Let's move on to the next reaction.
And actually, these next two are going to be both considered partial saturation.
So what that means is that for these next two I'm not saturating all the way to an alkane.
In fact what I'm doing is I'm taking a triple bond and I'm only reacting it to a double
bond.
So I am adding two hydrogens, but I'm stopping there.
And the first of these – really the biggest difference between them other than the reagents
which are totally different, are the stereochemistry.
Notice that my first one is anti addition, notice that my second one is syn addition.
That's going to have a difference in what my products look like.
So let's look at this first one.
The first one's called dissolving metal reduction.
Dissolving metal reduction is just another name for these reagents.
What it is is that we have some kind of metal, lithium or sodium, your professor can use
either one, and some kind of liquid amine source.
Many times that amine is going to be NH3, but there's a lot of other things it can be.
It could be NH2CH3 or it could be just any kind of usually primary or secondary amine.
All right?
Cool.
So we've got that part down.
What does it do?
Well, we learned – I just taught you guys that partial saturation means that it's only
working on triple bonds.
So notice that I have the same or very similar molecule here and what I'm wondering is, how
should I draw this.
I have the same sigma framework, but for example, should I draw the double bond or should I
leave it there.
I should leave it there because it turns out that since this is partial saturation it has
no effect on alkenes.
No effect on alkenes.
It's only going to work on triple bonds.
So now I've got this triple bond.
Is it going to work on this one?
Absolutely.
Well, what does anti addition mean?
Anti addition, if you remember that just means I have trans products.
So what that means is that I would expect that my hydrogens are going to add from different
sides of this double bond.
What that means is that I should actually draw the double bond in a trans position.
Why?
Because the two hydrogens that I added must have added on different sides.
I have different sides of the double bond, which means that they were anti addition.
Is that making sense?
So what that means is that dissolving metal reduction is important because this is a way
that we can turn triple bonds into trans-double bonds.
That's going to be your product.
Trans-double bonds.
Pretty cool, right?
So that's something you have to keep in mind because for synthesis later when we start
trying to make molecules, that's going to be really the only way that we know how to
turn a triple bond and make it trans.
Cool?
So now let's go to this next one which I'm sure you guys can kind of already anticipate
what's going to happen.
Notice that this one's called Lindlar's catalyst.
Lindlar's catalyst has a lot of different reagents that you could use.
Unfortunately, different textbooks, different professors have their own way of writing it.
Some professors are going to be really easy.
They're going to be like, you know what?
F all of these reagents.
We're just going to write Lindlar's.
They're going to put H2 and Lindlar's and you're going to know that that's Lindlar's.
But some professors are going to use the actual reagents that are in the textbook, so some
textbooks have this as the P-2 catalyst, so the P-2 catalyst is one form of Lindlar's.
Another form is actually, with lead acetate and quinoline.
These are reagents that you just have to recognize that they are Lindlar's.
You don't need to know exactly what they look like or draw them or even know the mechanism,
but what you do need to know is recognize that these are Lindlar's.
These are also, just so you guys know, another term that you may hear is that these are poisoned
catalysts.
Now what could – oops, catalysts.
Wow.
Okay.
Sorry, guys.
Spelling isn't my forte.
So poisoned catalysts.
What does poisoned mean?
What it means is that these catalysts have been created in such a way that they're not
going to reduce all the way or they're not going to hydrogenate all the way, instead,
they're going to stop at a given point.
That's why they're called poisoned.
So these are three different ways that you could see this reagent just be aware of them.
Personally, I hope that your professor just writes Lindlar's, but you may see these other
two reagents come up.
So now let's finally get into the products.
What should I draw for my product?
Well, what I should draw is that I should still keep that double bond because whenever
you have partial saturation that means you're going to have no effect on double bonds.
That means the only difference – the only thing that I'm really touching here is the
triple bond.
So how should I draw it?
Well, in this case, I have syn addition and what syn addition means is that I'm going
to get cis products.
So what I would expect is that I have to draw this double bond now in a way that has my
R group cis to whatever that double bond is.
So I'm going to get out of the way here and draw this for you.
I would actually want to draw my stick down this time.
Why?
Because if I create like a little fence parallel to this double bond, what I see is that now
both of my groups are going to be on the same side of that fence, so this creates cis double
bonds.
So I know that was a lot of reagents.
I'm just going to stay here for a little bit so that you guys can write that down.
But anyway, I know that seemed like a lot of reagents, but really it's not that bad,
especially when you're not having to consider any mechanisms at this point yet.
I just want you to know the reagents, recognize what they do and be able to draw them.
Be able to draw the products.
And keep in mind the stereochemistry because the stereochemistry really is the most important
part of these reactions.
So let's move on to the next topic.