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.
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 Keq 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. MAPD interferes with the catalytic polymerization of propene.
Laboratory methods Propyne can also be synthesized on laboratory scale by reducing 1-propanol, allyl alcohol or acetone vapors over magnesium.
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 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.
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. Whereas purified propyne is expensive, MAPP gas could be used to cheaply generate large amounts of the reagent. Propyne, along with 2-butyne, is also used to synthesize alkylated hydroquinones in the total synthesis of vitamin E. 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.
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. Propyne has been proposed to act as a precursor molecule to the formation of PAHs in space, such as indene. 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 and on Saturn in 1997, both using the Infrared Space Observatory; on Titan in 1981 using Voyager's IRIS instrument; and on the ice giants Uranus in 2006 and on Neptune in 2008 using the Spitzer space telescope.
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References
External links NIST Chemistry WebBook page for propyne German Aerospace Center Nova Chemicals CDC - NIOSH Pocket Guide to Chemical Hazards
