Now we're going to talk about a way that we can make double bonds out of alcohols and
this is through a mechanism called acid catalyzed dehydration.
So how does this work?
Remember back to when I talked about leaving groups in the past and leaving group is even
a concept from acid/base chemistry.
All it means is that it's something that really doesn't want to leave, something that really
doesn't want to get a negative charge.
Another word for leaving group is just conjugate base.
So if you maybe don't really remember exactly what a leaving group is, just think conjugate
base, back to acids and bases.
Basically, alcohols are really bad leaving groups or they're really bad conjugate bases.
They hate to become OH-.
Why?
Because OH- is actually a really strong base.
Remember that you always want to go from stronger to weaker?
If we're making OH- hydroxide, that's a really strong base.
So this is not going to be very favored to just leave a molecule.
But it turns out that there is one thing we can do to make alcohol a better leaving group
and that is to use acid.
If we can use some kind of acid, we can actually convert alcohol into an awesome leaving group.
That awesome leaving group would just be that we add an H to it, we protonate it, so it
turns into water.
And water is an awesome leaving group because it's neutral.
It loves to leave and it loves to just be by itself in solution.
So here's the general formula.
I'm not going to show you the mechanism just yet.
But basically what we have is we have some kind of alcohol and we have some kind of acid
over water.
Now, in this case, I just put the general HA to mean really any acid, but your common
acids are going to be – sorry abut that, any of the strong acids, so H2SO4 is seen
all the time Any of the hydrogen halide strong acids.
I've also seen phosphoric acid.
This is not really a strong acid, but it's still strong enough to make the reaction go.
These are all very frequently used acids for acid catalyzed dehydration.
What we're basically doing is we're taking an alcohol.
We're going to be removing two sigma bonds.
We're going to be removing the alcohol and a beta-hydrogen so this is my alpha, this
is my beta-carbon, alpha and beta relative to the alcohol.
We're going to be taking away two different sigma bonds and we're going to be making one
pi bond instead.
Now that general reaction of taking away two sigmas and making one pi is an elimination.
So this is actually going to be an elimination reaction.
Hope that's making sense so far.
Now there's another reaction that you may have already learned or that you will learn
soon and that's actually the opposite.
It's called acid catalyzed hydration.
Acid catalyzed hydration is a reaction where we go from a double bond and we go back to
the alcohol.
This is actually what's called an addition reaction.
If you remember back to the general types of reactions we talked about, that when you
take one bond and you make it into two, that's actually addition.
When you take two bonds and you make it into one, that's elimination.
So hopefully, that makes sense.
So basically, the addition part is going to be the hydration.
The elimination part is the dehydration.
How do you know which one it is?
How do you know if it's going to be a hydration or a dehydration?
You just look at what you're starting with.
In this case, since I have my alcohol, I know that I'm starting with an alcohol and I'm
going to try to eliminate it with an acid to become a double bond.
However, if I was starting with a double bond and I used acid, then I could add water to
it and that would become a hydration.
In this step, in this video, we're just going to learn about dehydration, but I'm just asking
you to keep this in the back of your mind later on for when you have to do practice
problems with hydration that this is the way the way that we tell the two reactions apart.
So let's just talk more in depth about dehydration.
There's a few facts I want you to know.
First of all, the more R groups on that alcohol, the easier it's going to be to dehydrate.
This is just a fact that might come up on maybe a conceptual part of your exam or your
professor might even ask you – give you four different alcohols, which one's the easiest
to dehydrate.
Tertiary would be the easiest, secondary, primary and then – is the worst and then
– oops, sorry.
I don't know what I'm trying to draw here.
But basically, that methyl can't even happen because if it's a methyl alcohol or methanol
that can't even eliminate because it's only got one carbon.
So basically, the easiest one is tertiary, the worst one is primary.
That's the first thing.
The second thing is that the specific elimination mechanism that we use is going to depend on
how easily the molecule's going to form a carbocation.
And the understanding of carbocations is kind of essential to these two mechanisms.
All that means is that if you guys remember back to the trend of carbocation stability
that tertiary carbocations are the most stable.
And primary carbocations are the worst.
And then obviously methyl is even worse than that, but like I said, methyl doesn't even
get counted because we can't use it.
So it's really the same trend as the one about ease.
Basically, what we're saying is that the easier it is to make a carbocation, a positive charge
on a tertiary carbon, the easier – I'm sorry, not only the easier it's going to be to make
it, but also it's going to change the mechanism as well.
So what I want you to know for this section is that secondary and tertiaries make pretty
good carbocations.
And then primaries make pretty bad carbocations.
So I would say these two are good and that primary is bad and then obviously methyl is
out of here.