What I want to talk about now is a way that we can make triple bonds into strong nucleophiles.
Now that might sound weird because if you remember triple bonds are hydrocarbons and
usually hydrocarbons aren't really good nucleophiles or electrophiles.
They kind of just sit there.
But it turns out that triple bonds are uniquely acidic because of a principle that we talked
about way back when we talked about acid and base chemistry.
And we can use that to our advantage to make them strong nucleophiles.
And these are called alkynides.
So let's go ahead and talk about how we can do that.
Basically, remember that we talked about in the acid and base chapter, so if you didn't
watch that chapter, it's fine, I'm just going to remind you now that terminal alkynes, the
alkynes that have one H at the end are uniquely acidic due to an effect called the hybridization
effect.
Now when we talked abut acids and bases we talked about that there were basically five
effects that made things more acidic.
There was like the element effect, inductive effect, resonance, solvation and there was
hybridization.
I know this might be bringing back some bad memories for you guys or maybe you just completely
have no clue what I'm talking about.
But hybridization was one of the things that could make something more acidic.
And here's an example of a terminal alkyne right there and let's just go through the
hybridization.
Basically, what the principle said was that the more s character that an atom has, the
more acidic it's going to be.
And notice that this carbon right here has what kind of hybridization?
It has sp hybridization.
The reason it's sp is because remember that the way we figured out hybridization was you
count up the number of groups or what we called bond sites and that would tell you what the
hybridization is.
So, in this case, I would just have one bond over here, a second bond over here.
That's two.
Two bond sites equals sp.
So we know that the hybridization is sp.
Then out of the entire hybrid orbital, what percentage of that is s?
That's actually really easy to figure out because you just say how many hybrid orbitals
are there total.
Two.
There's the s and there's the p.
S would be 50% of that.
Now just so you guys know that's pretty high s character because, for example, sp3, that's
a hybridization you should be really familiar with at this point.
Sp3 has one s and 3 p's.
So that one has a 25% s character.
So that one's a lot less.
So having 50% is really good.
That must mean this is pretty acidic.
And it turns out that it is.
Back in acids and bases, I made you guys memorize that the pKa of a triple bond is 25, which
is unusually low for a hydrocarbon.
Like I said, hydrocarbons are really sucky acids usually.
For this to be 25, that's not bad.
So how are we going to use this to our advantage?
Well, it turns out that if we can use a strong enough base, we can pull off this H and give
a negative charge to the terminal alkyne.
So we're going to use a strong base to deprotonate that H and usually the bases that we use are
either NaH, sodium hydride, or NaNH4, sodium anide, and both of these are very strong small
bases that are going to be able to pull that hydrogen off of the terminal alkyne, so I'm
talking abut that green hydrogen above me.
That's going to create a strong nucleophile that we're going to call a sodium alkynide.
What the sodium alkynide looks like is it's just going to be a negative charge on that
triple bond.
So if I were just to erase some of this stuff and show you what the acid/base reaction would
look like for this reaction it would be that I have let's say NaH, right?
Well, that's going to dissociate, right?
That would dissociate into Na+ plus H-.
Now the H- is what's going to react, that's the base, it's going to reach with the H.
So I would pull off the H and donate those electrons to the triple bond giving the triple
bond now a negative charge.