Enolate Chemistry · Section 54 of 64

Aldol reactions

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The aldol reaction joins two carbonyl molecules together at the alpha carbon of one and the carbonyl carbon of the other. It is the most important carbon–carbon bond-forming reaction in biology — glycolysis runs it backwards, and photosynthesis runs it forwards — and it is the reason the previous section's enolate chemistry matters.

The enolate as a carbon nucleophile

The carbon-centered contributor of an enolate gives the alpha carbon genuine nucleophilic character, even though most of the formal charge sits on oxygen. Given an electrophile, that alpha carbon behaves like any other carbon nucleophile in this course — an acetylide, a Grignard — and attacks.

What makes the aldol special is that the electrophile is another carbonyl molecule. The same compound is both nucleophile and electrophile, in different molecules, at the same time.

The aldol reaction, step by step

enolate (nucleophile)carbonyl (electrophile)O⁻CCalpha carbonOCcarbonyl carbonthe new C–C bondOOHCCCC1carbonylC2alphaC3betathe OH lands herebeta-hydroxycarbonyl
Trace the bond and the product names itself. The alpha carbon of the enolate attacks the carbonyl carbon of the other molecule, and that is the only new bond in the reaction.Count from the surviving carbonyl: it is C1, the alpha carbon that did the attacking is C2, and the new hydroxyl sits on C3 — the beta carbon. Hence beta-hydroxy carbonyl, and hence an alpha hydrogen sitting between two electron-withdrawing groups, which is exactly the setup the condensation step needs.

Step 1. Base removes an alpha hydrogen from one carbonyl molecule, forming its enolate. Note that the base is deliberately weak — hydroxide or an alkoxide — so that only a small fraction is deprotonated and plenty of un-ionized carbonyl remains available as the electrophile.

Step 2. The enolate's alpha carbon attacks the carbonyl carbon of a second molecule, in ordinary nucleophilic addition: pi electrons move to oxygen, giving a tetrahedral alkoxide.

Step 3. That alkoxide is protonated, by water or by the solvent, giving the product: a beta-hydroxy carbonyl, with a new OH exactly two carbons from the retained carbonyl.

Every step is reversible. The reverse — a retro-aldol — is a real reaction and a real strategy; the aldolase enzyme in glycolysis uses it to cleave a six-carbon sugar into two three-carbon fragments.

The name describes the product: ALD(ehyde) + (alcoh)OL, a molecule containing both. The reaction keeps the name with ketone starting materials too, even when no aldehyde is present anywhere.

Aldol condensation: heat pushes further

the enolate, as a carbon nucleophileO−CCRit attacks from HEREit adds to a second carbonylRHOCordinary nucleophilic addition —the Module 9 mechanism, unchangedthe ALDOL productROCCCOHRthe new C–C bonda β-hydroxy carbonyl: an OH three atomsalong from a C=Oheat−H₂Othe CONDENSATION productROCCCRan α,β-unsaturated carbonyl — the C=Cis now conjugated with the C=O
The aldol reaction, and the extra step heat buys you. Nothing in the middle is new: an enolate is a carbon nucleophile, and it does ordinary Module 9 addition to a second carbonyl. What is new is the product — a β-hydroxy carbonyl, with a new carbon–carbon bond in it. Warm it and it loses water, moving the new double bond into conjugation with the carbonyl it already had. That conjugation is why the elimination happens at all.Stop cold and you keep the β-hydroxy carbonyl. Heat it and the molecule eliminates water to put the new double bond into conjugation with the old carbonyl — which is more stable, and is what "condensation" means.

The beta-hydroxy carbonyl still has an alpha hydrogen, now sitting between the carbonyl and the new hydroxyl — a setup primed for elimination. Under heat, or with continued catalysis and time, an E1cb pathway removes that alpha proton and expels the beta-hydroxyl, giving an enone: a carbonyl conjugated with a C=C.

The driving force is conjugation. The enone's extended pi system is substantially more stable than the isolated aldol product, which is what pulls an otherwise unfavourable dehydration forward. The two-stage sequence — aldol addition, then dehydration — is the aldol condensation, and "condensation" here means a reaction that joins two molecules and expels a small one, in this case water.

Whether you stop at the beta-hydroxy carbonyl or continue to the enone is a matter of conditions: mild temperature and a short reaction time give the addition product, while heat gives the condensation product.

Worked example — acetaldehyde with itself

Step 1: hydroxide removes an alpha hydrogen from CH₃CHO, giving ⁻CH₂CHO.

Step 2: that carbon attacks the carbonyl of a second acetaldehyde. The tetrahedral alkoxide forms.

Step 3: protonation gives 3-hydroxybutanal — the aldol, four carbons from two molecules of two.

With heat: dehydration gives but-2-enal, the conjugated enone.

Notice that both new bonds came from the carbonyls themselves. Nothing was added except a catalytic base.

Crossed aldols and why they need control

Mix two different carbonyl compounds, each with alpha hydrogens, and in principle four products result: either compound's enolate can attack either compound's carbonyl. That is a statistical mess, and a crossed aldol run naively is not a useful reaction.

Two strategies give control.

Use one partner with no alpha hydrogens. Benzaldehyde, formaldehyde and benzophenone cannot form enolates, so they can only be the electrophile. A crossed aldol between benzaldehyde and a ketone gives one product cleanly, and because the resulting enone is conjugated with the ring it dehydrates readily. This special case is called a Claisen–Schmidt condensation.

Use LDA at low temperature. Convert one partner completely and irreversibly to its enolate at −78 °C, then add the second partner. Since the first compound is entirely deprotonated, it cannot act as an electrophile, and since the second is added afterwards, it never sees the base. This is the general solution and the modern one.

Match the base to the intent. A classic aldol needs a weak base, because the reaction requires both the enolate and the un-ionized carbonyl in the same flask. A crossed aldol needs a strong one, because the whole point is to deprotonate one partner completely before the other arrives. Using hydroxide for a crossed aldol gives a mixture; using LDA for a self-aldol gives nothing to attack.

Conjugate addition: the other place a nucleophile can attack

The enone produced by an aldol condensation is itself electrophilic — and at two positions. A nucleophile can attack the carbonyl carbon directly (1,2-addition) or the beta carbon (1,4- or conjugate addition), which is electrophilic because of the resonance contributor described back in Module 2.

Which one happens depends on the nucleophile. Hard, reactive nucleophiles — organolithiums, hydride — tend to attack the carbonyl directly. Softer, more stabilized ones — cuprates (R₂CuLi), enolates, amines, thiolates — add conjugately. A conjugate addition using an enolate as the nucleophile is the Michael reaction, and combining a Michael addition with a subsequent intramolecular aldol condensation is the Robinson annulation, the standard way to build a new six-membered ring.

What carries forward

The aldol pattern — make an enolate, attack a carbonyl, join two fragments — is the template for the Claisen condensation in the next section and for a great deal of biological chemistry. Retro-aldol cleavage is how sugars are broken down. And the 1,2 versus 1,4 choice is the last of this course's recurring competitions between two sites on one molecule.