# Aldol
**Aldol** is the trivial name for 3-hydroxybutanal (C₄H₈O₂), a β-hydroxy aldehyde that is both a specific industrial chemical and the namesake of the aldol reaction, the base- or acid-catalyzed coupling of two carbonyl compounds through an enolate or enol to give a β-hydroxy aldehyde or ketone.[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol) • [2](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction) The name combines "aldehyde" and "alcohol": the molecule carries both functional groups, and it was so called by the chemist Wurtz in later work following Alexandre Borodin's studies of the condensation of aldehydes.[3](https://www.scielo.br/j/qn/a/7pYmRTCftL3qrSHC9pSgdwK/)
| Key fact | Detail |
|---|---|
| Identity | 3-Hydroxybutanal (3-hydroxybutyraldehyde, acetaldol); formula C₄H₈O₂; molecular weight 88.11 g/mol; CAS 107-89-1[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol) |
| Appearance | Clear white to yellow syrupy liquid, denser than water; moderately toxic by ingestion, inhalation, and skin absorption[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol) |
| Formation | Rapid, reversible base-catalyzed dimerization of acetaldehyde (sodium ethoxide or sodium hydroxide in a protic solvent)[2](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction) |
| Main use | Preparation of crotonaldehyde (2-butenal), the α,β-unsaturated aldehyde formed by dehydration[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol) • [4](https://openstax.org/books/organic-chemistry/pages/23-3-dehydration-of-aldol-products-synthesis-of-enones) |
| Reaction family | The aldol reaction: enolate of one carbonyl partner adds to the carbonyl carbon of a second, forming a new C–C bond and a β-hydroxy carbonyl compound[2](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction) |
| Industrial scale | Butyraldehyde aldol condensation to 2-ethylhexenal with aqueous NaOH, then hydrogenation, is the commercial route to 2-ethylhexanol, a plasticizer alcohol[5](https://bureau-industrial-transformation.jrc.ec.europa.eu/sites/default/files/2020-03/superseded_lvoC_bref_0203.pdf) • [6](https://openstax.org/books/organic-chemistry/pages/23-4-using-aldol-reactions-in-synthesis) |
## The compound
Aldol is a four-carbon molecule in which an aldehyde group and a hydroxyl group sit on carbons separated by one methylene, the defining arrangement of a β-hydroxy aldehyde. PubChem lists synonyms including 3-hydroxybutyraldehyde, oxybutanal, and acetaldol, and identifiers such as EC number 203-530-2 and [UN number](https://www.edgepedia.org/un-number) 2839.[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol) It is a syrupy liquid that irritates skin and eyes, and occupational exposure occurs through inhalation and dermal contact where it is produced or used.[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol)
The compound is rarely isolated and stored; it is prepared from acetaldehyde and used without isolation. Its principal outlet is dehydration to crotonaldehyde, which is itself a starting material for sorbic acid chemistry and other products; PubChem also notes the manufacture of substituted quinaldines, used for dyes and pigments.[1](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol)
## The reaction that carries its name
The aldol reaction is a carbonyl condensation: one partner is converted into an enolate-ion nucleophile, which adds to the electrophilic carbonyl group of the second partner. After protonation the product is a β-hydroxy carbonyl compound, with the two oxygen atoms in a 1,3 relationship.[2](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction) At least one reactant must possess α hydrogens, since enolate formation requires removal of a proton from the carbon next to the carbonyl.[7](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/22%3A_Carbonyl_Condensation_Reactions/22.01%3A_The_Aldol_Reaction) In base the sequence is enolate formation, nucleophilic attack on the second carbonyl, and protonation; in acid the enol is the nucleophile and protonation activates the electrophilic carbonyl.[7](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/22%3A_Carbonyl_Condensation_Reactions/22.01%3A_The_Aldol_Reaction)
The reaction is reversible, and the position of the equilibrium depends on substrate structure. It generally favors the condensation product for aldehydes with no α substituent (RCH₂CHO) but favors the reactants for disubstituted aldehydes (R₂CHCHO) and for most ketones, an effect attributed to steric congestion in the aldol product.[2](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction) [Phenylacetaldehyde](https://www.edgepedia.org/phenylacetaldehyde) dimerizes to its β-hydroxy aldehyde in 90% yield under sodium hydroxide in ethanol, while cyclohexanone reaches only 22% of the addition product under comparable conditions.[2](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction)
## Dehydration: aldol condensation
Aldol products often lose water on heating, eliminating the α-hydrogen and the β-hydroxyl group to give an α,β-unsaturated aldehyde or ketone, an enone. The double bond always forms in conjugation with the carbonyl, and the added stability of the conjugated system drives many condensations to completion.[7](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/22%3A_Carbonyl_Condensation_Reactions/22.01%3A_The_Aldol_Reaction) For acetaldehyde the sequence gives 3-hydroxybutanal and then 2-butenal (crotonaldehyde) plus water.[4](https://openstax.org/books/organic-chemistry/pages/23-3-dehydration-of-aldol-products-synthesis-of-enones)
Dehydration is what makes ketone aldols practical. Even when the initial addition step is unfavorable, removing water from the reaction mixture pulls the equilibrium toward product: cyclohexanone gives cyclohexylidenecyclohexanone in 92% yield despite an unfavorable initial equilibrium.[4](https://openstax.org/books/organic-chemistry/pages/23-3-dehydration-of-aldol-products-synthesis-of-enones)
## Variants and stereocontrol
Two useful extensions broaden the reaction's reach. In a crossed aldol, two different carbonyl compounds are combined; because the conventional reaction is reversible and prone to self-condensation and polycondensation, control requires careful partner selection or preformed enolates.[8](https://www.organicreactions.org/pubchapter/the-directed-aldol-reaction/) In an intramolecular aldol, a dicarbonyl compound cyclizes, favoring five- and six-membered rings.[9](https://kpu.pressbooks.pub/organicchemistry2/chapter/6-4-aldol-reactions/)
Stereochemistry is a major preoccupation of modern aldol methodology. Enolate geometry matters: under kinetic conditions E- or Z-enolates form, and enolate geometry largely determines the syn or anti relationship in the product through a cyclic transition state.[10](https://www.sciencedirect.com/science/article/abs/pii/S0040402099009643) With preformed lithium enolates the addition can be essentially irreversible, with a forward-to-reverse rate difference of roughly ten powers of ten based on pKₐ values, and rate-controlled conditions can favor the syn diastereomer while equilibrium conditions favor the anti isomer.[11](https://organicchemistrydata.org/reusch/virtualtext/aldol-reactions/) Reviews of asymmetric variants describe strategies in which any single stereoisomeric aldol product can be accessed by choosing the appropriate procedure, ranging from chiral auxiliaries and Lewis-acid catalysis of silyl enolates (the Mukaiyama reaction) to direct catalysis of unmodified ketones by chiral metal complexes or small organic molecules.[12](https://pubs.rsc.org/en/content/articlelanding/2004/cs/b202901d)
## Industrial significance
The largest-scale application is the production of 2-ethylhexanol, an alcohol used in synthesizing plasticizers for polymers. Working backward from the product, 2-ethyl-1-hexanol comes from 2-ethylhexanal by reduction, which comes from 2-ethyl-2-hexenal, the aldol condensation product of butanal.[6](https://openstax.org/books/organic-chemistry/pages/23-4-using-aldol-reactions-in-synthesis) In the plant process, the aldol condensation of butyraldehyde to 2-ethylhexenal proceeds rapidly with aqueous sodium hydroxide catalyst, and the dehydration is carried out promptly because the aldol is unstable and can impair product quality and yield.[5](https://bureau-industrial-transformation.jrc.ec.europa.eu/sites/default/files/2020-03/superseded_lvoC_bref_0203.pdf) • [13](https://p2infohouse.org/ref/21/20562.pdf) The enal is then hydrogenated to the alcohol.[6](https://openstax.org/books/organic-chemistry/pages/23-4-using-aldol-reactions-in-synthesis)
The same logic of recognizing β-hydroxy carbonyl or enone motifs in target molecules makes the aldol reaction a standard retrosynthetic disconnection, and it is taught at the university level partly because it illustrates enolate mechanisms so clearly.[6](https://openstax.org/books/organic-chemistry/pages/23-4-using-aldol-reactions-in-synthesis) • [14](https://dalalinstitute.com/chemistry/wp-content/uploads/sites/2/Books/A-Textbook-of-Organic-Chemistry-Volume-1/ATOOCV1-12-4-Mechanism-of-Condensation-Reactions-Involving-Enolates-Aldol-Knoevenagel-Claisen-Mannich-Benzoin-Perkin-and-Stobbe-Reactions.pdf)
## References
1. [3-Hydroxybutanal (Aldol), CID 7897, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/Aldol)
2. [23.1 Carbonyl Condensations: The Aldol Reaction, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/23-1-carbonyl-condensations-the-aldol-reaction)
3. [Entre a batuta e o tubo de ensaio: a carreira admirável de Alexandre Borodin, Química Nova (SciELO)](https://www.scielo.br/j/qn/a/7pYmRTCftL3qrSHC9pSgdwK/)
4. [23.3 Dehydration of Aldol Products: Synthesis of Enones, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/23-3-dehydration-of-aldol-products-synthesis-of-enones)
5. [Superseded LVOC BREF, European Commission JRC](https://bureau-industrial-transformation.jrc.ec.europa.eu/sites/default/files/2020-03/superseded_lvoC_bref_0203.pdf)
6. [23.4 Using Aldol Reactions in Synthesis, OpenStax Organic Chemistry](https://openstax.org/books/organic-chemistry/pages/23-4-using-aldol-reactions-in-synthesis)
7. [22.1: The Aldol Reaction, Chemistry LibreTexts](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/22%3A_Carbonyl_Condensation_Reactions/22.01%3A_The_Aldol_Reaction)
8. [The Directed Aldol Reaction, Organic Reactions](https://www.organicreactions.org/pubchapter/the-directed-aldol-reaction/)
9. [6.4 Aldol Reactions, Organic Chemistry II (KPU Pressbooks)](https://kpu.pressbooks.pub/organicchemistry2/chapter/6-4-aldol-reactions/)
10. [Advances in Asymmetric Enolate Methodology, Tetrahedron](https://www.sciencedirect.com/science/article/abs/pii/S0040402099009643)
11. [Aldol Reactions, Virtual Textbook, OrganicChemistryData.org](https://organicchemistrydata.org/reusch/virtualtext/aldol-reactions/)
12. [Current progress in the asymmetric aldol addition reaction, Chem. Soc. Rev. 2004, 33, 65](https://pubs.rsc.org/en/content/articlelanding/2004/cs/b202901d)
13. [Aldolisation and dehydration in 2-ethylhexanol production, Pollution Prevention reference document](https://p2infohouse.org/ref/21/20562.pdf)
14. [Mechanism of Condensation Reactions Involving Enolates, A Textbook of Organic Chemistry Volume I](https://dalalinstitute.com/chemistry/wp-content/uploads/sites/2/Books/A-Textbook-of-Organic-Chemistry-Volume-1/ATOOCV1-12-4-Mechanism-of-Condensation-Reactions-Involving-Enolates-Aldol-Knoevenagel-Claisen-Mannich-Benzoin-Perkin-and-Stobbe-Reactions.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Carbonyl and carboxyl chemistry › Aldehydes and ketones › Ketones › Hydroxy ketones*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 11, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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