Crotonic acid
[CAS 3724-65-0]

Identification

• Classification: Biochemical >> Biochemical reagent >> Biological dye
• Name: Crotonic acid
• Synonyms: 2-Butenoic acid; 3-Methylacrylic acid
• Molecular Formula: C₄H₆O₂
• Molecular Weight: 86.09
• CAS Registry Number: 3724-65-0
• EC Number: 223-077-4
• SMILES: C/C=C/C(=O)O

Properties

• Density: 1.0±0.1 g/cm³ Calc.^*
• Melting point: 73 °C (Expl.)
• Boiling point: 177.0±9.0 °C 760 mmHg (Calc.)^*
• Flash point: 87.8 °C (Calc.)^*
• Solubility: water soluble (Expl.)
• Index of refraction: 1.449 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS05;GHS07 Danger
• Risk Statements: H302-H312-H314-H318
• Safety Statements: P260-P264-P264+P265-P270-P280-P301+P317-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P321-P330-P362+P364-P363-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute toxicity	Acute Tox.	4	H302
Acute toxicity	Acute Tox.	4	H312
Skin corrosion	Skin Corr.	1C	H314
Serious eye damage	Eye Dam.	1	H318
Acute toxicity	Acute Tox.	3	H311
Skin irritation	Skin Irrit.	2	H315
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412
Skin corrosion	Skin Corr.	1B	H314
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410

• Transport Information: UN 2823

chemBlink Chemical Story

Crotonic acid, also known as trans-2-butenoic acid, is an unsaturated monocarboxylic acid with the molecular formula C₄H₆O₂. As the trans isomer of 2-butenoic acid, it is one of the simplest α,β-unsaturated carboxylic acids and has long served as both an important industrial intermediate and a model compound in organic chemistry. Although relatively simple in structure, crotonic acid has played a significant role in the development of structural organic chemistry, polymer science, and synthetic methodology.

The name "crotonic acid" originated in the nineteenth century because the compound was first obtained during investigations of croton oil, a vegetable oil extracted from the seeds of *Croton tiglium*. Early chemists recognized that the acid isolated from degradation products of croton oil differed from saturated butyric acid by containing a carbon-carbon double bond. During a period when the concepts of molecular structure and geometric isomerism were still being established, studies of crotonic acid and its related compounds contributed to the understanding of unsaturated organic molecules and the chemistry of carbon-carbon double bonds.

As organic structural theory matured, crotonic acid became one of the classical compounds used to investigate addition reactions, hydrogenation, hydration, halogenation, and esterification. Because its double bond is conjugated with the carboxyl group, the molecule exhibits characteristic chemical behavior that has made it a valuable model substrate in mechanistic studies. Numerous fundamental investigations conducted throughout the twentieth century employed crotonic acid to examine reaction kinetics and the influence of conjugated unsaturation on chemical reactivity.

Commercially, crotonic acid is produced primarily by oxidation of crotonaldehyde or through related industrial synthetic routes. It is manufactured on a moderate scale as a specialty chemical rather than a bulk commodity. Its principal value lies not in its direct consumption but in its versatility as an intermediate for preparing esters, amides, acid chlorides, and functional polymers.

One of the most important applications of crotonic acid is in polymer chemistry. The presence of both a vinyl group and a carboxyl group allows the molecule to participate in copolymerization while simultaneously introducing reactive functional groups into polymer chains. Crotonic acid is therefore incorporated into acrylic resins, vinyl resins, coatings, adhesives, inks, and specialty polymers to improve adhesion, hardness, pigment dispersion, and chemical resistance. Even relatively small amounts can significantly modify the physical properties of polymeric materials.

Crotonic acid derivatives are also used in the manufacture of plastic additives, surface coatings, textile finishing agents, paper treatment chemicals, and UV-curable materials. The reactivity of its unsaturated carboxylic acid structure provides opportunities for subsequent chemical modification, making it an attractive building block for functional materials.

In laboratory synthesis, crotonic acid serves as a convenient precursor for numerous organic transformations. The activated double bond readily undergoes hydrogenation, oxidation, epoxidation, Michael addition, and other reactions characteristic of α,β-unsaturated carboxylic acids. As a result, it is frequently employed as a model compound in studies of catalytic reactions and reaction mechanisms, as well as in the development of new synthetic methodologies.

Interest in crotonic acid has expanded further with the growing emphasis on sustainable chemistry. Because unsaturated carboxylic acids can potentially be obtained from renewable feedstocks through catalytic or biotechnological processes, crotonic acid has attracted attention as a possible bio-based platform chemical for advanced materials. Although conventional petrochemical production remains dominant, research into greener production methods continues.

More than a century after its discovery, crotonic acid remains an important industrial intermediate and a representative member of the α,β-unsaturated carboxylic acid family. Its historical contribution to the understanding of organic structure, together with its continuing importance in polymer chemistry and materials science, has ensured its lasting significance in both academic research and industrial applications.

References

1. Rogers, F.K. and Kennedy, D.J. (1960) 'Crotonic Acid', Industrial & Engineering Chemistry, 52(1), pp. 25–26.
https://doi.org/10.1021/ie50601a029

2. Pressman, D. and Lucas, H.J. (1939) 'The Hydration of Unsaturated Compounds. VII. The Rate of Hydration of Crotonic Acid; the Rate of Dehydration of β-Hydroxybutyric Acid; the Equilibrium between Crotonic Acid and β-Hydroxybutyric Acid in Dilute Aqueous Solution', Journal of the American Chemical Society, 61(9), pp. 2271–2277.
https://doi.org/10.1021/ja01878a003

3. National Institute of Standards and Technology (NIST) (2025) NIST Chemistry WebBook: Crotonic Acid.
https://webbook.nist.gov/cgi/cbook.cgi?ID=3724-65-0