Propyne
[CAS# 74-99-7]

Identification

• Classification: Chemical reagent >> Organic reagent >> Alkyne
• Name: Propyne
• Synonyms: Prop-1-yne; Allylene; Methylacetylene
• Molecular Formula: C₃H₄
• Molecular Weight: 40.06
• CAS Registry Number: 74-99-7
• EC Number: 200-828-4
• SMILES: CC#C

Properties

• Density: 0.6±0.1 g/cm³ Calc.^*
• Melting point: -102.7 °C (Expl.)
• Boiling point: -28.0±3.0 °C 760 mmHg (Calc.)^*, -23.2 °C (Expl.)
• Flash point: -91.1±1.4 °C (Calc.)^*, -51 °C (Expl.)
• Solubility: Insoluble (Expl.)
• Index of refraction: 1.364 (Calc.)^*

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

Safety Data

• Hazard Symbols: GHS02;GHS04;GHS07 Danger
• Risk Statements: H220-H280-H335-H336
• Safety Statements: P203-P210-P222-P261-P271-P280-P304+P340-P319-P377-P381-P403-P403+P233-P405-P410+P403-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Flammable gases	Flam. Gas	1	H220
Gases under pressure (liquid)	Press. Gas (Liq.)		H280
Specific target organ toxicity - single exposure	STOT SE	3	H335
Specific target organ toxicity - single exposure	STOT SE	3	H336
Gases under pressure (compressed)	Press. Gas (Comp.)		H280
Germ cell mutagenicity	Muta.	2	H341
Specific target organ toxicity - repeated exposure	STOT RE	2	H373

• Transport Information: UN 1954

Discovery and Applications

Propyne, also known as methylacetylene, is a hydrocarbon with the molecular formula C₃H₄. It is a colorless gas under standard conditions and is structurally characterized by a terminal alkyne group. Propyne exists as one of two isomers with the same molecular formula, the other being allene (propadiene). These two isomers are often found in mixtures and are collectively referred to as MAPD (methylacetylene-propadiene) in industrial settings.

The discovery and industrial relevance of propyne are tied to the development of thermal cracking processes in petroleum chemistry. During the steam cracking of propane to produce propylene, propyne and propadiene are generated as byproducts. While initially considered impurities in propylene production, these byproducts later found commercial value. MAPD mixtures have been used as fuel gases, particularly for oxy-gas welding and cutting, due to their relatively high flame temperatures and improved safety profile compared to acetylene. Unlike acetylene, propyne can be stored safely in standard cylinders without requiring acetone as a stabilizer.

In the laboratory, propyne is valued as a reactive terminal alkyne. The hydrogen atom on the terminal carbon is weakly acidic, which allows the formation of organometallic reagents such as propynyllithium upon treatment with strong bases like n-butyllithium. These reagents are used in carbon-carbon bond-forming reactions, particularly in the synthesis of propargylic alcohols and related structures. Propyne thus serves as a useful building block in organic synthesis.

Propyne also plays a role in catalytic and separation chemistry. It can be isomerized into propadiene using heterogeneous catalysts, with acid or base sites influencing reaction pathways. The equilibrium between propyne and propadiene is temperature-dependent and is exploited in processes where propadiene is desired for specialty chemical applications. However, separating propyne from propadiene poses a challenge due to their similar boiling points and physicochemical properties.

Recent advances have been made using microporous materials such as metal-organic frameworks (MOFs) for the selective adsorption and separation of propyne and propadiene. MOFs with open metal sites or specially tailored pore environments can preferentially adsorb one isomer over the other, offering an energy-efficient alternative to traditional distillation methods. This development is significant for improving the purity of chemical feedstocks in polymer and pharmaceutical industries.

Propyne also has a place in combustion chemistry. Studies of its flame behavior and reaction intermediates have contributed to understanding the formation of aromatic compounds in hydrocarbon flames. Propyne-enriched flames are used in model systems to study soot formation and hydrocarbon oxidation, both of which are relevant to engine performance and emissions control.

As a potential rocket fuel, propyne has attracted attention due to its combination of high performance, manageable storage conditions, and relatively high density. Although it has not been widely adopted in modern aerospace propulsion systems, its combustion characteristics when paired with liquid oxygen make it an option worth considering for future small-scale or experimental propulsion applications.

Propyne is classified as a flammable gas and must be handled with care. It can act as an asphyxiant in high concentrations and forms explosive mixtures with air. Therefore, its use requires proper ventilation and adherence to occupational safety standards. Exposure limits have been established in some jurisdictions to minimize health risks to workers.

Overall, propyne is a versatile chemical compound with applications that span fuel technology, synthetic organic chemistry, gas separation, and catalysis. Its reactivity and structural simplicity make it a fundamental material in both academic research and industrial processes.

References

2007. A crossed molecular beam study on the formation of hexenediynyl radicals (H2CCCCCCH; C6H3(X2A′)) via reactions of tricarbon molecules, C3(X1Σg+), with allene (H2CCCH2; X1A1) and methylacetylene (CH3CCH; X1A1). Physical Chemistry Chemical Physics, 9(16).
DOI: 10.1039/b618179a

2005. Excited-state decay of hydrocarbon radicals, investigated by femtosecond time-resolved photoionization: Ethyl, propargyl, and benzyl. The Journal of Chemical Physics, 122(8).
DOI: 10.1063/1.1857475

2001. An Unusual Ruthenium-Catalyzed Dimerization of Propargyl Alcohols. Journal of the American Chemical Society, 123(32).
DOI: 10.1021/ja0111636