Aldol Reactions (Addition/Condensation) - Organic Chemistry 2

SECTION 1

Historical Context & Motivation

The formation of carbon–carbon bonds has long been recognized as one of the most critical challenges in organic synthesis. In the mid-nineteenth century, chemists began to observe that certain aldehydes could spontaneously dimerize under basic conditions, producing a new class of compounds that contained both a hydroxyl group and a carbonyl moiety. These early observations laid the foundation for what would become one of the most versatile and widely employed reactions in organic chemistry—the aldol reaction. The name itself derives from the contraction of aldehyde and alcohol, reflecting the structural features of the initial product. Understanding this reaction opens the door to constructing complex molecular architectures from relatively simple starting materials, a principle that remains central to both academic research and industrial synthesis.

1838 • Liebig's Early Observations

Justus von Liebig observed that acetaldehyde undergoes self-condensation under basic conditions, though the reaction mechanism was not yet understood. This marked one of the first recorded instances of a carbon–carbon bond formation between carbonyl compounds.

1872 • Wurtz and Borodin Independently Characterize Aldol Products

Charles-Adolphe Wurtz and Alexander Borodin independently reported the base-catalyzed dimerization of acetaldehyde to form 3-hydroxybutanal (aldol). Wurtz coined the term 'aldol,' establishing the reaction's identity in the chemical literature.

1907 • Claisen–Schmidt Condensation

Ludwig Claisen and J. G. Schmidt developed the crossed aldol condensation between aromatic aldehydes and ketones, expanding the scope beyond self-condensation and demonstrating the reaction's utility in forming α,β-unsaturated carbonyl compounds.

1950s–1970s • Kinetic vs. Thermodynamic Enolates

Herbert House and others systematically studied the regioselective formation of lithium enolates using hindered bases such as LDA, enabling chemists to direct aldol reactions with unprecedented control over regiochemistry and stereochemistry.

1997–2000s • Organocatalytic Asymmetric Aldol Reactions

Benjamin List and Carlos Barbas III demonstrated that proline and other small organic molecules catalyze enantioselective aldol reactions, launching the modern field of organocatalysis. List's work contributed to his 2021 Nobel Prize in Chemistry.

The central question that the aldol reaction addresses is deceptively simple: how can we form a new carbon–carbon bond between two carbonyl compounds in a controlled, predictable manner? As we will see, the answer lies in the nucleophilic character of enolates and their ability to attack electrophilic carbonyl carbons, a reactivity pattern that can be manipulated through careful choice of base, solvent, temperature, and substrate.

SECTION 2

Core Principles & Definitions

The aldol reaction encompasses two closely related transformations. In the aldol addition, an enolate (or enol) from one carbonyl compound attacks the carbonyl carbon of a second molecule, generating a β-hydroxy carbonyl product. If the reaction proceeds further to eliminate water and form an α,β-unsaturated carbonyl compound, the overall process is termed an aldol condensation. These two steps can often be controlled independently: lower temperatures and careful quenching favor the addition product, while heating under basic or acidic conditions drives dehydration to the conjugated enone.

Alpha-Carbon Acidity

The C–H bonds on the α-carbon adjacent to a carbonyl are weakly acidic (pKₐ ≈ 19–20 for ketones, ≈17 for aldehydes). Deprotonation by a suitable base generates a resonance-stabilized enolate anion, which serves as the nucleophile in the aldol reaction.

Enolate as Nucleophile

The enolate anion possesses significant electron density on the α-carbon due to resonance delocalization with the carbonyl oxygen. This carbon-centered nucleophile attacks the electrophilic carbonyl carbon of a second molecule, forming the critical C–C bond.

Carbonyl as Electrophile

The second carbonyl compound acts as the electrophilic partner. The polarization of the C=O bond renders the carbonyl carbon electron-poor, making it susceptible to nucleophilic attack by the enolate. Aldehydes are generally more electrophilic than ketones due to reduced steric hindrance and less electron donation.

Dehydration to Enone

The initially formed β-hydroxy carbonyl can undergo E1cb elimination (under basic conditions) or E1/E2 elimination (under acidic conditions) to lose water and form a conjugated α,β-unsaturated carbonyl product. This step is thermodynamically favorable due to extended conjugation.

Equilibrium Considerations

Aldol additions of ketones are often thermodynamically unfavorable (Kₑq < 1), while aldehyde aldols are generally favorable. The aldol condensation product is almost always thermodynamically favored because loss of water and formation of the conjugated system provide a strong driving force.

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KEY TAKEAWAY

Think of the aldol reaction like a molecular handshake: one molecule donates an electron-rich 'hand' (the enolate nucleophile), while the other extends an electron-poor 'hand' (the electrophilic carbonyl). When they clasp—forming the new C–C bond—you get the β-hydroxy carbonyl product. Heating the handshake too vigorously causes part of it to break away (water leaves), locking the molecules into a tighter, conjugated embrace: the α,β-unsaturated carbonyl condensation product.

SECTION 3

Visual Explanation — The Aldol Mechanism

The base-catalyzed aldol reaction of acetaldehyde serves as the prototypical example. The mechanism proceeds through three key stages: enolate formation, nucleophilic addition, and protonation of the resulting alkoxide. When dehydration follows, an additional E1cb elimination step converts the β-hydroxy aldehyde into the α,β-unsaturated product. The following diagram illustrates the complete mechanistic pathway, with electron-flow arrows indicating bond-forming and bond-breaking events at each stage.

The base-catalyzed aldol mechanism of acetaldehyde. Step 1 shows α-deprotonation by hydroxide to form the enolate. Step 2 is the key C–C bond-forming nucleophilic addition. Step 3 regenerates hydroxide by protonation of the alkoxide intermediate. The optional Step 4 shows E1cb dehydration to the conjugated α,β-unsaturated aldehyde (aldol condensation product).

Several features of this mechanism merit close attention. First, the base is catalytic: hydroxide is consumed in Step 1 but regenerated in Step 3, so only a sub-stoichiometric amount is needed when the equilibrium is favorable. Second, the new C–C bond forms between the α-carbon of the enolate and the carbonyl carbon of the electrophilic partner. Third, the β-hydroxy carbonyl product contains a new stereocenter at the former carbonyl carbon, introducing stereochemical considerations that become important in directed aldol reactions. Finally, whether the reaction stops at the aldol addition product or proceeds to the condensation product depends on reaction conditions: mild, low-temperature conditions preserve the β-hydroxy carbonyl, while vigorous heating or excess base favors elimination.

SECTION 4

Acid-Catalyzed Mechanism & Enol Intermediates

While the base-catalyzed pathway proceeds through an enolate anion, the acid-catalyzed aldol reaction operates via a distinct mechanism involving the neutral enol tautomer as the nucleophile. In this pathway, a Brønsted acid protonates the carbonyl oxygen, increasing the electrophilicity of the carbonyl carbon and simultaneously promoting tautomerization of another substrate molecule to its enol form. The enol then attacks the protonated carbonyl, forming the new C–C bond. Deprotonation of the resulting oxocarbenium ion furnishes the β-hydroxy carbonyl product. Under acidic conditions, dehydration to the α,β-unsaturated product typically occurs readily because the acid catalyst promotes elimination.

Acid-Catalyzed Pathway Steps

Step 1 — Enol Formation: Acid-catalyzed keto-enol tautomerization generates the enol nucleophile. The equilibrium lies far toward the keto form, but even small amounts of enol are sufficient because it is continuously regenerated.
Step 2 — Carbonyl Protonation: A second molecule of the carbonyl compound is protonated on oxygen, activating it as a powerful electrophile.
Step 3 — C–C Bond Formation: The enol attacks the protonated carbonyl carbon. This is the rate-determining step, proceeding through a six-membered transition state in some models.
Step 4 — Deprotonation and Dehydration: Loss of a proton gives the β-hydroxy carbonyl, which under acidic conditions readily loses water to yield the conjugated α,β-unsaturated product.

Thermodynamics & Equilibrium

ALDOL EQUILIBRIUM — ALDEHYDES

2 CH₃CHO ⇌ CH₃CH(OH)CH₂CHO Kₑq ≈ 10² (favorable)

For simple aldehydes like acetaldehyde, the aldol addition equilibrium strongly favors the product. The less-substituted carbonyl carbon and the thermodynamic stability of the β-hydroxy aldehyde both contribute.

ALDOL EQUILIBRIUM — KETONES

2 CH₃COCH₃ ⇌ (CH₃)₂C(OH)CH₂COCH₃ Kₑq ≈ 10⁻¹ (unfavorable)

For ketones such as acetone, the equilibrium lies toward starting material. The increased steric strain in the product and reduced electrophilicity of the ketone carbonyl disfavor the addition. Strategies to drive the reaction include removal of product (distillation) or using the condensation pathway where dehydration provides a thermodynamic driving force.

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Why Dehydration Helps

Dehydration of the β-hydroxy carbonyl to the α,β-unsaturated product creates an extended conjugated π-system (C=C–C=O), which is thermodynamically more stable than the unconjugated aldol addition product. This conjugation energy, combined with the entropy gain from liberating a water molecule, shifts the overall equilibrium strongly toward the condensation product even for substrates where the addition alone is unfavorable.

SECTION 5

Types of Aldol Reactions & Selectivity Challenges

Aldol reactions fall into two broad categories. A self-aldol (or homo-aldol) involves two molecules of the same carbonyl compound—the classic acetaldehyde dimerization is the archetype. A crossed aldol (or mixed aldol) involves two different carbonyl compounds. The crossed aldol presents a significant selectivity challenge: with two different substrates, each capable of acting as either nucleophile or electrophile, up to four different aldol products can potentially form. The practical utility of the crossed aldol depends on strategies to favor one product over the others.

A summary of selectivity challenges in crossed aldol reactions and three key strategies for controlling product distribution: using a non-enolizable electrophile, preforming the enolate with a strong, non-nucleophilic base (e.g., LDA), or exploiting intramolecular cyclization. Named variants including the Claisen–Schmidt, Mukaiyama, and Evans aldol reactions are shown.

The Claisen–Schmidt condensation is perhaps the most commonly encountered crossed aldol in an undergraduate course. It succeeds because the aromatic aldehyde (benzaldehyde, for example) has no α-hydrogens and therefore cannot form an enolate—it can only serve as the electrophile. The ketone partner, which does have α-hydrogens, provides the enolate nucleophile. Because the aromatic aldehyde is typically more electrophilic than the ketone, selective cross-coupling occurs efficiently. In a directed aldol using LDA, both partners may have α-hydrogens, but the chemist selects which one is deprotonated by adding it to LDA first at low temperature (−78 °C), quantitatively generating its enolate before the electrophilic partner is introduced.

ℹ

Intramolecular Aldol: Ring Formation

When a 1,4-diketone, 1,5-diketone, or 1,6-diketone is treated with base, the intramolecular aldol cyclization typically follows Baldwin's rules: 5-exo-trig and 6-exo-trig closures are strongly favored. Thus, 2,5-hexanedione cyclizes to give a five-membered ring (cyclopentenone after dehydration), while 2,6-heptanedione gives a six-membered ring (cyclohexenone). The intramolecular pathway is entropically favored over intermolecular alternatives when the resulting ring size is 5 or 6.

SECTION 6

Worked Example — Crossed Aldol with Benzaldehyde

Let us work through a classic Claisen–Schmidt condensation. The problem: predict the major product when benzaldehyde (PhCHO) is treated with acetone (CH₃COCH₃) in the presence of aqueous NaOH at elevated temperature.

Claisen–Schmidt Condensation: PhCHO + Acetone / NaOH / Δ

1
Step 1 — Identify the Enolizable Partner

Benzaldehyde has no α-hydrogens (the carbonyl carbon is directly bonded to the aromatic ring). Therefore, benzaldehyde cannot form an enolate and must serve exclusively as the electrophile. Acetone has six equivalent α-hydrogens (two sets of three on the methyl groups), so it is the enolizable partner that will provide the enolate nucleophile.

Nucleophile: enolate of acetone. Electrophile: benzaldehyde.
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Step 2 — Form the Enolate of Acetone

Hydroxide deprotonates one α-hydrogen of acetone to generate the resonance-stabilized enolate: ⁻CH₂COCH₃ ↔ CH₂=C(O⁻)CH₃. In aqueous NaOH, this is an equilibrium process—only a small fraction of acetone is in the enolate form at any given time, but this is sufficient because the subsequent C–C bond-forming step continuously draws the equilibrium forward.

Enolate: ⁻CH₂COCH₃
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Step 3 — Nucleophilic Addition (C–C Bond Formation)

The α-carbon of the acetone enolate attacks the electrophilic carbonyl carbon of benzaldehyde. The carbonyl π-bond of benzaldehyde breaks, with the electrons going to oxygen to form the alkoxide. This generates the β-hydroxy ketone intermediate: PhCH(O⁻)CH₂COCH₃.

Alkoxide intermediate: PhCH(O⁻)CH₂COCH₃
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Step 4 — Protonation to Give Aldol Addition Product

The alkoxide is protonated by water to yield the β-hydroxy ketone: PhCH(OH)CH₂COCH₃. This is the aldol addition product. Under mild conditions (room temperature, dilute base), the reaction could be stopped here.

Aldol addition product: PhCH(OH)CH₂COCH₃
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Step 5 — Dehydration (E1cb Elimination) to Give Condensation Product

Because the problem specifies heating with NaOH, dehydration occurs. Hydroxide removes an α-proton from the carbon between the OH and the C=O, forming an enolate that then expels hydroxide as a leaving group (E1cb mechanism). The resulting α,β-unsaturated ketone is PhCH=CHCOCH₃ (trans-4-phenyl-3-buten-2-one, also called benzalacetone). The trans (E) configuration is preferred because it minimizes steric interactions between the phenyl group and the carbonyl.

Major product: (E)-PhCH=CHCOCH₃ (benzalacetone) + H₂O
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Step 6 — Consider Double Condensation

With excess benzaldehyde, a second Claisen–Schmidt condensation can occur on the other α-position of acetone, giving dibenzylideneacetone (dba): PhCH=CHCOCH=CHPh. This product is significant in organometallic chemistry as a ligand in Pd₂(dba)₃. The stoichiometry of benzaldehyde relative to acetone determines whether the mono- or bis-condensation product predominates.

With 2 equivalents PhCHO: PhCH=CHCOCH=CHPh (dba)

SECTION 7

Aldol vs. Related Condensation Reactions

The aldol reaction belongs to a family of carbonyl condensation reactions that share the common theme of α-carbon reactivity. However, each differs in the nature of the electrophile, the leaving group, and the final product. Understanding these distinctions is crucial for predicting reaction outcomes and selecting the appropriate transformation in a synthetic context.

Reaction	Nucleophile	Electrophile	Product	Key Feature
Aldol Addition	Enolate of aldehyde/ketone	Aldehyde or ketone	β-Hydroxy carbonyl	C–C bond + new OH
Aldol Condensation	Enolate of aldehyde/ketone	Aldehyde or ketone	α,β-Unsaturated carbonyl	Aldol + dehydration (−H₂O)
Claisen Condensation	Enolate of ester	Ester (acyl substitution)	β-Keto ester	Leaves −OR instead of −OH
Knoevenagel	Active methylene compound	Aldehyde or ketone	α,β-Unsaturated dicarbonyl	More stabilized nucleophile (pKₐ < 13)
Mannich	Enolate or enol	Iminium ion (from CH₂O + amine)	β-Amino carbonyl	Nitrogen instead of oxygen electrophile

Comparison of major carbonyl condensation reactions

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KEY TAKEAWAY

All of these reactions share the same fundamental logic: generate a nucleophilic carbon at the α-position, then use it to attack an electrophilic carbon. The aldol is the 'parent' reaction of this family—once you master it, the Claisen, Knoevenagel, and Mannich reactions become variations on a theme, analogous to how learning one programming language makes it easier to pick up related languages. The differences lie in the electrophilic partner (aldehyde/ketone for aldol, ester for Claisen, iminium for Mannich) and the resulting functional group in the product.

SECTION 8

Stereochemistry & Modern Asymmetric Aldol Reactions

The aldol addition creates up to two new stereocenters in a single step—the α-carbon (from the enolate) and the β-carbon (from the electrophilic carbonyl). This means four possible stereoisomeric products can form, organized into two diastereomeric pairs: syn (or erythro) and anti (or threo). Controlling this diastereoselectivity—and ultimately enantioselectivity—is one of the major achievements of modern synthetic organic chemistry.

Method	Enolate Geometry	Diastereoselectivity	Enantioselectivity	Typical Application
Zimmerman–Traxler (Li, boron enolates)	(Z)-enolate → syn; (E)-enolate → anti	High (>95:5 dr possible)	Racemic unless chiral auxiliary used	Polyketide synthesis
Evans Oxazolidinone	(Z)-boron enolate	>98:2 syn selectivity	>99% ee (chiral auxiliary)	Natural product total synthesis
Mukaiyama Aldol (chiral Lewis acid)	Silyl enol ether (geometry varies)	Variable; catalyst-controlled	>90% ee with chiral catalysts	Pharmaceutical synthesis
Proline Organocatalysis	Enamine intermediate	Typically anti-selective	>95% ee	Green chemistry, simple substrates

Modern stereoselective aldol methods

The Zimmerman–Traxler model provides the conceptual framework for understanding diastereoselectivity. It proposes that the aldol reaction proceeds through a six-membered chair-like transition state in which the metal (Li, B, Ti) coordinates both the enolate oxygen and the carbonyl oxygen of the electrophile. In this transition state, substituents preferentially adopt equatorial positions to minimize 1,3-diaxial interactions. A (Z)-enolate places the α-substituent in a pseudo-equatorial position in the transition state that leads to the syn product, while an (E)-enolate favors anti. This powerful predictive model—reminiscent of cyclohexane conformational analysis—has been validated across hundreds of substrate combinations.

Looking forward, the aldol reaction connects directly to several advanced topics you will encounter in graduate-level synthesis courses and the research literature: the Reformatsky reaction (zinc enolates of α-halo esters), the Mukaiyama aldol with chiral Lewis acid catalysts, and the biomimetic polyketide biosynthesis by type I and type II polyketide synthases, which use iterative aldol reactions to construct macrolide antibiotics like erythromycin.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain why a self-aldol reaction of acetone under thermodynamic (equilibrium) conditions gives very little aldol addition product, yet the aldol condensation product can often be obtained in good yield. Relate your answer to the concepts of Kₑq and the thermodynamic driving force for dehydration.

Show answer

The self-aldol addition of acetone is thermodynamically unfavorable (Kₑq < 1) because the product, a β-hydroxy ketone, suffers from increased steric strain around the newly formed C–C bond, and the loss of the highly stabilized ketone carbonyl is not compensated by sufficient product stability. However, when the reaction is pushed to the condensation stage by heating or using excess base, the dehydration step produces an α,β-unsaturated ketone (mesityl oxide, (CH₃)₂C=CHCOCH₃). Formation of the conjugated C=C–C=O system releases significant stabilization energy (~15–20 kJ/mol) and generates water, an entropy-increasing process. The overall free energy change for the condensation (addition + dehydration) is therefore more favorable, and removal of water (e.g., by distillation) further drives the equilibrium to the right by Le Chatelier's principle.

Question 2: BASIC CALCULATION

Draw the product of the self-aldol condensation of propanal (CH₃CH₂CHO) under NaOH/Δ conditions. Identify the new C–C bond, and assign E/Z stereochemistry to the product.

Show answer

Propanal undergoes self-aldol reaction: the enolate forms at the α-carbon (C2), and this carbon attacks the carbonyl carbon of a second propanal molecule. The initial aldol addition product is 3-hydroxy-2-methylpentanal: CH₃CH₂CH(OH)CH(CH₃)CHO. Under heating with NaOH, E1cb dehydration eliminates water from the β-hydroxy carbonyl to give the α,β-unsaturated aldehyde: (E)-2-methyl-2-pentenal (CH₃CH₂CH=C(CH₃)CHO). The new C–C bond connects C2 and C3 of the product. The (E) configuration is favored because placing the ethyl group trans to the aldehyde minimizes A¹·³ strain. Note that the α-carbon bears a methyl substituent from the enolate, making this a trisubstituted alkene.

Question 3: INTERMEDIATE

When a mixture of benzaldehyde and cyclohexanone is treated with NaOH followed by heating, one major product is obtained. Draw this product and explain why no self-condensation products of cyclohexanone are observed under these conditions. Would using HCl instead of NaOH change the product?

Show answer

The major product is 2-benzylidenecyclohexanone (PhCH=C(cyclohexanone)), resulting from Claisen–Schmidt condensation. Benzaldehyde cannot self-condense (no α-H) and acts solely as the electrophile. Cyclohexanone's self-aldol is disfavored because ketone self-aldols have unfavorable equilibria (Kₑq < 1). However, the crossed product benefits from extended conjugation through the phenyl ring, the newly formed C=C bond, and the ketone C=O, making it thermodynamically highly favorable. Using HCl instead of NaOH would still give the same major product (acid-catalyzed pathway via enol nucleophile and protonated benzaldehyde electrophile), though the acid pathway typically gives the condensation product directly since acid readily catalyzes dehydration of the β-hydroxy intermediate.

Question 4: APPLIED

2,5-Hexanedione is treated with aqueous NaOH. Predict the product of the intramolecular aldol condensation. Explain why a five-membered ring product is formed rather than a three-membered ring, and identify which carbonyl acts as the nucleophile versus the electrophile.

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2,5-Hexanedione (CH₃COCH₂CH₂COCH₃) undergoes intramolecular aldol condensation to form 3-methyl-2-cyclopentenone. In this molecule, an enolate forms at the α-carbon adjacent to one ketone (say C1), and this nucleophilic carbon attacks the electrophilic carbonyl carbon of the other ketone (C5 position), forming a five-membered ring (5-exo-trig cyclization per Baldwin's rules). A three-membered ring would require 3-exo-trig cyclization, which, while not strictly forbidden by Baldwin's rules, is disfavored because of the enormous ring strain in a cyclopropanone and the geometric difficulty of achieving the required orbital overlap. The five-membered ring closure is kinetically and thermodynamically preferred. After aldol addition gives the β-hydroxy ketone, dehydration under the basic conditions gives the conjugated cyclopentenone, driven by the thermodynamic stability of the enone system within the ring.

Question 5: CRITICAL THINKING

A chemist wants to perform a crossed aldol addition (not condensation) between the kinetic enolate of 2-butanone and benzaldehyde to obtain a single diastereomer. Propose reagents and conditions (including base, metal, solvent, and temperature) that would favor: (a) the syn (erythro) β-hydroxy ketone, and (b) the anti (threo) β-hydroxy ketone. Justify your choices using the Zimmerman–Traxler model.

Show answer

(a) For syn selectivity: Generate the (Z)-boron enolate of 2-butanone by treating it with dicyclohexylboron chloride (Cy₂BCl) and Et₃N at 0 °C in Et₂O. The boron enolate of methyl ketones with Cy₂BCl is preferentially (Z). Then add benzaldehyde at −78 °C. In the Zimmerman–Traxler chair transition state, the boron coordinates both oxygens, and the (Z)-enolate geometry places the methyl group equatorial and the hydrogen axial. The aldehyde R group (Ph) also adopts an equatorial position, leading to the syn aldol product. Work up with H₂O₂/NaOH to cleave the boron. (b) For anti selectivity: Form the (E)-lithium enolate using LDA in THF at −78 °C (kinetic deprotonation, which for 2-butanone gives the less-substituted, kinetic enolate with E-geometry when using a non-chelating solvent). In the Zimmerman–Traxler chair with Li coordination, the (E)-enolate forces the α-substituent into a pseudo-axial position to avoid gauche interactions with the enolate oxygen, and the aldehyde substituent takes an equatorial orientation, favoring the anti product. Alternatively, the Mukaiyama aldol with (E)-silyl enol ether and BF₃·OEt₂ at −78 °C in CH₂Cl₂ also gives anti selectivity through an open (Felkin–Anh) transition state.

SUMMARY

Aldol Reactions — Summary

Summary

The aldol reaction is a cornerstone C–C bond-forming reaction in which an enolate nucleophile (or enol, under acidic conditions) attacks the electrophilic carbonyl carbon of an aldehyde or ketone. The aldol addition produces a β-hydroxy carbonyl product; further dehydration (E1cb elimination) converts this into an α,β-unsaturated carbonyl compound—the aldol condensation product. Aldehyde self-aldols are thermodynamically favorable, while ketone self-aldols typically require driving conditions such as product removal or the condensation pathway.

Selectivity in crossed aldol reactions is achieved by using a non-enolizable electrophile (as in the Claisen–Schmidt condensation), by preforming a specific enolate with LDA at −78 °C, or through intramolecular cyclization of dicarbonyl substrates. Stereochemical control is rationalized by the Zimmerman–Traxler model: (Z)-enolates give syn aldol products and (E)-enolates give anti aldol products through chair-like transition states. Modern asymmetric methods—including the Evans oxazolidinone and proline organocatalysis—achieve exceptional enantioselectivities, cementing the aldol reaction as one of the most powerful tools in the synthetic chemist's arsenal.