Chapter 21: Alpha Carbon Chemistry: Enols and Enolates

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Have you ever paused to wonder how your body performs that incredible feat of converting

the food you just ate into the raw energy powering your muscles?

It's a fundamental biological process, right?

And the chemistry behind it, well, it dives deep into a really fascinating corner of organic chemistry.

Yeah, and it's not just theory, it's the foundation for designing new drugs, building complex molecules.

And as we'll see, it's crucial for life itself.

Welcome to the Deep Dive.

We take complex information, distill it down, and hopefully give you a shortcut to being truly well -informed.

Today, we're cracking open a really key chapter,

Alpha Carbon Chemistry, Enols and Enoliths from David Klein's Organic Chemistry.

Our mission today is clear.

Explain the key concepts, the mechanisms behind these powerful reactions, and the sort of problem -solving techniques that make this chemistry so versatile and useful.

We're going to explore a whole suite of reactions that let chemists form carbon bonds, basically building molecules piece by piece.

Exactly.

It's like a toolkit for synthetic chemists offering incredible precision.

So, get ready to expand your understanding of chemical synthesis.

We'll touch on practical applications and some surprising connections.

Let's unpack this.

All right.

So our journey begins with the star of the show, the alpha carbon.

Okay.

In organic chemistry, when you have a carbonyl group that's C double bond O, we use Greek letters, right, to map out the neighbors.

Alpha, beta, gamma.

Exactly.

The carbon atom or atoms sitting right next to that carbonyl, those are the alpha carbons.

The next ones that are beta, then gamma, and so on.

So what's the big deal about the hydrogens attached to these alpha carbons?

Why are they the key players here?

Because those alpha protons, as we call them, are surprisingly acidic, much more than typical CH bonds.

Ah, okay.

So they can be removed.

Precisely.

They can be plucked off.

And that allows the alpha carbon to transform into this highly reactive hotspot.

And this reactivity shows up mainly in two forms,

enols and enolates.

Let's start with enols, then.

What exactly is an enol?

An enol is what we call a tautomer of a ketone or an aldehyde.

Tautomer.

Think of tautomers as constitutional isomers that rapidly interconvert.

Same atoms, just arranged differently, specifically.

The position of a proton in a double bond shifts.

And crucially, they are not resonance structures, right, because atoms actually move.

Exactly right.

Atoms move, not just electrons.

Now, typically, the ketone or aldehyde form is overwhelmingly favored in this equilibrium.

Like massively favored.

Oh, yeah.

For cyclohexanone, for instance, it's over 99 .99 % ketone.

So you might think enols are just these fleeting ghosts, you know, too unstable to be really useful.

There's always a but in chemistry.

There often is.

And here's where it gets really interesting.

Some enols are stable enough to hang around in significant amounts.

Why?

What makes them stable?

Well, take beta diketones, like 2 -phi -phi -4 -pen -and -anion.

The enol form can be anywhere from 70 % to 90 % of the mixture.

Wow, that's a huge difference.

It is.

And this stability comes down to two main things.

First, a conjugated pi system.

The double bonds line up, letting electrons spread out, which lowers the energy.

Think of it like a superhighway for charge.

Makes it more stable, happier.

Exactly.

And second, intramolecular hydrogen bonding.

The enols OH group forms a hydrogen bond with a nearby carbonyl oxygen.

It's like the molecule is hugging itself into this

stable Six -membered ring neat and you mentioned phenol earlier as an extreme example, right phenol is technically an enol But it's part of an aromatic ring that aromaticity makes it incredibly stable.

So the ketone form is basically non -existent So whether formed an acid or base the alpha position of an enol is electron -rich making it nucleophilic Ready to attack stuff.

That's the key takeaway.

It's a nucleophile, but while enols are reactive Enolates are the true powerhouses here.

Okay, enolates How do they form if you treat a ketone or aldehyde with a strong base you completely remove that acidic alpha proton What you're left with is a negatively charged species the enolate and it's resonance stabilized resonance stabilized Meaning the negative charge isn't stuck on the carbon.

Correct.

It's shared between the alpha carbon and the oxygen atom This stabilization is key and you called them ambident nucleophiles meaning they can attack from two places Yep, a bit in two teeth.

The oxygen can act as a nucleophile or the alpha carbon can For most of the reactions we care about today It's the carbon attack that forms new carbon carbon bonds, which is often what we want in synthesis so compared to enols enol it's are just

Stronger much more reactive.

Yeah, because they have that full negative charge And unlike enols, which are usually just transient intermediates Enol it's can actually be prepared and you know hang around long enough to be used deliberately in reactions They're incredibly useful building blocks.

Okay, this brings up a crucial point

Choosing the right base you set a strong base for enol.

It's why does it matter so much?

It matters a lot Aldehydes and ketones their alpha protons typically have pK values around 16 to 20.

That's roughly similar to alcohols So if you use a milder base like an alkoxide ion say so do you meet oxide you only set up an equilibrium You know some starting ketones some base some antelit some alcohol.

It's a mix and that's bad It can be very problematic Because the intellect you just formed is a nucleophile and the starting ketones still floating around is an electrophile They can react with each other in unwanted ways like self L reactions.

We'll see later messy stuff Oh, I see.

So you need something stronger to avoid that equilibrium mess.

Exactly That's where strong non nucleophilic bases come in think sodium hydride ANH or very commonly lithium disapropyl amide LDA LDA hear that one a lot you do these bases react Irreversibly and completely convert the ketone or aldehyde into its antelit Now H generates hydrogen gas which bubbles away driving the reaction forward with LDA The deep rodent is just complete only the antelit is present clean no starting material left to cause trouble precisely It avoids those equilibrium issues and side reactions.

And what about those beta dike tones?

You mentioned earlier the ones with stable, you know, yes They're a special case their alpha protons are way more acidic with pK values around 9 That's about a million times more acidic than a regular ketone Wow Why so acidic because the antelite they form is doubly stabilized That negative charge gets spread over two oxygen atoms and the carbon in between through resonance.

So it's super stable Extremely stable which means you don't even need a super strong base like LDA Even mild bases like hydroxide or alkoxides can completely deprotonate them to form the antelite quantitatively So understand the pico and choosing the right base

fundamental stuff Absolutely critical for controlling these reactions.

Okay, let's move on to actually using these enols and antelites How about alpha halogenation putting a halogen on that alpha spot, right under acidic conditions?

Ketones and aldehydes react with halogens chlorine bromine iodine at the alpha carbon What's the mechanism look like it actually goes via the enol first the carbonyl oxygen gets protonated by the acid Then a base like water removes an alpha proton to form the enol intermediate the enol we talked about earlier That's the one and this enol formation is usually the slow rate determining step That's why the reaction rate doesn't depend on how much halogen you add which is kind of interesting, huh?

So the enol forms slowly then reacts fast Exactly the enol being nucleophilic at the alpha carbon then attacks the halogen molecule like br2

Finally another base removes a proton to give you the alpha halogenated product and you mentioned something about auto catalysis Yeah A neat little detail if you use bromine the reaction produces HBR as a byproduct that HBR is an acid So it actually catalyzes the reaction further speeding it up as it goes along clever What happens if the ketone isn't symmetrical if it has two different alpha carbons good question, that's a common challenge

Halogenation tends to happen mainly at the more substituted alpha carbon Why there because that position forms the more stable enol intermediate remember more substituted double bonds are generally more stable So you often get a mixture but favoring the more substituted side, okay, and these alpha halo ketones can be used for other things Yeah, one common follow -up is to treat them with a base This can cause an elimination reaction kicking out HX and forming a double bond between the alpha and beta carbons That gives you an apogo unsaturated carbonyl though.

Sometimes yields aren't great right now What about perboxylic acids you said they don't analize easily they don't so standard acid catalyzed halogenation doesn't work Well for them, we have a specific named reaction the Helvohort -Zelinski reaction or

HVZ this uses bromine br2 and a catalytic amount of phosphorus tribromide PBR3 followed by water.

How does that work around the enol ization problem?

The trick is PBR3 first converts the carboxylic acid into an acid bromide

Acid halides do enol ize much more readily than carboxylic acids Ah, okay So it forms the acid bromide that enol izes and gets Halogenated at the alpha position exactly and then the final water step hydrolyzes the acid bromide back to the alpha bromo carboxylic acid It's a neat workaround very clever.

Okay, so that's acid conditions What about alpha halogenation and base basic conditions lead to something quite different, especially if you have multiple alpha protons This takes us to the haliform reaction Haliform like chloroform precisely so under basic conditions a base pulls off an alpha proton to make the enolate Which then attacks the halogen simple enough it but

If you have more than one alpha proton on that carbon It's almost impossible to stop the reaction after adding just one halogen Why not because once you add that first electron withdrawing halogen it makes the remaining alpha protons on that same carbon Even more acidic.

Oh, I see so they get ripped off even faster by the base exactly So the second halogenation is faster than the first and the third is faster than the second if possible You get poly halogenation usually all alpha protons on one side get replaced Okay So how does that lead to the haliform reaction the haliform reaction specifically applies to methyl ketones?

Compounds with a ch3 group right next to the carbonyl like acetone acetone is the simplest example Yes If you treat a methyl ketone with excess base and excess halogen like iodine or bromine All three protons on that methyl group get replaced by halogens forming a cx3 group.

Okay, try halogenated

Then what then the hydroxide base attacks the carbonyl carbon normally this would just be an equilibrium But here the cx3 group is actually a decent leaving group.

Wait a carbon anion leaving isn't that usually a big no -no It usually is carbon anions are typically terrible leaving groups But this cx3 anion is an exception those three electron withdrawing halogens Stabilize the negative charge on the carbon enough to make it leave Wow Okay, so the cx3 group leaves and you form a carboxylic acid or rather its carboxylate salt under the basic conditions The cx3 anion then gets protonated to form chx3 the halo form chloroform chcl3 Bromiform chbr3 or iota form ch i3 iota form is a yellow solid So it's sometimes used as a test for methyl ketones So the overall result is chopping off the methyl group and turning the methyl ketone into a carboxylic acid Exactly, it's a synthetically useful way to do that transformation specifically fascinating stuff All right Let's switch gears to one of the most fundamental carbon carbon bond forming reactions all all reactions Absolutely central an aldol addition you treat an aldehyde or sometimes a ketone with a catalytic amount of base Often hydroxide a milder base this time generally yes compared to LDA the base Generates a small amount of the enolate that enolate then acts as a nucleophile and attacks the carbonyl carbon of another molecule of the Starting aldehyde or ketone ah so it reacts with itself it does The result after protonation is a molecule that contains both an aldehyde or ketone Functional group and an alcohol functional group on the beta carbon relative to the carbonyl beta hydroxy carbonyl Which we call an aldol a portmanteau of aldehyde and alcohol makes sense.

What's the mechanism again?

It's usually three equilibrium steps one base removes an alpha proton to form the enolate to The enolate attacks the carbonyl of another aldehyde molecule Yeah, three the resulting alkoxide intermediate gets protonated usually by water or the conjugate acid of the base To give the final aldol product and you said equilibrium does it favor the product for simple aldehydes like ascaldehyde?

The equilibrium often does favor the aldol product But for many ketones the equilibrium actually lies towards the starting materials Sterics often play a role ketones are bulkier So aldol additions aren't always high yielding for t -tones often not the reverse reaction called the retro aldol reaction Can be significant especially for ketones retro aldol just the reverse process exactly base removes the hydroxyl proton the cc bond breaks reforming the carbonyl and kicking out an enolate ion as the leaving group another Carbon anion leaving group.

Yeah, and again It's allowed because the enolate is resonance stabilized making it stable enough to depart and this isn't just lab chemistry, right?

You mentioned muscles earlier.

Absolutely.

The retro aldol reaction is biologically crucial It's a key step in glycolysis the pathway your body uses to break down glucose for energy How so an enzyme called aldolase?

catalyzes a retro aldol reaction on fructose 1 carose 6 bisphosphate A 6 carbon sugar into two three carbon pieces.

This is fundamental to generating ATP for muscle contractions So that burst of energy for a sprint Partly powered by retro aldol chemistry in a very real sense.

Yes, it's happening in your cells right now.

Cool Okay back to the lab.

What happens after the aldol addition?

Is that the end of the story often not if you heat the aldol addition product, especially under acidic or basic conditions It can undergo dehydration lose a molecule of water Dehydration forming a double bond exactly water is eliminated from the alpha and beta positions forming a double bond conjugated with the carbonyl group The product is an unsaturated aldehyde or ketone and this whole two -step process addition Then dehydration is called that's the aldol condensation condensation because water is lost What drives the dehydration part?

The main driving force is the formation of that stable conjugated pi system that extended electron highway We talked about it provides significant stabilization So even if the initial aldol addition equilibrium isn't great the condensation can pull it forward often Yes, the dehydration can be favorable enough to give good yield to the condensation product even if the aldol intermediate itself is hard to isolate and Cicerically the trans or e isomer of double bond is usually favored Okay Now what if you try to react two different aldehydes or ketones both having alpha protons?

That's the challenge of crossed aldol reactions if both components can form an enolate and both can be attacked You can potentially get four different aldol products a messy mixture not synthetically useful.

Then how do you control it?

There are a couple of main strategies One is to choose one reactant that cannot form an enolate because it has no alpha protons like formaldehyde or benzaldehyde

exactly formaldehyde HCHO or benzaldehyde PHCHO are common choices.

They can only act as the electrophile the one being attacked Yeah So if you react one of these with a ketone or aldehyde that can form an enolate you generally get only one major Crossed product makes sense only one component makes the nucleophile right and particularly with benzaldehyde the resulting aldol often dehydrate Spontaneously to the condensation product because the conjugation extends into the benzene ring making it very stable What's the other strategy for control?

The other way is a directed aldol addition using LDA.

LDA again?

Yep, you take one carbonyl compound treated with LDA irreversibly to form its enolate first get all of it converted Then you slowly add the second carbonyl compound which acts as the electrophile This way you control exactly which enolate forms and what it attacks much cleaner more control Definitely it's a very powerful technique.

Can these reactions happen within the same molecule?

They certainly can if you have a molecule with two carbonyl groups spaced appropriately you can get an intermolecular Aldol reaction forming a ring forming a ring for example a one seroide four decarbonyl can form a five -membered ring and A 1005 dicarbonyl can form a six -membered ring and are those ring sizes preferred very much So just like in other cyclization reactions forming stable five and six -membered rings is strongly favored over smaller or larger rings Due to wing strain considerations.

Okay that covers aldols pretty well.

Now, what about the Claisen condensation?

You said it's the ester version That's a good way to think about it esters can also react to via their enolates in a condensation reaction Analogous to the aldol condensation.

What kind of product do you get?

The product is a beta keto ester a Molecule with a ketone group at the beta position relative to an ester group and what base do you use for this?

Can you use hydroxide like in some aldols?

No, you cannot use hydroxide here Hydroxide would simply hydrolyze the ester back to a carboxylic acid saponification Ah, right.

So what base then you need to use an alkoxide base and critically it must match the alkoxy group of the ester match So if you have an ethyl ester, you must use sodium ethoxide No way if you use say sodium methoxide Naomi you get trans ester vacation Swapping the ethyl group for a methyl group competing with the condensation using the matching alkoxide avoids this complication Okay, matching alkoxide base.

How does the mechanism work?

It's similar to the aldol the alkoxide base removes an alpha proton from one ester molecule to form the ester enolate This enolate then attacks the carbonyl carbon of a second ester molecule forming a tetrahedral intermediate Right and then unlike the aldol where an alkoxide intermediate gets protonated here The tetrahedral intermediate collapses kicking out an alkoxide ion as a leaving group to reform a carbonyl This gives you the beta keto ester product, but there's a catch right?

You mentioned something about driving forces for Claisen Yes a crucial point the overall equilibrium for the steps I just described isn't actually very favorable What really drives the Claisen condensation to completion is the final step?

The beta keto ester product has alpha protons between the two carbonyl groups These protons are quite acidic PK around 11 because the resulting enolate is doubly stabilized like the beta diketones earlier Exactly, so the alkoxide base used in the reaction irreversibly deprotonates the beta keto ester product as it's formed This final favorable deprotonation step pulls the whole equilibrium over to the product side It acts like a thermodynamic sink.

So you actually form the enolate of the product Yes Which means you need to do a final acid workup step adding dilute acid to protonate that enolate and get your neutral beta keto ester product Got it that final deprotonation is key and can you do crossed Claisen reactions?

You can and they face the same selectivity challenges as cross algals If you mix two different esters both with alpha protons, you risk getting a mixture of four products So similar solutions use one ester without alpha protons.

Yeah, that's one strategy esters like ethyl formate or ethyl benzoate lack alpha protons and can only act as the electrophile or Again, you can use a directed approach with LDA to form one ester enolate selectively first then add the second ester and Intramolecular Claisen does that work too?

It does it's called the Dykman cyclization or Dykman condensation If you have a diester it can undergo an intramolecular Claisen to form a cyclic beta keto ester and I bet it prefers five and six membered rings you guessed it same principle applies Formation of those stable ring sizes is strongly favored.

Okay, so aldols and Claisen's build complexity What about just adding a simple alkyl group to the alpha position?

That's alpha alkylation a very important transformation It's typically a two -step process step one step one form the enolate

Quantitatively using a strong non nucleophilic base like LDA We want complete conversion to avoid side reactions.

Okay, make the enolate cleanly step two step two add an alkyl halate RX the enolate being nucleophilic at the alpha carbon attacks the alkyl halate in an SN2 reaction Displacing the halite and forming a new carbon carbon bond Attaching the R group to the alpha position SN2 that means there are restrictions on the alkyl halate, right?

Absolute critical point the SN2 reaction works best with methyl halides and primary alkyl halides Secondary halides react slower and often give competing elimination reactions tertiary halides basically only give elimination Why elimination?

Because the enolate is also a strong base with hindered secondary or tertiary halides instead of attacking the carbon substitution The enolate is more likely to just pull off a proton from the alkyl halide leading to an alky E2 elimination So stick to methyl and primary halides for alpha alkylation via enolates generally Yes for good yields and using LDA is beneficial here Not just for clean enolate formation But also because LDA itself is too bulky to easily attack the alkyl halide further preventing side reactions now What about those unsymmetrical ketones again where you could form two different enolates?

How does that play out in alkylation?

That's where we get into the fascinating concept of kinetic versus thermodynamic control of enolate formation kinetic versus thermodynamic Explain, okay an unsymmetrical ketone has two different sets of alpha protons

Removing a proton from the less hindered side is usually faster.

That's the kinetic pathway leading to the kinetic enolate

Removing a proton from the more substituted side is slower but leads to a more stable enolate more substituted double bond character That's the thermodynamic enolate Less hindered faster kinetic more substituted more stable thermodynamic.

Got it.

Can we choose which one we make?

Yes, we can to favor the kinetic enolate less substituted You use a strong bulky base like LDA at low temperature typically amount of 78 degrees C Why LDA in low temp?

LDA is bulky so preferentially grabs the more accessible less hindered proton The low temperature prevents the kinetic enolate once formed from equilibrating to the more stable thermodynamic enolate It freezes the faster product.

Okay and to get the thermodynamic one to favor the thermodynamic enolate more substituted more stable You use conditions that allow equilibrium to be established that means a strong but possibly less hindered base like sodium hydride and ah and Typically room temperature or even slight heating the reaction can then reach equilibrium favoring the most stable species So you can actually direct the alkyl group to either the more substituted or less substituted alpha carbon just by picking your base and temperature Exactly.

It gives chemists amazing control over where they build new CC bonds.

That's really powerful Now that there are also some classic named syntheses related to alkylation, right?

Like the Melonic ester synthesis Ah, yes the classics the Melonic ester synthesis is a reliable method for taking an alkyl halide Rx and converting it into a carboxylic acid with two extra carbons our ch2 Co H do extra carbons Oh, the key region is diethyl malonate.

It's a diester with a ch2 group in between a beta diester Those alpha protons are very acidic pKa to 13 because the enolate is doubly stabilized by both ester groups So easy to do perfect.

Yep, you treat diethyl malonate with a base like sodium methoxide Since it's an ethyl ester to form the enolate then you add your alkyl halide

Rx methyl or primary again SN2 rules the enolate attacks the alkyl halide putting the R group onto that central carbon Okay So now you have R attached to the Melonic ester then what then you hydrolyze both ester groups to carboxylic acid groups Using aqueous acid in heat This gives you a substitute of Melonic acid a molecule with two Co H groups attached to the same carbon One of which also bears the R group a one fervor a three Dicarboxylic acid exactly and one three dicarboxylic acids have a special property when you heat them They readily undergo decarboxylation one of the carboxyl groups breaks off is co2 gas leaving just one co OH group precisely Leaving you with our ch2 co OH So you started with Rx and you ended with the carboxylic acid that has two more carbons the ch2 and the co OH from the Malonate framework very neat and you could potentially add two different R groups.

You absolutely can you could do the deprotonation alkylation sequence twice Using different alkyl halides each time before the hydrolysis and decarboxylation steps very versatile cool And there's a similar one for making ketones.

Yes, the acetoacetic ester synthesis It's analogous, but it starts with ethyl acetoacetate, which is a beta keto ester also has acidic alpha protons between the carbons Right pK 11.

So same frequency deprotonate with sodium methoxide Alkalate with Rx methyl or primary this puts the R group on the alpha carbon between the ketone and the ester then hydrolyze and Decarboxylate exactly hydrolysis with acid converts the ester to a carboxylic acid giving you a beta keto acid and beta keto acids Just like one's got their three diacids readily decarboxylate upon heating using co2 losing co2 and leaving you with our ch2 co Ch3 a methyl ketone where you've installed the R ch2 group So the synthesis takes Rx and converts it into a methyl ketone with three extra carbons The ch2 the co and the ch3 from the acetoacetic ester framework Why use these elaborate methods instead of just trying to alkylate acetone directly good question Directly alkylating simple analytes like that from acetone can be messy You can get over alkylation adding more than one R group self alkyl reactions and poor control The malonic ester and acetoacetic ester synthesis provide much more reliable and controlled routes to these specific Types of carboxylic acids and methyl ketones they avoid the pitfalls of direct alkylation make sense Exactly, they use the enhanced acidity the beta carbonyl starting materials to ensure clean and a light formation and alkylation All right, let's shift focus slightly to conjugate addition reactions the beta attack, right?

This involves those ospa unsaturated aldehydes and ketones.

We formed earlier via aldol condensation We often call them unknowns or annals and they're special because they have to electrophilic sites precisely due to resonance Not only is the carbonyl carbon electrophilic like in any ketone aldehyde But the beta carbon is also electron deficient making it electrophilic too So nucleophiles have a choice attack the carbonyl carbon position 2 or the beta carbon position 4 exactly Attack of the carbonyl carbon is called 1 4 radiation attack The beta carbon is called 1 4 4 sorting addition or more commonly conjugate addition And what determines where the attack happens?

It often depends on the type of nucleophile very reactive hard nucleophiles like Grignard reagents arm GX or Organo lithiums are a lie tend to favor 1 4 2 addition direct attack at the carbonyl Okay And conjugate addition less reactive softer nucleophiles tend to favor 1 over 4 conjugate addition a prime example is lithium dial Kyle cuprates are to see lie also knows Gilman reagents They almost exclusively add to the beta position cuprates for 1 4 your 4 addition got it Where does the Michael reaction fit in the Michael reaction is a specific type of conjugate addition where the nucleophile is a relatively stable Weakly basic enolate typically one that's doubly stabilized like the enolates from melonic ester or acetoacetic ester exactly Those enolates along with enolates from beta dike tones beta keto nitriles nitrile canes.

These are called Michael donors They're soft nucleophiles and react reliably via 1 over a 4 addition with unsaturated carbonyl compounds Which we call Michael acceptors so Michael reaction doubly stabilized enolate adding one more 4 to an anonyl.

That's the core idea Yes It's a very powerful way to form CC bonds further down the chain from the carbonyl and you mentioned this connects to biology again Detoxification it absolutely does yeah Michael addition is a key reaction and biological detoxification pathways Your body contains a molecule called glutathione, which is a tripeptide with a very nucleophilic the old DSH group Okay Harmful electrophiles including some drug metabolites or environmental toxins that have Michael acceptor structures can react with glutathione via Michael addition So glutathione adds to the harmful molecule.

Yes tagging it for excretion and preventing it from reacting with crucial biomolecules like DNA or proteins It's a major defense mechanism and this relates to acetaminophen overdose It does a toxic metabolite of acetaminophen paracetamol is a Michael acceptor

Normally glutathione handles it but in an overdose glutathione supplies get depleted the toxic metabolite then reacts with liver proteins Causing severe damage.

That's why overdose is so dangerous.

It overwhelms this Michael addition detox pathway Wow Okay vital chemistry now What if you want to do a conjugate addition?

But your nucleophile is just a regular ketone enolate not one of those doubly stabilized Michael donors Good question regular ketone enolates are generally too reactive and too basic They tend to do one ready to addition or just act as bases rather than undergoing clean one mode for Michael addition So another workaround needed.

Yep, and the clever workaround here is the stork and amine synthesis and an amines What are those you form an enamine by reacting a ketone or aldehyde?

With the secondary amine R2NH under slightly acidic conditions for moving water and then amine has a C -fee double bond Adjacent to a nitrogen atom.

Okay CCNS.

How does that help and amines are electronically similar to enols and enolates?

They have resonance structures showing that the alpha carbon is nucleophilic But crucially enamines are less reactive than enolates because they are neutral molecules not onions They don't carry that strong negative charge.

Ah less reactive.

So they're softer nucleophile Exactly, this reduced reactivity makes them perfect candidates for acting as Michael donors in conjugate additions They add cleanly 1 ,4 to Michael acceptors where a simple enolate might fail.

So the stork synthesis Make the amine from your ketone react that with the Michael acceptor 1 ,4 ,4 addition And then what then you just add water and acid hydrolysis to convert the intermediate back into a carbonyl group So the net result is the conjugate addition of your ketones alpha position to the Michael acceptor Brilliant strategy very clever indeed Okay, one more big one the Robinson annulation sounds impressive.

It is impressive annulation means ring forming The Robinson annulation is a powerful sequence that combines two reactions We've already discussed to build a six -membered ring onto an existing molecule, which two reactions It's a Michael addition followed immediately by an intermolecular aldol condensation.

Wow putting it all together exactly Typically you react the ketone enolate often formed under conditions allowing Michael addition Maybe using an enamine with an aliga unsaturated ketone like methyl vinyl ketone MVK The Michael addition happens first connecting the two molecules creating a 1 3r5 Dicarbonyl compound precisely and that 1 3r5 Dicarbonyl compound is perfectly set up to undergo an intermolecular

Attila condensation base catalyzed often just using the same conditions to form a new six -membered ring containing an unlawful unsaturated ketone a Cyclohexanone ring.

Yep.

It's a cornerstone reaction for synthesizing polycyclic compounds, especially steroids and terpenes found in nature Really a workhorse reaction.

Okay, that's a lot of reactions.

How do we start putting this together strategically?

Let's talk synthesis strategies right when you're faced with synthesizing a target molecule Especially one with multiple functional groups You need to think retro synthetically work backwards look at the function groups and their relative positions The relationship between them tells you which reaction might have formed them often.

Yes Here's a key takeaway based on the relative positions and numbering pattern Okay if your target molecule has functional groups in a 1005 relationship like a 1005 diatone or something derived from that you should immediately think Michael addition or maybe stork and amin synthesis those reactions create that one cell five pattern One thousand five means Michael or stork got it What about one fill three if your target has functional groups in a one four to three relationship like a beta hydroxycarbonyl and a Boiton saturated carbonyl a beta keto ester or a 143 diatone Then you should be thinking all doll addition condensation or Claisen condensation 143 means all doll or Claisen How do you pick between all doll and Claisen if it's 143 look closely at the oxidation states of the two functional groups Although our reactions typically connect a carbonyl to what becomes or started as an alcohol Claisen reactions connect to carbonyl level functional groups like a ketone and an ester that Difference often points you to the right disconnection.

That's a really useful way to think backwards.

Look at the one three or one four five pattern It's a very powerful heuristic for planning syntheses involving these types of compounds It helps you choose the right tool from the toolkit We've just discussed and can we combine some of these ideas like Michael addition and alkylation?

Absolutely,

remember that the immediate product of a Michael addition after the one one a few attack is actually an enolate ion Ah, right before it gets protonated exactly So instead of just quenching that enolate with water or acid you can trap it in situ with an alkyl halide So you do a Michael addition to add a group at the beta position and then immediately Alkylate the resulting enolate at the alpha position precisely.

It's called tandem Michael addition alkylation It allows you to install one group at the beta carbon and a second group Potentially different at the alpha carbon all in one pot Incredible efficiency and control Wow, so putting it all together these alpha Corbin reactions They really give you fine control over building molecules immense control From simple alkylations to forming rings with L doll or Robinson to the strategic placement using kinetic thermodynamic Analytes or conjugate addition.

It's the foundation of much of modern organic synthesis

Understanding ain't alls and enolates unlocks the ability to build complex structures with precision So what does this all mean for you the listener?

It means mastering these fundamental reactions alpha carbon chemistry enol's analytes Halogenation aldol Claisen Michael stork Robinson gives you this incredible versatility You can build complex molecules strategically place multiple carbon chains and really understand how chemists construct them like your world And thank you for diving deep with us today We've journeyed through this dynamic world of alpha carbon chemistry uncovering how these really clever reactions Let chemists build intricate structures from small chains to complex rings We've hopefully clarified key concepts and problem -solving techniques And what's truly amazing as we saw is how these very same chemical principles are humming away inside your own body right now powering your muscles through glycolysis Detoxifying things with glutathione its fundamental chemistry at work We really hope this deep dive helps you unlock new understanding and appreciate the intricate dance of molecules all around us and within us Sparking maybe even more curiosity and thank you as always for being part of our last -minute lecture family