Chapter 20: Formation and Reactions of Enols and Enolates

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Welcome to the Deep Dive.

Today we're exploring a really interesting aspect of organic chemistry compounds that kind of have a split personality.

You probably know about carbonyl compounds, right, those C double bondo molecules, and how they often act as electrophiles.

They're electron seeking.

We've talked about that quite a bit.

We have.

But here's the puzzle.

These same compounds can sometimes act as nucleophiles, electron donors.

How does that work?

How can they be both?

That's exactly the paradox we're diving into.

And the answer, well, it lies in this key idea that carbonyl compounds aren't just stuck in one form.

They exist in this dynamic balance between the keto form, that's the electrophilic one you mentioned, and this less famous but really powerful enol form, which is the nucleophilic one.

And for this deep dive, we're drawing heavily from chapter 20 of Clayton, Greaves, and Warren's Organic Chemistry, second edition.

Our mission really is to unpack how these enols, and their even more reactive relatives, the enolates, are made, how they react, and why they're so crucial for actually building and changing carbon structures in synthesis.

And this isn't just theory, right?

You can actually see this happening.

Like, imagine you buy some dimedone that's five halogen, five dimethylcyclohexane, one villiol -3 -dion.

A common lab chemical.

Exactly.

You run an NMR spectrum to check its purity, and bam, it looks like a mixture.

My first thought would be, hey, this isn't pure.

What's really going on?

That's a great example because the dimedone is pure.

It's just showing you that equilibrium in action.

A pretty big chunk of it, maybe a third in certain solvents like CDCl3, exists as the enol.

A third.

That's a lot.

It is unusually high, yeah.

And the name enol tells you what it is.

Enol for the double bond, ol for the alcohol group, right on that double bond.

This switcheroo between the dione, the keto form, and the enol is just a proton hopping around inside the molecule.

It's called tautomerism.

Tautomerism, okay.

We see similar proton shifts in other places, like carboxylic acids or even some biological molecules like imidazole rings.

Okay, dimedone is a clear case, but why don't we see this for, say, plain old acetone or cyclohexanone?

If I run an NMR of acetone, I just see acetone, not this enol mixture.

Right, and that's because for most simple ketones and aldehydes, the balance point, the equilibrium,

strongly favors the keto form.

For acetone, it's tiny, like maybe one molecule in a million is the enol at any given moment.

The keto form is just a bit more stable thermodynamically speaking.

The bond energies work out slightly better.

The CO and CH bonds are just a touch stronger overall than the CCNOH and the enol.

Just slightly more stable, but it makes a huge difference in what you see.

It does in terms of concentration,

but, and this is super important, even if it's a tiny amount, that enolization is always happening.

It's a constant back and forth.

So if it's practically invisible for most compounds, how did chemists even figure this out?

How do we know it's happening if we can't see it?

Ah, well that's where a really neat experiment comes in deuterium exchange.

Imagine taking a ketone, say, one -fetal propane 1 ,1, and dissolving it in D2O.

That's heavy water, water made with deuterium instead of regular hydrogen.

Now you watch it with NMR over time, slowly, you'd see the signals for the protons next to the carbonyl group.

The alpha protons, they just fade away.

They disappear.

They disappear from the proton NMR, yeah.

And if you analyze the compound by mass spec, you find it's gotten heavier.

Those hydrogens have been swapped out for deuterium.

How does that happen?

It happens via the enol.

The ketone briefly flips to its enol form.

That enol now has an OD bond because it's in D2O.

When it flips back to the keto form, it doesn't just put the deuterium back on oxygen.

It can pick up a deuteron from the D2O solvent and put it onto the alpha carbon.

Ah, I see.

So the enol is the bridge for the swap.

Exactly.

The enol intermediate is the key.

This exchange proves that the keto -enol conversion is constantly happening even if the enol concentration is too low to detect directly.

It might be slow in just water, but it's happening.

Okay.

But slow isn't always great for chemists trying to make things efficiently.

How do you speed this process up?

You use catalysts.

Either acid or base works brilliantly.

How do they work?

Under acid catalysis, the first step is protonating the carbonyl oxygen.

That makes the alpha protons more acidic, easier to remove.

So then a base, like water, can pluck off an alpha proton to form the enol.

Okay.

Proton on, proton off.

Right.

And under base catalysis, it's the reverse order.

The base rips off the alpha proton first.

This creates a really important intermediate called the enolate ion.

The enolate.

Okay, I've heard of that.

Yeah, it's crucial.

Then that enolate can pick up a proton on its oxygen, maybe from water, to form the enol.

But the key is, in both cases, the acid or base isn't consumed.

It gets regenerated, so it's truly catalytic.

You mentioned that intermediate and the base route, the enolate ion.

What's so special about it?

Why is it important?

The enolate is, well, it's the real powerhouse here.

It's the conjugate base of the enol.

It's basically an anion, a negatively charged species.

Right.

And that charge isn't just sitting on one atom.

It's delocalized, spread out over the oxygen, the alpha carbon, and the carbonyl carbon.

Think of it like an allyl anion system, three atoms sharing electrons.

So the negative charge is shared.

Exactly.

We draw resonance structures, one with the charge mostly on oxygen, the oxyanion, and one with it on carbon, the carbanion.

Now, even though oxygen is more electronegative and holds most of the negative charge density, it's actually the carbon atom that has more of highest energy electrons, the HOMO.

And that often makes the carbon the more reactive site for forming new carbon bonds.

So charge on oxygen,

but reactivity often at carbon.

That's interesting.

It is.

We talk about the oxygen being a hard nucleophilic center, good for reacting with hard, highly charged electrophiles.

And the carbon is a soft center, better for reacting with softer electrophiles, where orbital interactions are more important.

Hard and soft,

like Pearson's HSAB principle.

Exactly like that.

This hard -soft distinction is key to controlling where the enolate reacts.

You can react it at oxygen, for example, with acyl chlorides to get enol esters.

Or you react it at carbon without the halides to make new CC bonds, which is often the main goal.

This is really versatile.

So we focused on ketones and aldehydes.

What other types of compounds can form enols or enolates?

Is it pretty widespread?

It's fairly broad.

Yes, ketones and aldehydes are the classic examples.

And if a ketone isn't symmetrical, it can potentially form two different enols or enolates, depending on which alpha proton is removed.

We call those regioasomers.

But some carbonyls just can't enolize.

Things like benzophenone or formaldehyde, they simply don't have any alpha hydrogens to remove.

No alpha hydrogens, no enolization.

Makes sense.

Then you have carboxylic acid derivatives.

Esters form enolates pretty readily, especially if you use the right base, like the corresponding alkoxide, to avoid messing up the ester group itself.

Acyl chlorides.

They can enolize, but the intermediate enolate is unstable and tends to lose chloride to form something called a ketene.

Very reactive species.

Ketene's right.

Unstable.

Carboxylic acids and imotomides, though, generally don't enolize at the alpha carbon under basic conditions because they have much more acidic protons elsewhere.

The OH or NH protons get pulled off first.

Priority goes to the most acidic proton.

Always.

And the concept even extends beyond oxygen.

You have nitrogen analogs.

Imomides can tautomerize to enamines, kind of like ketoenol for nitrogen.

Deprotonating an enamine gives in a NASA enolate.

Even nitrile canes and nitriles form similar enolate -type anions.

Wow.

Okay, so what's the general rule?

What does the molecule need to have?

The basic requirement is an electron withdrawing group, usually with a pi bond like CO or CN or NO,

attached to a saturated carbon, and it's P3 carbon, that has at least one hydrogen atom on it.

That hydrogen is the one that needs to be removed.

Got it.

Electron withdrawing group next to a CH bond.

So most enols we've talked about are fleeting, just transient intermediates.

But dimedone, that one hangs around.

What makes some enols so much more stable?

Right.

Dimedone breaks them all to bit.

There are really two flavors of stable enols.

First, you can have kinetically stable enols.

Kinetically stable, meaning slow to react.

Exactly.

If you're incredibly careful and exclude all traces of acid or base catalysts, even simple enols like vinyl alcohol, the enol of acetaldehyde can actually be isolated and studied, maybe at very low temperatures, or if they're sterically hindered, so the proton transfer is difficult.

They're not thermodynamically preferred, but they stick around because the pathway back is slow.

Okay, so that's one way.

What's the other?

The more common scenario is thermodynamic stability.

This is where the enol form is genuinely lower in energy, or almost as low as the keto form.

And the prime examples are those 1, 2, or 3 dicarbonyl compounds.

Like dimedone and acetylacetone.

Precisely.

In these, the enol form benefits hugely from conjugation.

The Ceto double bond is conjugated with the remaining CO group spreading out the electrons.

And often, like in acetylacetone, there's a strong intramolecular hydrogen bond forming a stable 6 -membered ring.

That makes the enol incredibly favorable.

Acetylacetone is almost 100 % an enol.

That hydrogen bond really helps lock it in.

It really does.

And you see these stable enols in real -world molecules, too.

The anti -inflammatory drug peroxacam has one.

A natural herbicide called leptospormone is basically a collection of stable enol tautomers.

And don't forget, vitamin C, ascorbic acid, it's a vital nutrient, and it's structurally an endial 2 -enol groups, which makes it acidic because the resulting anion is highly stabilized by resonance.

Vitamin C is an enol.

And maybe the ultimate stable enols are phenols aromatic alcohols.

A phenol is technically the enol form of acyclohexidinone, but the keto form would destroy the aromaticity of the benzene ring, which is a huge energy penalty.

Right, aromaticity is a big deal.

A very big deal.

So the equilibrium lies completely, totally on the side of the phenol, the enol form.

It's fascinating how these subtle shifts have big consequences.

Beyond just making stable forms, how else does this enolization equilibrium affect molecules?

You mentioned things like structure, chirality.

Absolutely.

It has some really key consequences.

One is conjugation.

If you have a carbonyl compound with a double bond that isn't conjugated, meaning it's not right next to the CO, just add a trace of acid or base,

it will, I summarize, it'll move that double bond into conjugation because the conjugated system is more stable, lower energy.

The enol or enolate provides the pathway for that double bond migration.

We often label carbons alpha, beta, gamma away from the carbonyl, and this leads to stable alpha, beta unsaturated carbonyls.

So it finds the most stable arrangement.

Yeah.

What about chirality?

You said it affects 3D structure.

This is critical, especially in synthesis and biology.

If you have a chiral center, a carbon, with four different groups making the molecule handed, and that chiral center is the alpha carbon right next to the carbonyl, well, it's in danger.

Danger.

Why?

Because when it analyzes that alpha carbon becomes part of the CfLb -SeqC double bond, double bonds are flat, planar.

So the intermediate enol or enolate is acryl at that position, it's lost its 3D information.

Okay, could go flat.

Then when it tonomerizes back to the keto form, the proton can come back in from either face of that flat double bond.

So you can get back the original enantiomer, or you can get its mirror image.

Ah, so you end up with both.

You end up with a mixture, often a 50 .50 mixture, which means you've lost all the optical activity.

That's called racemization.

It's a huge issue if you're trying to make or preserve a single enantiomer.

If your only chiral center is alpha to a carbonyl, it's generally going to racemize easily under acidic or basic conditions.

That's a major drawback.

Are there examples?

Sure.

Beta -keto esters, where the chiral center is sandwiched between two carbonyls, are easily racemized.

Even alpha amino acids, the building blocks of life.

Normally they're stable.

In base, the acid group is a carboxylate anion.

In acid, the amino group is protonated.

These charged forms resist enolization.

Okay, so they're usually safe.

Usually.

But if you, say, make an N -acetyl derivative, you neutralize that amino group then you can racemize them more easily with base.

This is actually used sometimes if you make a specific amino acid enantiomer and have the unwanted one left over.

You can racemize the unwanted one and potentially recycle it.

That's clever.

Using the problem to your advantage.

And here's a really common example, ibuprofen, the painkiller.

Only one enantiomer, the S -form, is biologically active, but it's often sold as a racemate, a 50 .5U mix of S and R.

Why sell the inactive half?

Because your body contains enzymes that actually catalyze the enolization of the inactive ibuprofen.

It gets racemized in vivo, effectively converting some of the inactive form into the active S -form.

The body does the racemization.

That's incredible.

It is.

It's using this fundamental organic chemistry process.

Amazing.

Okay, so we understand the equilibrium stability consequences.

How do chemists actually use enols and enolates to make specific changes, to add new groups?

Right, let's get into reactions.

A classic one is alpha -halogenation, adding bromine or chlorine to that alpha carbon.

If you do this under acidic conditions, it's usually quite controlled.

The ketone forms its enol, and the electron -rich double bond of the enol attacks the electrophilic halogen, like Br2.

A proton is lost, and you get the alpha -halo ketone.

Too straightforward.

And interestingly, under acid, the reaction often prefers the more substituted alpha carbon, if there's a choice.

This relates to the stability of the intermediate steps.

More substituted side for acid.

Got it.

What about base?

Base -promoted halogenation is messier.

You form the enolate first, which attacks the bromine.

That works.

But here's the catch.

Adding that first halogen makes the remaining alpha protons on that same carbon even more acidic.

Oh, so it gets easier to remove the next proton?

Much easier.

So the reaction doesn't stop.

You get dehalogenation, trihalogenation very quickly.

And if you have a methyl ketone, a CH3 group next to the CO, you can get complete halogenation of the methyl group, followed by cleavage of the carbon -carbon bond.

Cleavage.

The molecule breaks apart.

Yep.

This is the basis of the famous iotaform test for methyl ketones.

You use iodine in base, form triodomethane, iotaform, which is a yellow solid, and the carboxylate of the rest of the molecule.

So breast conditions lead to overreaction and potential cleavage.

So acid for a controlled monohalogenation base for something else, or if you want cleavage?

Generally, yes.

Acid is preferred for putting just one halogen on, typically at the more substituted position.

Base leads to polyhalogenation, and with methyl ketones, the haliform reaction.

And interestingly, the initial enolate formation under base often happens faster at the less substituted carbon, though the subsequent reactions complicate things.

Okay.

Halogenation.

What else?

Another useful one is nitrosation.

Here, you react the enol with an electrophilic nitrogen species, NO plus azo, which you can generate from noitrous acid.

NO plus O.

Okay.

The enol attacks the NO plus tear, adding a nitroso group in XNO at the alpha carbon.

This initial product is usually unstable and tautomerizes to a more stable oxime.

Oxime.

C -N -O -H.

Right.

And that oxime can then be hydrolyzed, broken down with water and acid, to reveal a second carbonyl group.

So the net result is you've introduced another CO group alpha to the original one.

Like halogenation, this also tends to happen at the more substituted alpha carbon in unsymmetrical ketones.

So another way to functionalize that alpha position, it seems the core principle is always the enol or enolate reacting with an electrophilic carbon.

That's the dominant pathway.

Absolutely.

Reactivity at the alpha carbon is the name of the game.

Now, these free enolates formed with simple bases, they sound very reactive, may be hard to control precisely, especially if you have multiple acidic sites.

How do chemists get finer control?

That's a key challenge in synthesis.

You need ways to generate the enolate you want, cleanly and completely, and have it wait patiently for you to add the electrophile.

This leads to the idea of stable enolate equivalents.

Stable equivalents.

Yeah, ways to handle that reactivity.

One major strategy is using lithium enolates.

Instead of a simple base like hydroxide, you use a very strong, very bulky non -nucleophilic base.

The classic is LDA lithium disypropylamide.

LDA.

Heard of that one too.

It's super strong, so it rips off the alpha proton quantitatively, basically converts all your ketone to the enolate.

It's bulky, so it doesn't tend to add to the carbonyl itself, which would be a side reaction, and you usually do this at very low temperature, like MADISC 78 degrees C in an inert solvent.

MADISC 78.

Really cold.

Yeah.

Keeps things controlled.

This gives you a stable solution of the lithium enolate, ready to react cleanly when you add your desired electrophile, and because LDA is so bulky, it often selectively removes the proton from the less sterically hindered alpha carbon, if there's a choice.

Ah, so LDA gives you control over which enolate you form too.

Often, yes.

It's a cornerstone of modern synthesis.

Another approach is using silyl enol ethers.

Silyl enol ethers.

What are they?

These are even more stable.

You can often purify them, put them in a bottle.

They're formed by reacting an enolate, maybe a lithium enolate, formed in situ, with a silicon electrophile, like trimethylsil chloride, TMSCl.

Silicon.

Okay.

Now, silicon loves oxygen.

It forms very strong SiO bonds.

So, unlike alkylation, which prefers carbon, silylaration happens almost exclusively at the enolate's oxygen atom.

You trap the enolate as its O -sily derivative.

So you react to oxygen to make these.

Yes.

Due to silicon's hard nature and the strong bond it forms,

these sily enol ethers are less reactive than lithium enolates, but are still nucleophilic at the alpha carbon.

And you can convert them back to lithium enolates, if needed, or react them directly.

Okay.

So we have lithium enolates and sily enol ethers as controlled versions.

What about just plain carbon -based enol ethers, like an O -methyl group on the enol double bond?

You mentioned sily goes on oxygen.

Is it hard to get carbon groups onto the oxidative?

It is generally harder, yeah.

Direct O -alkylation of enolates often competes unfavorably with the preferred SiO, unless you use very specific hard alkylating agents and conditions.

So, not the usual way.

Not usually.

A more reliable method to make simple enol ethers is actually indirect.

You can start with an acetyl -protected form of a ketone or aldehyde and treat it with acid without any water around.

It eliminates alcohol via an E1 -like mechanism to give the enol ether.

Clever workaround.

And how do these enol ethers react, sily enol ethers, specifically?

Well, firstly, they're much more easily hydrolyzed than regular ethers.

Just adding aqueous acid rapidly converts them back to the starting carbonyl compound.

The mechanism involves protonation on the alpha carbon, creating a very reactive intermediate that water attacks.

Protonation on carbon.

Interesting.

Yeah, it reflects where the electron density is highest.

But more usefully, they react with electrophiles at that alpha carbon, similar to enols.

You can react sily enol ethers with halogens like Br2 or Cl2 or sulfur electrophiles like PhSCl.

So, alpha halogenation again.

Yes, but often much cleaner than base promoted halogenation.

You typically get monohalogenation without the overreaction issues.

And crucially, if you made your sily enol ether selectively from the less substituted side using LDA, then the halogen goes on the less substituted side.

Exactly.

Which is the opposite outcome to catalyzed halogenation.

So, having access to both lithium enol and sily enol ethers gives chemists fantastic control over where reactions happen.

What an incredible deep dive.

Yeah.

It's amazing how this keto enol balancing act, which seems so subtle for simple ketones, becomes the foundation for so much synthetic chemistry.

It really is central.

Understanding how enols and enolates form, their stability, how they react to carbon versus oxygen, how we can control them with LDA, or by making sily enol ethers, it allows chemists to behead complex molecules with real provision, add groups, and even control stereochemistry.

Absolutely.

Mastering enol and enolate chemistry, as laid out so well in Clayton Greaves and Warren, isn't just about memorizing mechanisms, it's really about understanding this fundamental reactivity principle and how to harness it.

From making natural products to designing drugs like peroxacam or understanding how ibuprofen works in the body, controlling that alpha -carbon chemistry is key.

It really makes you wonder, doesn't it, what other subtle dynamic equilibria are out there quietly shaping the chemical world, just waiting for us to uncover their full impact?

A fantastic question to end on.

Thank you for joining us on this deep dive into the world of enols and enolates.

Keep exploring the amazing transformations chemistry makes possible.

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