Propyne (74-99-7) Physical and Chemical Properties
Propyne
A small terminal alkyne supplied as a liquefied compressed gas and used commercially as a specialty fuel and chemical feedstock.
| CAS Number | 74-99-7 |
| Family | Alkynes (terminal acetylenes) |
| Typical Form | Liquefied compressed gas |
| Common Grades | BP, EP, USP |
Propyne is a simple aliphatic unsaturated hydrocarbon of the terminal alkyne class (molecular formula C3H4). Structurally it is a methyl-substituted acetylene (SMILES: CC#C, IUPAC: prop-1-yne). The carbon–carbon triple bond confers a linear geometry at the acetylenic carbon atoms with predominant sp hybridization of the two terminal carbons and an sp2-like character for the methyl-bearing carbon in terms of steric environment. The molecule is nonpolar overall (topological polar surface area = 0) and lacks hydrogen-bond donors or acceptors; consequently its intermolecular interactions are dominated by dispersion forces and weak dipolar contributions associated with the C≡C bond.
As a light, volatile terminal alkyne, propyne behaves as a flammable, low-boiling gas that is liquefiable under pressure. The terminal acetylenic hydrogen is relatively acidic compared with aliphatic C–H bonds, which is relevant for metal–acetylide formation in the presence of reactive metal salts. Propyne is chemically reducing toward strong oxidizers, can form unstable acetylides with certain metals, and under compression or heating may decompose or detonate under specific conditions. Its moderate lipophilicity (log Kow ≈ 0.94; computed XLogP ≈ 0.9) and low aqueous solubility behavior lead to rapid volatilization when released to water or soil and a low tendency for bioaccumulation.
Common commercial grades reported for this substance include: BP, EP, USP.
Basic Physical Properties
Density
Reported liquid and gas density descriptors vary with state and reference conditions. Examples from experimental reports include: - Density: 0.607 at \(25\,^\circ\mathrm{C}/4\,^\circ\mathrm{C}\) - Relative density (water = 1): 0.70 - Relative vapor (gas) density (air = 1): 1.41
These values indicate that in the vapor phase propyne is heavier than air and in the condensed (liquid) state has a subunity specific gravity relative to water.
Melting Point
- Reported values: \(-102.7\,^\circ\mathrm{C}\) and \(-153\,^\circ\mathrm{F}\).
If a single, definitive experimental melting point is required, no single consolidated value beyond these reported measurements is established in the current data context.
Boiling Point
- Reported values: \(-23.2\,^\circ\mathrm{C}\) and \(-10\,^\circ\mathrm{F}\) at \(760\ \mathrm{mmHg}\).
Propyne’s low boiling point is consistent with its handling as a liquefied compressed gas for storage and transport.
Vapor Pressure
- Reported vapor pressure: \(5.2\,\mathrm{atm}\).
- Equivalent experimental representations: \(4.31\times 10^{3}\,\mathrm{mmHg}\) at \(25\,^\circ\mathrm{C}\); vapor pressure reported as \(521\,\mathrm{kPa}\) at \(20\,^\circ\mathrm{C}\).
The very high vapor pressure at ambient temperature explains the need for pressurized containment for the liquid form and the rapid partitioning to the gas phase upon release.
Flash Point
- Classified as a flammable gas; flash point not applicable for a gas.
Propyne will readily form flammable and potentially explosive mixtures with air within its flammability limits (see Safety section).
Chemical Properties
Solubility and Phase Behavior
Experimental solubility reports are internally consistent with a volatile, low-polarity gas: - Water solubility reported as \(3{,}640\,\mathrm{mg}\,\mathrm{L}^{-1}\) at \(25\,^\circ\mathrm{C}\) (also reported as g/100 mL: 0.36). - Stated as “insoluble” in some qualitative descriptors; also described as “very soluble in alcohol; soluble in chloroform and benzene.”
The apparent discrepancy between qualitative “insoluble” statements and a numeric solubility reflects the gas-phase behavior and limited thermodynamic solubility compared with typical liquids: propyne partitions strongly to the vapor phase and volatilizes rapidly from aqueous media (estimated Henry’s law constant ≈ \(1.1\times 10^{-2}\,\mathrm{atm}\,\mathrm{m}^{3}\,\mathrm{mol}^{-1}\) derived from reported data), producing short aqueous residence times.
Phase behavior: exists as a gas at ambient conditions; stored as a liquefied compressed gas under pressure. Vapor is denser than air and can collect in low areas when released.
Reactivity and Stability
Propyne is chemically reactive toward strong oxidizing agents (exothermic reactions) and can form sensitive metal–acetylide compounds with certain metals/salts (e.g., silver, copper); such metal acetylides can be shock-sensitive. Reported hazardous behaviors include: - Detonation or explosive decomposition under elevated pressures (reported potential to decompose explosively at roughly 4.5–5.6 atmospheres under certain conditions). - Exothermic reactions with oxidizers and incompatibility with some metal alloys (high-copper alloys) and certain polymers. - Decomposition on heating emits irritating and toxic combustion products (e.g., carbon monoxide).
Given the acetylenic functionality, reactions such as selective hydrogenation, addition across the triple bond, or metal-catalyzed coupling are chemically plausible under controlled conditions; uncontrolled contact with oxidizers or reactive metal salts should be avoided.
Thermodynamic Data
Standard Enthalpies and Heat Capacity
Available thermochemical properties reported include: - Heat of formation: \(185.6\,\mathrm{kJ}\,\mathrm{mol}^{-1}\). - Heat of combustion: \(-1.8496\times 10^{9}\,\mathrm{J}\,\mathrm{kmol}^{-1}\). - Heat capacity (reported): \(67.6\,\mathrm{J}\,\mathrm{mol}^{-1}\,\mathrm{K}^{-1}\) at \(25\,^\circ\mathrm{C}\).
These values indicate a highly endothermic formation and a large exotherm upon complete combustion, consistent with light hydrocarbon fuels.
Other transport and thermal properties reported: - Ionization potential: \(10.36\,\mathrm{eV}\). - Thermal conductivity (gas, 1 atm, 298 K): \(0.018\,\mathrm{W}\,\mathrm{m}^{-1}\,\mathrm{K}^{-1}\); liquid at 233 K: \(0.16\,\mathrm{W}\,\mathrm{m}^{-1}\,\mathrm{K}^{-1}\). - Viscosity: gas at 298 K: \(8.4\times 10^{-3}\,\mathrm{mPa}\,\mathrm{s}\); liquid at 233 K: \(0.26\,\mathrm{mPa}\,\mathrm{s}\).
Molecular Parameters
Molecular Weight and Formula
- Molecular formula: C3H4
- Molecular weight: \(40.06\) (nominal; units conventionally \( \mathrm{g}\,\mathrm{mol}^{-1}\))
Computed exact/monoisotopic mass: \(40.0313001276\).
LogP and Polarity
- Reported log Kow: \(0.94\).
- Computed XLogP: \(0.9\).
- Topological polar surface area (TPSA): 0.
These metrics indicate low polarity and low-to-moderate lipophilicity relative to larger hydrocarbons.
Structural Features
- SMILES: CC#C
- InChI: InChI=1S/C3H4/c1-3-2/h1H,2H3
- InChIKey: MWWATHDPGQKSAR-UHFFFAOYSA-N
Propyne is a terminal alkyne (prop-1-yne) with a methyl substituent on one acetylenic carbon and a terminal acetylenic C–H. The triple bond imparts linearity about the acetylenic carbon atoms and contributes to characteristic spectroscopic signatures (IR C≡C stretch, 13C NMR shifts consistent with acetylenic carbons). The terminal C–H has enhanced acidity relative to typical sp3 C–H bonds and can engage in metal–acetylide chemistry.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS number: 74-99-7
- EC number: 200-828-4
- UN Numbers reported (as used in transport contexts): 3161 (1-PROPYNE) and 1954
- UN Hazard Class: 2.1 (flammable gas)
- UNII: 086L40ET1B
Additional registry identifiers and database-specific IDs are recorded in various institutional registries.
Synonyms and Structural Names
Common synonyms appearing in nomenclature lists include: - Propyne - Methylacetylene - prop-1-yne - 1-propyne - Methyl acetylene - Propynylidyne - Allylene
These synonyms reflect trivial, systematic, and historical naming conventions for the same C3H4 connectivity.
Industrial and Commercial Applications
Representative Uses and Industry Sectors
Propyne is used as a specialty fuel and as a component in fuel gas mixtures (e.g., certain welding/torch fuel blends). It is also used as a chemical intermediate in petrochemical processing and in small-scale organic synthesis where an acetylenic building block is required. Industry sectors with potential occupational exposure include petrochemical manufacturing, welding and cutting operations, and chemical intermediates production.
Role in Synthesis or Formulations
As a terminal alkyne, propyne can act as a reactive C3 building block in alkynylation and hydrogenation chemistry. It is recoverable from cracked gas streams and may be separated from C3 mixtures by solvent extraction and selective hydrogenation strategies in refining and chemical manufacturing contexts. Its primary practical role in formulated products is as a fuel component where the energy content and combustion characteristics are advantageous.
If a concise application summary at product specification level is required, the substance is typically selected for fuel applications or as an acetylenic intermediate based on its volatility, energy density, and reactivity described above.
Safety and Handling Overview
Acute and Occupational Toxicity
Propyne is an asphyxiant at high concentrations and a respiratory irritant; central nervous system depressant effects (dizziness, headache, nausea, unconsciousness) are reported with acute inhalation exposure. Reported occupational exposure limits and toxicity thresholds include: - Permissible/Recommended 8‑hour TWA exposure: \(1000\ \mathrm{ppm}\) (reported). - Immediately Dangerous to Life or Health (IDLH) benchmark (safety-based): \(1700\ \mathrm{ppm}\) (based on a percentage of the lower explosive limit as a precautionary criterion). - Lower and upper flammable (explosive) limits in air: lower ≈ \(1.7\%\) by volume; upper ≈ \(11.7\%\) by volume.
Animal inhalation studies indicate that very high concentrations produce anesthesia and pulmonary irritation; effects include transient central nervous system depression and pulmonary pathology at sufficiently high exposures. Contact with the liquefied gas can cause cryogenic burns (frostbite).
First-aid: for inhalation, move to fresh air and provide respiratory support as indicated; for frostbite from liquid contact, rinse affected areas gently and seek medical attention.
Storage and Handling Considerations
- Storage: maintain cylinders and vessels in cool, well-ventilated, fireproof areas away from ignition sources; store upright and secure against mechanical damage. Keep containers closed and protected from heat to prevent pressure buildup.
- Handling: use closed systems, explosion-proof electrical equipment and grounding/bonding when transferring liquid to minimize electrostatic hazards. Avoid contact with incompatible materials (strong oxidizers, certain copper-containing alloys, and reactive metal salts that can form sensitive acetylides).
- Fire and release response: propyne forms flammable mixtures with air; vapors are heavier than air and can travel to ignition sources and flash back. In case of large releases, evacuate downwind and prevent ignition sources; for fires involving pressurized containers, cool exposed containers with water spray while maintaining a safe distance.
- Personal protective equipment: use appropriate respiratory protection for potential concentrations approaching or exceeding occupational exposure limits; employ cryogenic-resistant gloves and face/eye protection for handling liquefied gas to prevent frostbite injuries; implement local exhaust ventilation where vapors may accumulate.
For detailed regulatory, transport classification, and emergency response guidance, consult the product-specific Safety Data Sheet (SDS) and applicable local legislation.
Storage and Handling Considerations
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Compressed gas considerations and handling precautions reiterate that propyne is typically shipped and stored as a liquefied compressed gas. Specific recommendations include: - Grounding and bonding all transfer equipment to avoid static accumulation when handling liquefied material. - Eliminating ignition sources in areas where vapors may accumulate. - Avoiding use of incompatible materials for piping or containment (notably certain copper alloys and reactive metal salts). - Provision of emergency ventilation, gas detection, and appropriate firefighting measures; do not direct unconfined water jets at spills of liquefied gas.
For exact procedures on cylinder handling, transfer rates, and emergency isolation distances, refer to facility emergency plans and the product SDS.
