to count this as my alpha-carbon.
My beta-carbon is the one right next to it.
Any carbon right next to the alpha is beta.
That has a hydrogen on it.
Can I eliminate?
Yes.
So now I'm going to do a concerted three-step mechanism where I do my nucleophile to the
H. Make a double bond.
Kick out the leaving group.
And what I'm going to get at the end is just a double bond plus, check it out, an acid
that I originally had and now a molecule of water.
The molecule of water came from the leaving group.
The acid came from the nucleophile that attacked that H.
Now, what's important about the acid is that notice that the name of this reaction was
acid catalyzed hydration.
That means – remember the definition of a catalyst is that it's something that can
neither be consumed nor destroyed in the reaction.
All it does is it lowers the activation energy of the actual reaction.
So it's really important that at the end of this reaction, you're ending up with the same
acid that you started with.
So if I started with H3O+, I have to make sure to get one molecule of H3O+ at the end.
Why?
Well, because if you just didn't have H3O+ present at the end, it wouldn't be a catalyst
anymore.
It would be something else.
It would be a reagent.
But since I'm regenerating it, not you know it's a catalyst.
I hope that makes sense.
That was the E2 mechanism.
That only applies, just so you guys know, that only applies to primary alcohols.
So if it's a primary, you'd use this.
But what about if it's a secondary or a tertiary, well, you're going to have to find out.
That's what our next topic is all about.
So let's go ahead and see what happens when you have a secondary or a tertiary alcohol.