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Hi! I’m Deboki Chakravarti and welcome to Crash Course Organic Chemistry!
Double-bond-containing compounds are pretty abundant in nature.
But compounds with alkynes, triple-bonded carbons, are not.
There are a few naturally occurring alkynes, like cicutoxin in water hemlock,
and histrionicotoxin that the harlequin poison frog uses to protect itself.
But many alkynes aren't poisonous and aren't naturally occurring.
They're made in a lab, like two common ingredients in birth control pills:
ethynylestradiol, a synthetic estrogen, and norethindrone, a synthetic progesterone.
Natural estrogen and progesterone are expensive, hard to purify, and relatively scarce.
Plus, we need a higher concentration of the natural compounds to get the same effects as their synthetic replacements.
So even though they're not as common, alkyne-containing molecules are really important.
Organic chemistry is really about how chemical reactions can make compounds that make human lives better --
from medicines to materials.
We're trying to build on the building blocks that nature provides!
So with that as motivation, let's learn about alkyne reactions.
[Theme Music]
Remember that the carbons in a triple bond have sp hybridization.
And if an sp-hybridized alkyne is found at the end of a molecule,
it's called a terminal alkyne.
Terminal alkynes always have a hydrogen atom attached to them.
And the increased s-character of the bond means that a hydrogen attached to those carbons
is very weakly acidic, with pKas around 25.
So if we mix a super strong base, like the anion of ammonia, with a terminal alkyne,
we can make negatively-charged carbon-containing compounds called carbanions.
You might also hear them called acetylide anions -- after the simplest alkyne, acetylene.
Carbanions have a formal negative charge, so they’re nucleophiles!
But they're especially cool nucleophiles because they can make carbon-carbon bonds.
Even though organic chemistry is full of carbon-carbon bonds, making one is a pretty big deal!
It's not easy.
This might not be very obvious until later in this series, but many organic chemists aim to make huge molecules with lots of carbon-carbon bonds,
so the reactivity of these carbanions can help.
One way they form carbon-carbon bonds is an alkylation reaction, which looks like this:
Also, carbanions can attack carbonyl groups, which are those polar carbon-oxygen double bonds.
The lone pair of electrons attacks the partially positive carbon, forming a carbon-carbon bond,
and pushing electrons onto the oxygen.
We have a charged intermediate here, so as a second step we add in a source of protons,
water, in this case, and form an alcohol.
The acidity of hydrogen atoms on carbon-carbon triple bonds makes terminal alkynes unique.
But like alkenes, alkynes have pi bonds that are also nucleophilic.
So some alkyne reactions are pretty similar to the alkene reactions we've learned in the past two episodes.
Remember halogenation, where we added halogens like bromine or chlorine across a double bond?
They can also be added across a triple bond too!
Look at the reaction mechanism,
we can see that the nucleophilic triple bond attacks the electrophilic molecular bromine.
And even though there's an extra carbon-carbon bond in there,
a bromonium ion still contributes to the resonance structure and stabilizes the positive charge.
Like with alkenes, the bromonium ion structure blocks one face of the compound
and our major product is formed by anti-addition of the second bromide ion.
But we're not done yet!
These products are alkenes, which also react with halogens.
So there’s basically nothing stopping this halogenation reaction from going and going until it forms a tetrabromide product.
We can control side reactions a little bit with the amount of halogen we add, like adding
one or two molecules of dibromide to one alkyne molecule.
But it's pretty tricky to control alkyne halogenation reactions.
On the other hand, the addition of hydrogen halides is a bit more predictable with certain substrates.
We've spent a lot of time with this reaction in alkenes,
like when we were first exploring addition reactions in episode 14.
When we have an alkyne and hydrogen bromide, the nucleophilic attack happens like we'd expect
and the hydrogen bromide adds across the triple bond.
If the alkyne is a terminal alkyne, this addition will follow Markovnikov’s Rule.
As a reminder, this means that the proton will add to the side of the triple bond with the hydrogen atom.
To unpack that a little more, the carbon of the triple bond on the left side has a methyl substituent and no hydrogens.
And the carbon on the right side of the triple bond is bonded to one hydrogen.
So the proton from hydrogen bromide will add regioselectively to the right,
which forms a more stable secondary vinyl carbocation, as opposed to a primary one.
This is called a vinyl carbocation because the positive charge sits on a carbon-carbon double bond.
The attack step creates a vinylhalide:
a term we use to describe a halogen directly attached to the carbon of a double bond.
And remember, alkenes are still reactive!
So if we start with two molecules of hydrogen bromide to one molecule of alkyne,
the reaction will keep going.
We add a second hydrogen bromide across the double bond, also following Markovnikov addition.
We’re showing terminal alkynes here because this reaction is regioselective.
When the alkyne isn't at the end of the molecule, called an internal alkyne, the reaction is hard to control.
We get complex mixtures of products because there's no difference in carbocation stability!
OK, cool, we have two alkyne reactions in our back pocket,
both of which were similar to the halogen and alkene reactions!
We can add water using similar reactions too.
For example, we can do a mercury-catalyzed hydration reaction.
The name tells us a couple things:
we’ll be using a mercury compound, specifically mercury (II) sulfate, HgSO4,
which we’ll get back unchanged at the end because it’s a catalyst.
And we’ll be adding some water across the triple bond.
That's the hydration.
The first step of this reaction is the alkyne attacking the mercury cation with a positive two charge,
while the sulfate anion just floats around as a spectator.
The mercury adds so that the more stable carbocation forms.
We get a secondary vinyl carbocation, stabilized by resonance with a mercurinium ion,
as opposed to a primary carbocation.
Next, a water molecule attacks our newly-formed vinyl carbocation.
And with a one-two punch, another molecule of water deprotonates this ion, forming an alcohol.
This is a pit stop and our water molecule is officially added!
But we're not done with the whole reaction mechanism.
We still need to get rid of the mercury to restore our catalyst.
So the alkene attacks the hydronium ion that we just made.
This forms a carbocation on the side that has resonance stabilization,
thanks to oxygen being all generous and sharing a pair of electrons.
This structure has a lot of positive charge built up, so mercury is happy to get out of there,
leaving its electrons behind.
And this reforms the double bond, the alcohol group, and our free-floating catalyst.
Our final product has a special duo:
a double bond next to an alcohol.
So we steal parts of the words alkene and alcohol to call it an enol.
This is the summary of our long and winding road map for a mercury-catalyzed hydration reaction,
adding water to a terminal alkyne.
Internal alkynes would again give us complex mixtures of products!
But there's still more fun to be had with enols.
Specifically, there's a type of constitutional isomer called tautomers, which differ in the location
of the double bond and just one hydrogen.
If the double bond is between two carbons, it's the enol form.
And if the double bond is between a carbon and the oxygen, it’s the keto form.
In fact, the enol we just made quickly undergoes tautomerization to become a ketone.
This happens because the sum of the bond energies in the keto form is greater than the sum of the bond energies for the enol form.
Because the keto form makes stronger bonds, we know it's more stable.
Looking at our reaction mechanism, we have some hydronium ions, which are acidic.
So an acid-catalyzed tautomerization can happen here.
First, the double bond in the enol attacks a hydronium ion, and adds a proton so that the positive charge goes next to oxygen,
where it can be resonance stabilized.
At this point, there are two resonance forms:
the positive charge can hang out on a carbon, or the oxygen.
So to make the keto form, a water molecule deprotonates the oxonium ion.
Overall, we added a proton on one side of the enol, and removed one from the other side.
So it's not too complicated:
tautomerization is basically some double bonds shifting and a single proton being moved around.
In fact, tautomerization reactions have a similar pattern as resonance, which we learned in episode 10.
If we place a block around the double bond, along with the atom that has electrons in a p-orbital next door,
we find our three-atom pattern.
Now, swap the stuff at atoms one and three.
We move the double bond and hydrogen, and switch between our keto and enol forms!
So we can make a ketone from an alkyne using a mercury-containing catalyst and some water.
But that's not the only way to add water across a double bond…
we can do hydroboration too!
Looking at the name, you might remember that's a hydration reaction with some help from carbon’s periodic-table-next-door-neighbor boron.
Hydroboration happens the same way it did with an alkene:
our nucleophilic alkyne attacks the boron reagent, we get a pit-stop-intermediate thanks to anti-Markovnikov addition,
and then an oxidation reaction gives us the enol.
Whew!
If you want that in more detail, watch episode 17.
Like the enol formed in mercury-catalyzed hydration reactions,
this one will tautomerize to produce the more stable keto form.
But because we have sodium hydroxide ions floating around, which are basic,
this is base-catalyzed tautomerization.
And the mechanism looks a little different.
First, the hydroxide ion attacks the alcohol of the enol and deprotonates it.
This puts a negative charge on our molecule, which has two resonance forms:
one with the negative charge on the oxygen, and one with the negative charge on the carbon.
We use the carbanion resonance form, attack the proton on a water molecule,
and reform the base to make our keto form.
Done!
Even though the reaction mechanism was a little different, the overall tautomerization reaction was similar:
adding a proton on one side of the enol, and removing one from the other side.
Now that we've seen acid-catalyzed and base-catalyzed tautomerization,
I want to point out another pattern that we'll see throughout this series:
When we're doing reactions in acids, we'll generally have positive charges lying around.
And when we're doing reactions in bases, we'll generally have negative charges around.
Look back at our two reactions and see what I mean!
And to round out this episode, let's explore one last tool that we used on alkenes too: reduction.
Reduction by hydrogenation is when we add hydrogen across pi bonds to make an alkane,
with the help of a metal catalyst like platinum, palladium, or nickel.
We can also manage to do a partial hydrogenation on an alkyne to form an alkene and stop the reaction early.
Lindlar’s catalyst is a complex palladium-based catalyst where some of the reactive sites on the palladium are unavailable.
This keeps the reaction from going all the way to an alkane.
With Lindlar's catalyst, the reaction happens on a metal surface once again,
so the hydrogens can’t help but add to "ze Zame Zide” of the alkyne, giving us the Z-isomer.
But, if we want the hydrogen to add "E-cross" to get the E-isomer,
we can use a different type of reaction and do a metal-ammonia reduction.
This mechanism involves radical chemistry.
Like, single-electrons radical… not "these gnarly waves" radical.
So we’ll save it for the next episode.
But the important takeaway here is that a radical allows the R groups to position themselves
far away from each other during the mechanism and form the E-alkene.
In this episode, we learned that:
The acidity of terminal alkynes allows us to form carbon-carbon bonds
Many of the reactions that we used for alkenes also happen for alkynes
Terminal alkynes can have good regioselectivity, while internal alkynes often form mixtures of products.
But reactions with the triple bond can also lead to products resulting from additions to both pi bonds, like tetrahalides
And hydration reactions of alkynes can form enols, which tautomerize to keto compounds
Next time we’ll get into radical chemistry, which is pretty wild,
but also pretty fascinating because it gets those couch potato alkanes up and moving.
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