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Propyne
Names
	Preferred IUPAC name Propyne
	Other names Methylacetylene; Methyl acetylene; Allylene
Identifiers
CAS Number	74-99-7 ;
3D model (JSmol)	Interactive image;
Beilstein Reference	878138
ChEBI	CHEBI:48086 ;
ChEMBL	ChEMBL116902 ;
ChemSpider	6095;
ECHA InfoCard	100.000.754
EC Number	200-828-4;
MeSH	C022030
PubChem CID	6335;
UNII	086L40ET1B ;
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	C3H4
Molar mass	40.0639 g/mol
Appearance	Colorless gas
Odor	Sweet
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)
Hazards
Explosive limits	1.7%-?
	NIOSH (US health exposure limits):
PEL (Permissible)	TWA 1000 ppm (1650 mg/m3)
REL (Recommended)	TWA 1000 ppm (1650 mg/m3)
IDLH (Immediate danger)	1700 ppm
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) 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:

{\ce {H3CC#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.

Laboratory methods

Propyne can also be synthesized on laboratory scale by reducing 1-propanol,^[4] allyl alcohol or acetone^[5] 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^{[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]

Notes

^ "Prop-1-yne" mistake fixed in the errata Archived 2019-08-01 at the Wayback Machine. The locant is omitted according to P-14.3.4.2 (d), p. 31 for propene and P-31.1.1.1, Examples, p. 374 for propyne.

References

^ "Characteristic (Functional) and Substituent Groups". Nomenclature of Organic Chemistry. IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 374. doi:10.1039/9781849733069-00372. ISBN 978-0-85404-182-4.
^ ^a ^b ^c ^d ^e ^f ^g NIOSH Pocket Guide to Chemical Hazards. "#0392". National Institute for Occupational Safety and Health (NIOSH).
^ ^a ^b Peter Pässler, Werner Hefner, Klaus Buckl, Helmut Meinass, Andreas Meiswinkel, Hans-Jürgen Wernicke, Günter Ebersberg, Richard Müller, Jürgen Bässler, Hartmut Behringer, Dieter Mayer, "Acetylene" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim 2007 (doi:10.1002/14356007.a01_097.pub2).
^ Keiser, Edward & Breed, Mary (1895). "The Action of Magnesium Upon the Vapors of the Alcohols and a New Method of Preparing Allylene". Journal of the Franklin Institute. CXXXIX (4): 304–309. doi:10.1016/0016-0032(85)90206-6. Retrieved 20 February 2014.
^ Reiser, Edward II. (1896). "The preparation of Allylene, and the Action of Magnesium upon Organic Compounds". The Chemical News and Journal of Industrial Science. LXXIV: 78–80. Retrieved 20 February 2014.
^ Valentian, Dominique; Sippel, Martin; Grönland, Tor-Arne; Baker, Adam; van Den Meulen, Jaap; Fratacci, Georges; Caramelli, Fabio (2004). "Green propellants options for launchers, manned capsules and interplanetary missions" (PDF). la.dlr.de. DLR Lampoldshausen. Archived from the original (PDF) on 2006-01-10.
^ Michael J. Taschner; Terry Rosen; Clayton H. Heathcock (1990). "Ethyl Isocrotonate". Organic Syntheses; Collected Volumes, vol. 7, p. 226.
^ US patent 5744071, Philip Franklin Sims, Anne Pautard-Cooper, "Processes for preparing alkynyl ketones and precursors thereof", issued 1996-11-19
^ Reppe, Walter; Kutepow, N & Magin, A (1969). "Cyclization of Acetylenic Compounds". Angewandte Chemie International Edition in English. 8 (10): 727–733. doi:10.1002/anie.196907271.
^ Loudon, Marc; Parise, Jim (2015-08-26). Organic chemistry. Parise, Jim, 1978- (Sixth ed.). Greenwood Village, Colorado: W. H. Freeman. ISBN 9781936221349. OCLC 907161629.