Propadiene

Propadiene (/proʊpəˈdaɪiːn/) or allene (/ˈæliːn/) is the organic compound with the formula H₂C=C=CH₂. It is the simplest allene, i.e. a compound with two adjacent carbon double bonds.^[3] As a constituent of MAPP gas, it has been used as a fuel for specialized welding.

Propadiene

Stereo structural formula of propadiene with explicit hydrogens Spacefill model of propadiene
Ball and stick model of propadiene
Names
Preferred IUPAC name Propadiene[1]
Other names Allene[1] Propadiene
Identifiers
CAS Number	463-49-0 Y
3D model (JSmol)	Interactive image
Beilstein Reference	1730774
ChEBI	CHEBI:37601 Y
ChEMBL	ChEMBL116960 Y
ChemSpider	9642 Y
ECHA InfoCard	100.006.670
EC Number	207-335-3
Gmelin Reference	860
MeSH	Propadiene
PubChem CID	10037
UNII	4AV0LZ8QKB Y
UN number	2200
CompTox Dashboard (EPA)	DTXSID1029178
InChI InChI=1S/C3H4/c1-3-2/h1-2H2 Y Key: IYABWNGZIDDRAK-UHFFFAOYSA-N Y
SMILES C=C=C
Properties
Chemical formula	C3H4
Molar mass	40.065 g·mol−1
Appearance	Colorless gas
Melting point	−136 °C (−213 °F; 137 K)
Boiling point	−34 °C (−29 °F; 239 K)
log P	1.45
Hazards
GHS labelling:
Pictograms	[2]
Signal word	Danger
Hazard statements	H220[2]
Precautionary statements	P210, P377, P381, P410+P403[2]
NFPA 704 (fire diamond)	0 4 3
Explosive limits	13%
Safety data sheet (SDS)	External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Production and equilibrium with methylacetylene

Propadiene exists in equilibrium with methylacetylene (propyne) and the mixture is sometimes called MAPD for methylacetylene-propadiene:

H₃C−C≡CH ⇌ H₂C=C=CH₂

for which K_eq = 0.22 at 270 °C or 0.1 at 5 °C.

MAPD is produced as a side product, often an undesirable one, of dehydrogenation of propane to produce propene, an important feedstock in the chemical industry. MAPD interferes with the catalytic polymerization of propene.^[4]

Occurrence in Space

In 2019 it was announced that propadiene had been detected in the atmosphere of Saturn's moon Titan using the NASA Infrared Telescope Facility.^[5] This was the first time that propadiene had been detected in space, and the second structural isomeric pair (paired with propyne) detected in Titan's atmosphere, after HCN-HNC.^[6][7]