2-Butynoic acid
[CAS# 590-93-2]

Identification

• Classification: Chemical reagent >> Organic reagent >> Fatty acid
• Name: 2-Butynoic acid
• Synonyms: Tetrolic acid
• Molecular Formula: C₄H₄O₂
• Molecular Weight: 84.07
• CAS Registry Number: 590-93-2
• EC Number: 209-695-7
• SMILES: CC#CC(=O)O

Properties

• Density: 1.2±0.1 g/cm³ Calc.^*, 0.964 g/mL (Expl.)
• Melting point: 78 - 80 °C (Expl.)
• Boiling point: 203.0±9.0 °C 760 mmHg (Calc.)^*, 200 - 203 °C (Expl.)
• Flash point: 90.9±15.2 °C (Calc.)^*
• Index of refraction: 1.468 (Calc.)^*

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

Safety Data

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

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1B	H314
Serious eye damage	Eye Dam.	1	H318
Skin corrosion	Skin Corr.	1C	H314
Acute toxicity	Acute Tox.	4	H302
Skin corrosion	Skin Corr.	1	H314
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Acute toxicity	Acute Tox.	4	H312
Acute toxicity	Acute Tox.	4	H332

• Transport Information: UN 3261

Discovery and Applications

2-Butynoic acid is a short-chain unsaturated carboxylic acid featuring a terminal carboxyl group and a triple bond between the second and third carbon atoms. Its molecular formula is C₄H₄O₂, with a molecular weight of approximately 84.07 g/mol. The compound combines the acidity of the carboxyl group with the electrophilic character of the alkyne, rendering it a versatile intermediate for chemical reactions including nucleophilic additions, condensations, and metal-catalyzed transformations.

Synthesis of 2-butynoic acid can be achieved through oxidation of propargyl alcohols or via carboxylation of terminal alkynes under suitable conditions. One common method involves the treatment of propargyl halides with carbon dioxide in the presence of a strong base, yielding the corresponding alkynoic acid. Reaction parameters such as solvent, temperature, and choice of base are carefully optimized to ensure high yield and to prevent polymerization or side reactions of the reactive triple bond.

Chemically, 2-butynoic acid exhibits reactivity both at the carboxyl group and at the alkyne. The carboxyl functionality can participate in standard reactions such as esterification, amidation, and formation of anhydrides. The alkyne can undergo nucleophilic additions, cycloaddition reactions, and transition-metal-catalyzed couplings, allowing formation of functionalized derivatives and conjugated systems. The proximity of the carboxyl group to the alkyne also enables intramolecular reactions and facilitates the synthesis of cyclic or heterocyclic compounds.

The compound is generally a colorless liquid at room temperature and is soluble in polar solvents such as water, ethanol, and acetone. It is chemically stable under neutral conditions but may undergo polymerization, hydration, or oxidation under strongly acidic, basic, or oxidative environments. Handling requires standard precautions for alkynoic acids, including avoidance of strong oxidants and control of temperature to prevent decomposition.

In practical applications, 2-butynoic acid is used as a building block in organic synthesis and medicinal chemistry. Its dual functionality allows construction of complex molecules, including pharmaceuticals, agrochemicals, and specialty materials. It serves as a precursor for substituted alkynoic acids, conjugated systems, and heterocyclic scaffolds. Additionally, the alkyne group is exploited in “click” chemistry and other metal-catalyzed transformations, providing a versatile tool for chemical derivatization and materials design.

Overall, 2-butynoic acid is a versatile unsaturated carboxylic acid with reactive alkyne and carboxyl functionalities. Its predictable chemical behavior, solubility profile, and dual reactivity make it an important intermediate for the synthesis of functionalized organic molecules, heterocycles, and specialized derivatives in both research and applied chemistry.

References

2019. Enzymatic control of cycloadduct conformation ensures reversible 1,3-dipolar cycloaddition in a prFMN-dependent decarboxylase. Nature Chemistry.
DOI: 10.1038/s41557-019-0324-8