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Welcome back to the Deep Dive.
Today we're jumping into something really fascinating in organic chemistry.
Yeah, we're looking at alkynes.
Exactly, alkynes.
These are molecules with that carbon -carbon triple bond.
Now, maybe they aren't quite as common as, say, alkenes, but they've got some really, really interesting features and reactivities that are kind of unique just to them.
Absolutely, and that's our goal today, right?
We want to sort of break down alkynes for you, make them simpler.
Yeah, cut through the jargon.
We'll cover the basics,
the main reaction types, get into some key mechanisms, and importantly, give you some practical tips, things to watch out for.
Pitfalls.
Always good to know the pitfalls.
Definitely.
So by the time we're done here, you should have a really solid handle on their properties, how to name them, how chemists actually make them, and how they turn into other functional groups.
It's kind of a shortcut to getting up to speed on alkynes.
Okay, let's unpack this then.
So basics first.
Alkynes mean carbon -carbon triple bond.
Got it.
But how do we start differentiating them, like with names?
Good place to start.
The systematic naming, the sort of official way, it's actually quite similar to how we name alkenes.
Okay.
The big change is the ending.
Instead of N, like an alkene, alkenes always end with pen.
Y -N -E.
Got it.
Exactly.
And just like with alkenes, you use a number out front in the prefix to show where that triple bond is located in the main carbon chain.
So like two pentine, five carbons, triple bond at carbon two.
Precisely.
That's the systematic way.
But chemists often use common names too.
Ah, the nicknames.
Sort of, yeah.
These are usually based on the simplest alkene, which is acetylene, C2H2.
Right.
So you just name the groups attached to the triple bond carbons as substituents of acetylene.
Like if you have two methyl groups, one on each side, it's dimethyl acetylene.
Makes sense.
Or like two isopropyl groups.
That's your propyl acetylene.
You got it.
It's a bit more informal, but you hear it a lot.
Okay.
Names down.
Now here's where I think it gets really interesting.
The structure, that triple bond must really dictate the shape, right?
Oh, absolutely.
It's fundamental.
The carbons in that triple bond, they're what we call
hybridized.
Hybridized.
Okay.
What does that mean practically?
It means each of those carbons is only bonded to two other things.
The other alkene carbon and one other atom or group.
Only two substituents.
Right.
And because of that, everything arranges itself to be as far apart as possible, which leads to a perfectly linear geometry.
180 degrees.
Straight line.
Exactly.
180 degrees between the bonds.
And the triple bond itself is made of one strong sigma bond plus two pi bonds.
Two pi bonds.
That's the difference from an alkene.
That's the key difference.
And those pi bonds come from the side by side overlap of p orbitals.
They're kind of exposed, sticking out above and below the sigma bond axis.
Which makes them reactive.
Very reactive.
Those pi electrons are the action zone.
And this whole electron cloud arrangement forces and keeps the molecule in that straight linear shape.
That's why we always draw them as a straight line.
So, okay.
Linear.
Rigidly linear.
What does that mean if you try to put an alkyne into a ring structure?
Great question.
It means trouble for small rings.
Big trouble.
Because you're trying to bend something that wants to be straight?
Precisely.
Think about it like this.
Imagine trying to grab your own leg and touch your big toe to your nose.
Oh, okay.
Yeah.
Forming a ring with my body.
Right.
The strain you'd feel.
That's kind of like the strain an alkyne feels if you force it into a small ring.
It just doesn't want to bend that way.
So it's highly unstable.
Extremely unstable in small rings.
We're talking rings smaller than seven carbons.
You just generally can't make or isolate them.
They're too strained.
Seven carbons is the threshold.
Pretty much.
Yeah.
Cycloheptine.
That's the seven carbon one.
It's actually the smallest cycloalkene that's been characterized.
Yeah.
But even it is super reactive.
It doesn't hang around long.
The bigger rings are okay.
Eight carbons or more.
Yeah.
Once you get to eight or more carbons, the ring is flexible enough to accommodate that linear alkyne unit without too much protest.
They become much more stable.
Fascinating.
Okay.
So we know what they are, how they're named, their shape.
How do chemists actually make these things?
How do you forge that triple bond?
Right.
Synthesis.
There are two main workhorse methods you really need to know.
Okay.
The first one involves elimination.
It's actually a double elimination reaction called dehydrohalogenation.
Dehydrohalogenation.
Losing hydrogen and halogen.
Twice.
Exactly.
You start with a molecule that has two halogens, usually bromines, on adjacent carbons.
That's a vicinal dibromide.
Okay.
Dibromide.
Then you treat it with two equivalents.
You need two of a really strong base.
Typically sodium amide, that's nanananshu, sometimes called sodiumide.
Strong base.
Why sodium amide?
It's just very effective at ripping off those protons and kicking out the bromides.
Concentrated sodium hydroxide, NaOH, can sometimes work too, but sodiumide is more common.
So two equivalents of base pull off two HBr molecules.
And leave you with a carbon triple bond.
Pop.
Alkyne formed.
Neat.
And you connect to alkenes.
It's a handy sequence.
You can take an alkene, add bromine across the double bond that gives you the dibromide, and then do this double elimination.
So alkene to alkene in two steps.
Clever.
What's the second method?
You said it highlights a difference from alkenes.
Ah, yes.
This one is really powerful.
It involves what we call acetylide chemistry.
Acetylide related to acetylene.
Exactly.
But first, we need to distinguish between terminal and internal alkynes.
Okay.
A terminal alkyne has the triple bond right at the end of the carbon chain.
So one of the alkyne carbons is bonded to a hydrogen.
Hdex C triple bond Cr.
Right.
An internal alkyne has the triple bond somewhere in the middle.
Rc triple bond Cr prime.
Gotcha.
Terminal versus internal.
Why does it matter?
Because that hydrogen on a terminal alkyne, it's actually somewhat acidic.
Really?
A C -H bond?
Acidic?
Relatively speaking, yes.
Its p -cash is around 25.
Now, that's not super acidic like say HCl or carboxylic acid.
No, definitely not.
But it's much more acidic than a regular alkyne C -H bond, which has a p -co up around 50.
And it's acidic enough that a very strong base, like our friend sodium amide again, can easily pull it off.
Sodium amide rips off the terminal proton.
Leaving behind a carbon with a negative charge.
This is the acetylide anion.
Okay.
A carbon anion C -.
And that acetylide anion is a fantastic nucleophile.
Remember, nucleophile means nucleus lover.
It's looking for something positive, an electrophile to attack.
Like an alkyl halide.
Exactly.
Specifically, a primary alkyl halide.
The acetylide anion attacks the carbon holding the halogen, kicks the halogen out.
And forms a new carbon bond.
Precisely.
You've coupled the alkyne fragment to the alkyl halide fragment, making a longer carbon chain, usually forming an internal alkyne.
This is huge because making C -T bonds is like the core business of organic synthesis.
Building bigger molecules.
That makes sense why it's so important.
Foundational stuff.
But here's a really crucial tip for you, especially when you're solving problems or planning a synthesis.
Okay, listening.
This acetylide attack only works well with primary alkyl halides.
That means the carbon with the halogen has to be a CH2 group or maybe a methyl group.
Not secondary or tertiary halides like where the carbon has two or three other carbons attached.
Nope.
If you try to use secondary or tertiary alkyl halides, the acetylide anion tends to act as a base instead of a nucleophile.
It causes elimination reactions, not the substitution you want.
That's a common trap.
Ah, good to know.
Primary alkyl halides only for the C -C bond formation.
Remember that one.
It'll save you points.
Okay, so we've built them using these cool methods.
Now let's switch gears again.
What can they become?
You said the reactions are like alkenes but maybe double?
Sort of, yeah.
That extra pi bond means they can often react twice where an alkene reacts once.
It's often double the fun, as you said.
So like bromination, alkenes add one Br2.
Alkenes can add two equivalents of Br2.
You react an alkene with excess bromine and you end up adding four bromine atoms total, making a tetrabromide.
Tetrabromide, wow.
Same mechanism as alkenes, that bromonium ion thing.
Pretty much, yeah.
It just happens sequentially.
First addition makes a dibromololken, then the second addition happens to that remaining double bond.
Okay, what about adding hydrogen reduction?
Hydrogenation, yep, alkenes do that too.
Right.
If you just take an alkene and bubble in hydrogen gas, H2, over a standard metal catalyst like palladium on carbon, PDC, or platinum Br.
The usual suspects.
Right, and you use enough hydrogen, like two equivalents, or just keep it going.
It'll reduce the alkene all the way down, pass the alkene stage straight to the alkene, fully saturated.
Triple bond to single bond, complete reduction.
Complete reduction, but often that's not what you want synthetically.
Right.
What if you want to stop halfway at the alkene?
Ah, now that's a very common goal, and it's possible, but you need special conditions because generally, alkenes are actually more reactive towards hydrogenation than alkenes.
So you need to kind of hobble the catalyst.
Hobble the catalyst.
How?
To stop at the cis alkene, you use something called Lindlarz catalyst.
Lindlarz?
Heard of that.
It's basically palladium metal that's been poisoned,
made less reactive by adding some lead compounds and quinoline.
Poisoned palladium.
Sounds dramatic.
It just tones down the reactivity enough so that it reacts with the alkene, adds one molecule of H2 in a syn fashion, meaning both H's add to the same phase.
Syn addition gives cis geometry.
Exactly.
It stops right there, giving you the cis alkene selectively.
Really clever trick.
Okay, so Lindlarz gives cis.
What if you want the trans alkene?
For the trans alkene, you need a completely different approach.
No palladium.
You use sodium metal dissolved in liquid ammonia.
Sodium metal in ammonia?
That sounds intense.
It is.
It's a dissolving metal reduction.
The mechanism is different, involving electron transfer, and leads to anti -addition of trans geometry.
You got it.
So Lindlarz for cis, sodium ammonia for trans,
two key ways to control alkene geometry from an alkene.
Very useful control.
Okay, what else?
Adding water,
like hydration?
Yes.
Hydration reactions are interesting with our kinds because of what forms initially and what it turns into.
Let's talk oxymercuration first.
Like with alkenes, but on a triple bond, using mercury.
Yep.
React the alkane with mercury, sulfate, water, and acid.
Just like with alkenes, it follows Markovnikov's rule.
Markovnikov.
The H goes to the carbon with more H's.
The OH goes to the more substituted carbon.
Right, but here, you initially form an alcohol group on a double bond.
This structure is called an enol.
Enol.
E -N -O -L.
Alkene plus alcohol.
Exactly.
And the key thing is, enols are usually unstable.
They don't like to stay that way.
Unstable?
What happens?
They rapidly rearrange into a more stable form, usually a ketone, through a process called tautomerization.
Tautomerization.
Sounds complex.
It's just an equilibrium where a proton and a double bond shift positions.
The enol tautomerizes to the ketone.
Think of them as isomers that interconvute really easily.
The ketone form is almost always way more stable.
So oxymercuration of an alkene gives you a key cone via an unstable enol intermediate.
Correct.
A Markovnikov ketone specifically.
Okay.
What about the other hydration method?
Hydroboration.
That's usually anti -Markovnikov.
Exactly right.
Hydroboration of alkynes also works similarly to alkenes.
You use borane, like BH3 or Ca2BH, followed by hydrogen peroxide and base.
And it gives the anti -Markovnikov product.
Yes.
So the initial enol that forms has the OH group on the least substituted carbon of the double bond.
The opposite regiochemistry.
Right.
And just like before, this anti -Markovnikov enol is also unstable, it tautomerizes.
But if it's anti -Markovnikov, does it still become a key tone?
Ah, good question.
No, because the OH was on the less substituted carbon, often the end carbon for a terminal alkyne.
When it tautomerizes, it becomes an aldehyde.
Aldehyde.
Okay.
So oxymercuration ketone Markovnikov.
Hydroboration aldehyde anti -Markovnikov via enol.
You've nailed it.
That distinction is super important.
And I guess this connects back to the terminal versus internal alkyne thing.
Absolutely.
This brings up a really important point for you, the listener, for practical use.
These two hydration reactions, oxymercuration and hydroboration, they are most useful, give you clean single products, mainly with terminal alkynes.
Why terminal?
Because with a terminal alkyne, there's a clear difference between the two carbons of the triple bond.
One is less substituted, the end one.
One is more substituted, the inner one.
So Markovnikov addition clearly gives a key tone and anti -Markovnikov clearly gives an aldehyde.
You get one major product.
Makes sense.
What happens with an internal alkyne?
With an internal alkyne, both carbons of the triple bond often have similar substitution.
So you add water.
It can add either way.
You usually end up getting a mixture of two different ketone products.
Ah, a mixture.
Messy.
Not ideal if you want one specific thing.
Exactly.
It's a potential pitfall in synthesis if you're not careful.
So for clean hydration, terminal alkynes are generally your best bet.
Wow, okay.
That covers a lot of ground.
We went from naming them, understanding that strict linear geometry.
And the ring strain issues.
Right.
Then how to make them that double elimination and the super useful acetylide coupling.
Especially for making those CC bonds.
For sure.
And then all these transformations.
Adding bromine twice, reducing all the way down, or stopping selectively at cis or trans alkenes.
Using lindlars or sodium ammonia.
And finally, hydration leading to ketones or aldehydes through those enol tautomerizations.
It's quite a journey.
It really is.
We've hit the fundamentals, the key reactions, mechanisms, and hopefully some useful tips for thinking about alkyne chemistry.
Definitely.
But stepping back for a second,
thinking beyond just these core reactions we discussed,
what does really understanding this unique reactivity, this triple bond chemistry actually imply?
Where are you going with this?
Well, think about creating new materials or designing complex pharmaceuticals.
How might knowing these specific ways to build and transform algens, understanding their rigidity, guide chemists in the future?
Towards making molecules with specific shapes or functions.
Exactly.
Could these principles lead to innovations we haven't even thought of yet?
Maybe new conductive polymers based on alkyne chains or drugs assembled using that precise acetylide coupling.
That's just something to think about, you know, where this fundamental knowledge might take us.
That's a great point.
Always good to connect the basics to the bigger picture and future possibilities.
Well, thank you for taking us through this deep dive today.
My pleasure.
And thank you for joining us.
Whether you're studying for an exam, trying to understand a reaction at work, or just feeding your curiosity, we hope this look at alkenes gave you some clarity and maybe a few aha moments for your shortcut to being well -informed.
Exactly.
Thanks for being part of our last minute lecture family.
We'll catch you on the next deep dive.