Propyne

Names
Preferred IUPAC name Propyne[1][note 1]
Other names Methylacetylene Methyl acetylene Allylene
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	C3H4
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:

${\displaystyle {\ce {H3CC#CH <=> H2C=C=CH2))}$

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 fuels that must be kept at extremely low temperatures. (See cryogenics.)

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.^[6] Whereas purified propyne is expensive, MAPP gas could be used to cheaply generate large amounts of the reagent.^[7]

Propyne, along with 2-butyne, is also used to synthesize alkylated hydroquinones in the total synthesis of vitamin E.^[8]

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.^[9]