Propyne

Names
Preferred IUPAC name Propyne[1][note 1]
Other names Methylacetylene Methyl acetylene Allylyne
Identifiers
CAS Number	74-99-7 Y
3D model (JSmol)	Interactive image
Beilstein Reference	878138
ChEBI	CHEBI:48086 Y
ChEMBL	ChEMBL116902 N
ChemSpider	6095
ECHA InfoCard	100.000.754
EC Number	200-828-4
MeSH	C022030
PubChem CID	6335
UNII	086L40ET1B Y
CompTox Dashboard (EPA)	DTXSID0026387
InChI InChI=1S/C3H4/c1-3-2/h1H,2H3 Key: MWWATHDPGQKSAR-UHFFFAOYSA-N InChI=1/C3H4/c1-3-2/h1H,2H3 Key: MWWATHDPGQKSAR-UHFFFAOYAI
SMILES CC#C
Properties
Chemical formula	CH3C≡CH
Molar mass	40.0639 g/mol
Appearance	Colorless gas[2]
Odor	Sweet[2]
Density	0.53 g/cm3
Melting point	−102.7 °C (−152.9 °F; 170.5 K)
Boiling point	−23.2 °C (−9.8 °F; 250.0 K)
Vapor pressure	5.2 atm (20°C)[2]
Hazards
Explosive limits	1.7%-?[2]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 1000 ppm (1650 mg/m3)[2]
REL (Recommended)	TWA 1000 ppm (1650 mg/m3)[2]
IDLH (Immediate danger)	1700 ppm[2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Propyne (methylacetylene) is an alkyne with the chemical formula CH₃C≡CH. It is a component of MAPD gas—along with its isomer propadiene (allene), which was commonly used in gas welding. Unlike acetylene, propyne can be safely condensed.^[3]

Production and equilibrium with propadiene

Propyne exists in equilibrium with propadiene, the mixture of propyne and propadiene being called MAPD:

H₃C−C≡CH ⇌ H₂C=C=CH₂

The coefficient of equilibrium K_eq is 0.22 at 270 °C or 0.1 at 5 °C. MAPD is produced as a side product, often an undesirable one, by cracking propane to produce propene, an important feedstock in the chemical industry.^[3] MAPD interferes with the catalytic polymerization of propene.

Use as a rocket fuel

European space companies have researched using light hydrocarbons with liquid oxygen, a relatively high performing liquid rocket propellant combination that would also be less toxic than the commonly used MMH/NTO (monomethylhydrazine/nitrogen tetroxide). Their research showed^{[citation needed]} that propyne would be highly advantageous as a rocket fuel for craft intended for low Earth orbital operations. They reached this conclusion based upon a specific impulse expected to reach 370 s with oxygen as the oxidizer, a high density and power density—and the moderate boiling point, which makes the chemical easier to store than cryogenic fuels that must be kept at extremely low temperatures.^[6]

Organic chemistry

Propyne is a convenient three-carbon building block for organic synthesis. Deprotonation with n-butyllithium gives propynyllithium. This nucleophilic reagent adds to carbonyl groups, producing alcohols and esters.^[7] Whereas purified propyne is expensive, MAPP gas could be used to cheaply generate large amounts of the reagent.^[8]

Propyne, along with 2-butyne, is also used to synthesize alkylated hydroquinones in the total synthesis of vitamin E.^[9]

The chemical shift of an alkynyl proton and propargylic proton generally occur in the same region of the ¹H NMR spectrum. In propyne, these two signals have almost exactly the same chemical shifts, leading to overlap of the signals, and the ¹H NMR spectrum of propyne, when recorded in deuteriochloroform on a 300 MHz instrument, consists of a single signal, a sharp singlet resonating at 1.8 ppm.^[10]

In astrophysics

Propyne has been detected in multiple astrophysical objects following its first observation in 1973 in the galactic center giant molecular cloud Sgr B2 using radio astronomy techniques.^[11] Propyne has been proposed to act as a precursor molecule to the formation of PAHs in space, such as indene.^[12]

Propyne has been detected by infrared spectroscopy in the chemically reducing atmospheres of the outer planets in the Solar System, including on Jupiter in 2000 ^[13] and on Saturn in 1997,^[14] both using the Infrared Space Observatory; on Titan in 1981 using Voyager's IRIS instrument;^[15] and on the ice giants Uranus in 2006 ^[16] and on Neptune in 2008 ^[17] using the Spitzer space telescope.

Notes

References

External links

• NIST Chemistry WebBook page for propyne
• German Aerospace Center
• Nova Chemicals
• CDC - NIOSH Pocket Guide to Chemical Hazards