Chapter 9: Alkynes

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Welcome to the Deep Dive.

We're here to take complex topics, break them down, and give you that shortcut to really understanding them.

That's the plan.

Today, we are diving into you know, those organic molecules with the carbon triple bonds.

Fascinating stuff.

It really is.

Did you know, for instance, that a simple triple bond is actually vital in treating Parkinson's disease?

Yeah.

In droves like seligulene, C triple C bond is key.

Exactly.

So our mission today is to unpack a whole chapter on alkynes, pull out the core ideas, and really get why these molecules are, so important in medicine, manufacturing, you name it.

Get ready for those aha moments.

Definitely.

Okay, so let's start at the heart of it.

What is it about these carbon -carbon triple bonds?

Right, the C bond.

It's actually made of three bonds, but they're not all the same.

You've got one strong sigma bond, sort of holding the carbons together directly.

Okay, the main link.

And then two weaker pi bonds.

These form above and below and side to side around the sigma bond.

Think of it like two clouds of electrons wrapping around that central sigma bond axis.

So like a cylinder of electron density.

Precisely.

A cylindrical region of high electron density.

That's the key phrase.

And that electron density, that must be where the action is, right?

Does it make them eager to react?

Absolutely.

That's exactly it.

That cloud of electrons makes alkynes quite reactive.

They're electron rich.

So they can act as bases?

Both bases, donating electrons and also as nucleophiles, seeking out positive charges to form new bonds.

It's really their defining chemical feature.

And this bonding,

it also dictates their shape.

They're not zigzaggy like alkanes.

Not at all.

They're perfectly linear.

The two carbons in the triple bond and the atoms directly attached form a straight line, 180 degree bond angles.

And that comes from the hybridization.

Yes, it's a direct result of some hybridization of those carbon atoms.

Very different from the B3 in alkanes or B2 in alkanes.

So rigid.

Can you even put a triple bond in a small ring?

Generally, no.

It puts way too much strain on the molecule.

You typically need a ring size of at least, say, nine carbons before a cycle kind becomes reasonably stable.

Wow.

Okay.

So comparing like acetylene with ethane and ethylene,

the bond lengths are shorter in acetylene, right?

Both CC and CH.

Why is that?

It all comes down to hybridization again, specifically something called cease character.

S character.

Remind me.

In SPEEP hybridization, the hybrid orbital has 50 % character from the S atomic orbital and 50 % from the P orbital.

That's way more S character than SP2, 33 % or SP3, 25%.

Okay.

And remember, S orbitals are closer to the nucleus than P orbitals.

So more S character means the electrons in that hybrid orbital are held closer, tighter to the positive nucleus.

Ah, pulling everything in tighter.

Makes sense.

Shorter bonds.

Exactly.

Shorter CC triple bond and shorter CH bonds attached to the SPEEP carbons.

Plus, you have three pairs of electrons between the carbons and the triple bond, which also helps pull them closer.

So this isn't just like a theoretical detail.

This structure has real world consequences.

Huge consequences.

Industrially, acetylene is still used as welding fuel.

That triple bond releases a lot of energy, creates a super hot flame.

Right.

The classic

And it's a starting point for making other more complex alkynes, but the biological side is maybe even more compelling.

Like the birth control example, ethaneostradio.

Precisely.

That triple bond adds rigidity, helps it bind effectively, makes it more potent.

Or natural toxins, like histrinototoxin from those poison dart frogs.

Yikes.

But let's go back to seligelin for Parkinson's.

How does the triple bond actually work there?

Okay.

So Parkinson's involves low dopamine.

Seligelin inhibits an enzyme called MAOB, which breaks down dopamine.

So it helps preserve dopamine levels.

Exactly.

But there's another enzyme, MAOA, that you don't want to inhibit too much because that can cause cardiovascular issues.

Seligelin is selective for MAOB.

And the triple bond is key to that selectivity.

Yes.

The linear shape imposed by the triple bond, combined with other parts of the molecule,

allows it to fit perfectly into the active site of MAOB, but not MAOA.

It's a beautiful example of structure -based drug design.

Amazing.

Okay.

So we know what they are, why they're important.

How do we name these things systematically?

It sounds like it follows the Ikean rules pretty closely.

It does.

Very closely.

Four steps, basically.

First, find the longest continuous carbon chain that includes the triple bond, that's your parent chain.

And you use the alpane suffix, right?

So heptane becomes heptine.

Correct.

Step two, identify and name any substituents, any branches off that main chain.

Got it.

Step three, number the parent chain.

And here's the rule.

Give the triple bond the lowest possible number.

The first carbon to the triple bond determines the number.

Even if that means a substituent gets a higher number.

Yes.

The triple bond takes priority for numbering.

You indicate its position with that number, like two heptane or heptuanane, both are acceptable IUPAC formats.

Okay.

And the last step.

Just assemble the full name, putting the substituents in alphabetical order.

Pretty straightforward if you know the alkenyalky rules.

And there are common names too, like acetylene for ethane.

Yeah, chemists use common names sometimes, especially for smaller ones.

Acetylene, methylene, things like that.

But one really crucial distinction we need to make is between terminal and internal alkynes.

Why is that so important?

It dramatically affects their reactivity, especially concerning acidity, which we'll get to.

A terminal alkyne has the triple bond at the end of a chain, so it has a hydrogen directly attached.

R -C -C -H.

Okay, one hydrogen on the triple bond.

An internal alkyne has the triple bond somewhere in the middle of the chain, so it has carbons attached to both sides.

R -C -C -A.

No hydrogen directly on the triple bond carbons.

Got it.

Terminal versus internal.

And you mentioned practical uses, polyacetylene conducting polymers.

Oh yeah, a quick but cool story.

Polyacetylene was like the first organic polymer found to conduct electricity.

Bit of a shocker at the time.

It basically launched the whole field of conducting polymers, won a Nobel Prize, and that technology eventually led to things like OLED screens in your phone or TV.

From a simple repeating alkyne unit.

Incredible.

Now let's tackle that acidity you mentioned.

Terminal alkynes are surprisingly acidic.

Surprisingly is an understatement.

Let's Alkyne is around 50.

Ethylene and alkene is 44.

So not acidic at all, really?

Practically not.

But acetylene, its pKa is 25.

25?

That's a huge jump.

How much more acidic is that?

It's about 10 to the power of 19 times more acidic than ethylene.

That's 10 followed by 19 zeros.

10 quadrillion times more acidic.

Oh, okay.

Why?

What makes that terminal hydrogen so removable?

It comes back to the hybridization and the stability of the resulting in the conjugate base.

When you remove that H plus foik, you're left with a negative charge on the carton.

The acetylide ion or alkanide ion.

Right.

And that negative charge sits in an A -dead hybrid orbital.

Remember how sprake orbitals have 50 % character and are held close to the nucleus?

Well, putting a negative charge, those extra electrons into an orbital that's closer to the positive nucleus, makes it much more stable than putting it in an sp2 or sp3 orbital, which are further away.

So the stability of the conjugate base drives the acidity.

Exactly.

A more stable conjugate base means the acid is stronger.

It's more willing to give up that proton.

Okay.

So this acidity isn't just a fun fact.

It's a gateway to doing stuff with these molecules.

Absolutely.

It's a synthetic superpower, but you need the right tool to unlock it.

You need a base strong enough to actually pull off that proton.

And not just any base will do, like sodium hydroxide?

Nope.

NaOH isn't strong enough.

It's conjugate acid water, has a pKa of about 15 .7.

Acetylene's pKa is 25.

The equilibrium heavily favors the starting materials.

So the base needs to be stronger than the acetylide ion you're forming.

Correct.

You need really strong bases where the negative charge is on something less electronegative than oxygen, like nitrogen, hydrogen, or carbon.

Sodium amide and NaH2 is a classic.

Sodium hydride, NaH, or organometallic bases like butyl lithium, buly, those will readily deprotonate a terminal alkan.

Okay, that makes sense.

So if we want to make an alkan in the lab, where do we usually start?

Often you start from an alkyl dehalide, a molecule with two halogen atoms, these can be geminal, with both halogens on the same carbon, or vicinal with halogens on adjacent carbons.

And you somehow remove the halogens and hydrogens to form the triple bond?

Essentially, yes.

It's done through two successive elimination reactions, specifically E2 eliminations.

You remove H and X twice.

Two eliminations.

Does that need special conditions?

You mentioned strong bases.

Yes, the second elimination especially requires a very strong base.

The go -to region is often excess sodium amide and NH2 dissolved in liquid ammonia.

Excess?

Why excess?

Good question.

You usually need three equivalents of the amide base.

Two equivalents are for the two E2 reactions themselves.

Okay, one for each elimination was the third one for?

If you form a terminal alkyne, the third equivalent of strong base immediately deprotonates it to form that stable alkanide ion we just talked about.

Ah, so that final deprotonation helps drive the whole reaction sequence to completion.

Exactly.

It's a thermodynamic sink pulling the equilibrium forward.

Then at the end you just add water or a mild acid to protonate the alkanide ion and get your neutral terminal alkyne product.

Clever.

Okay, so we can make them.

Now transforming them.

Let's talk reduction, adding hydrogen.

Right.

We can control this very precisely.

If you want to go all the way, reduce the alkene completely to an alkyne, you use standard catalytic hydrogenation.

H2 gas, metal catalysts like platinum, palladium, nickel.

Yep.

That adds four hydrogens in total, two equivalents of H2 across the trill bond taking you straight to the alkyne.

The tricky part is stopping halfway.

Because the intermediate alkyne is actually more reactive.

Usually, yes.

Under these conditions, it reacts faster than the starting alkyne so you can't easily isolate the alkyne.

But what if you want the alkyne?

And specifically, what if you want the cis alkyne?

Uh -huh.

For that, you need a special catalyst.

A poisoned catalyst.

Poisoned?

Sounds bad.

Yeah, it just means it's been partially deactivated.

It's less reactive.

Lindlar's catalyst is the classic example, palladium on calcium carbonate treated with lead acetate and quinoline.

Okay, so this weakened catalyst.

It's strong enough to reduce the alkyne to the alkyne, but not strong enough to reduce the alkyne further to the alkyne.

And crucially, it delivers both hydrogens to the same face of the triple bond syn addition, giving you exclusively the cis alkyne.

Wow, specific.

So that's cis.

How do you get the trans alkyne?

Completely different reaction.

You use dissolving metal reduction,

sodium metal, metal not sodium amide dissolved in liquid ammonia at low temperature.

Sodium metal in ammonia.

What does that do?

The sodium metal acts as a source of electrons.

The mechanism is different, involving single electron transfers,

forming a radical anion intermediate.

It's a stepwise process.

And this gives the trans product.

Yes.

The intermediate radical anion prefers a trans geometry to minimize repulsion between electron groups.

So when the second hydrogen adds, it adds to the opposite face anti addition, resulting specifically in the trans alkyne.

That's incredible control.

Alkyne, cis alkyne, or trans alkyne just by choosing the right reagents.

It's a cornerstone of synthetic strategy.

Really powerful.

Okay, moving on from reduction.

What about adding things like HX

hydrophilogenation?

Alkans react with HX like HCl or HBr, similar to alkenes.

It follows Markovnikov's rule.

Meaning the halogen adds to the more substituted carbon of the triple bond.

Correct.

If you use just one equivalent of HX, you get a vinyl halide.

But if you use excess HX, it adds twice.

Twice.

How does the second one add?

It also follows Markovnikov's rule.

So the second halogen adds to the same carbon as the first one.

You end up with a geminal dehalide, both halogens on the same carbon.

Okay.

Predictable.

Any way to get anti Markovnikov addition.

Yes.

But specifically with HBr and only in the presence of peroxides like ROR.

Just like with alkenes, this switches the mechanism to a radical pathway.

And the bronine adds to the less substituted carbon.

Exactly.

Anti Markovnikov.

You usually get a mixture of E and Z isomers in this case, though.

Right.

Okay.

How about adding water hydration?

Two main ways leading to different products, which is super useful.

First is acid catalyzed hydration.

You use aqueous sulfuric acid, usually with a mercury sulfate catalyst.

Mercury by mercury.

It helps activate the alkyne towards attack by water.

The initial product formed is actually an enol.

Enol.

That's the double bond plus alcohol thing.

That's right.

Alkyne plus alcohol enol.

But enols are usually unstable.

They rapidly rearrange into a ketone.

Through that tautomerization process.

Exactly.

Keto enol tautomerization.

It's an equilibrium, but it heavily favors the more stable ketone form.

Remember, tautomers are actual isomers that interconvert, not resonance structures.

Okay.

And the addition follows Markovnikov's rule too.

Yes.

So for a terminal alkyne, the oxygen adds to the internal carbon, meaning you always end up forming a methyl ketone, a ketone where one group is CH3.

Always a methyl ketone from a terminal alkyne.

What if you want an aldehyde instead?

Then you need the second hydration method.

Hydroboration oxidation.

Ah, the anti -Markovnikov one.

Precisely.

You use a borane regent first, often a bulky one like dysiamborane or 9 -BBN, to prevent adding twice, followed by hydrogen peroxide and sodium hydroxide.

And this also forms an enol initially.

It does, but this time the OH group adds to the less substituted carbon,

anti -Markovnikov.

And then the tautomerization happens under basic conditions from the second step.

And that leads to… An aldehyde from a terminal alkyne.

The enol tautomerizes to the aldehyde.

Fantastic.

So you can choose acid mercury for ketone or hydroboration for an aldehyde, starting from the same terminal alkyne.

That's the beauty of it.

Complete control over the regiochemistry of hydration.

What other reactions should we know?

Halogenation, adding Cl2 or Br2?

Yep, alkanes react with halogens too.

If you add excess X2, like two equivalents, it adds across the triple bond twice to give a tetrahalide.

All four halogens attached.

And if you use just one equivalent?

You add just one molecule of X2 across the triple bond, forming a dehalide.

This addition is typically anti, meaning the halogens add to opposite phases, so you predominantly get the e -isomer.

Okay.

And then there's ozonolysis, breaking the bond entirely.

Yes.

Oxidative cleavage with ozone, O3, followed by a water work out.

This reaction literally cuts the triple bond in two.

What do the pieces become?

For an internal alkyne, both fragments become carboxylic acids.

For a terminal alkyne, the internal part becomes a carboxylic acid, but the terminal carbon, the CHN, gets fully oxidized to carbon dioxide, CO2.

Useful for figuring out where the triple bond was?

Historically, yes.

Before fancy spectroscopy, it was a key method for structure determination.

Okay.

One last major reaction type,

alkylation, using that acidity of terminal alkyne.

Right.

This is a really powerful way to make new carbon bonds, which is fundamental to building complex molecules.

So you start by deprotonating the terminal alkyne.

Exactly.

Use a strong base like NNH2 to rip off that acidic proton, forming the alkanide ion.

Which is a good nucleophile.

A very good nucleophile.

It can then attack an alkyl halide like methyl iodide or ethyl bromide in an SN2 reaction.

The alkanide replaces the halide, forming a new CC bond.

Lengthening the carbon chain.

Precisely.

But there's a catch.

This only works well with methyl or primary alkyl halides.

Why not secondary or tertiary?

With bulkier secondary or tertiary alkyl halides, the alkanide ion tends to act as a base instead of a nucleophile, causing an elimination reaction on the alkyl halide, rather than the desired substitution.

So you don't form the CC bond you wanted.

Ah, okay.

Stick to methyl or primary halides for alkylation.

And what about acetylene itself?

It has two acidic protons.

Good point.

Acetylene, HTACH, can be alkylated twice.

You can do it stepwise.

Deprotonate one side, add one alkyl group, then deprotonate the other side, and add a second alkyl group, which could be the same or different from the first.

Very versatile for building internal alkynes with specific groups on either side.

Absolutely.

Gives you a lot of synthetic flexibility.

So putting it all together, we've built quite a toolbox.

We can make alkynes, reduce them selectively to alkenes, cisalkenase, or transalkenes.

Add HX or water with controlled regiochemistry, cleave them.

And use terminal alkynes to build longer carbon chains.

You mentioned converting alkenes to alkynes too.

Yes, that's a key transformation to have in your pocket.

You take an alkene, add bromine, Br2, across the double bond to make a vicinol D bromide.

Okay, two bromines on adjacent cargons?

Then you hit it with excess strong base, like NaNNH2, to do two eliminations, forming the alkyn.

Just like we discussed for preparing alkynes from dihalides.

So you can go alkynalkyne.

And we know we can go alkynal.

Alkynes, cis or trans, that really connects things.

It does.

You can start planning multi -step sequences.

Maybe start with an alkyn, make it an alkyn, alkylate it, then reduce it back down to exactly the stereoisomer of the alkyn you need for your target molecule.

It really feels like we're designing molecules now, not just learning reactions.

That's the goal.

Understanding how these structures behave and how to manipulate them.

So, quite the deep dive into alkynes today.

We went from the basic structure, that linear geometry, the electron density.

Through naming their unique acidity, how to make them.

All the way to the key reactions, reductions, additions like hydrophilogenation and hydration with that cool ketoenol tautomerization.

Hylogenation, ozonolysis, and a powerful C -C bond formation via alkylation.

It really hammers home how structure dictates function, dictates reactivity, and understanding that lets you control the outcome.

It really does.

That simple -looking C bond is just packed with chemical potential.

It makes you wonder.

Yeah.

Well, as you've seen, that seemingly simple triple bond holds immense power and versatility.

What other hidden functionalities might be lurking in everyday molecules, just waiting for us to uncover their deeper chemical secrets?

That's a great thought to end on.

Thank you for joining us on this deep dive, and thanks to all of you for listening and being part of the deep dive family.

Keep exploring the wonders of chemistry.