Propyne

Propyne (methylacetylene) is an alkyne with the chemical formula CH
3C≡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.^[1]

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

H
3C–C≡CH ⇌ H
2C=C=CH
2

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.^[1] MAPD interferes with the catalytic polymerization of propene.

Propyne can also be synthesized on laboratory scale by reducing 1-propanol,^[2] allyl alcohol or acetone^[3] vapors over magnesium.

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.^[4]

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.^[5] Whereas purified propyne is expensive, MAPP gas could be used to cheaply generate large amounts of the reagent.^[6]

Propyne, along with 2-butyne, is also used to synthesize alkylated hydroquinones in the total synthesis of vitamin E.^[7]

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.^[8]

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.^[9] Propyne has been proposed to act as a precursor molecule to the formation of PAHs in space, such as indene.^[10]

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 ^[11] and on Saturn in 1997,^[12] both using the Infrared Space Observatory; on Titan in 1981 using Voyager's IRIS instrument;^[13] and on the ice giants Uranus in 2006 ^[14] and on Neptune in 2008 ^[15] using the Spitzer space telescope.

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

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