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Propadiene or allene is the organic compound with the formula H2C=C=CH2. It is the simplest allene or compound with two adjacent carbon double bonds, and can also be identified as allene.[2] As a constituent of MAPP gas, it has been used as a fuel for specialized welding.

Stereo structural formula of propadiene with explicit hydrogens
Spacefill model of propadiene
Ball and stick model of propadiene
Preferred IUPAC name
Other names
3D model (JSmol)
ECHA InfoCard 100.006.670
EC Number 207-335-3
MeSH Propadiene
UN number 2200
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
Safety data sheet External MSDS
Extremely Flammable F+
R-phrases (outdated) R12
S-phrases (outdated) S9, S16, S33
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propaneHealth code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chlorideReactivity code 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g., fluorineSpecial hazards (white): no codeNFPA 704 four-colored diamond
Explosive limits 13%
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Production and equilibrium with methylacetyleneEdit

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


for which Keq = 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.[3]


  1. ^ a b Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 375. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4. The name allene, for CH2=C=CH2, is retained for general nomenclature only; substitution is allowed, but not by alkyl or any other group that extends the carbon chain, nor characteristic groups expressed by suffixes. The systematic name, propa-1,2-diene, is the preferred IUPAC name.
  2. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "allenes". doi:10.1351/goldbook.A00238
  3. ^ Klaus Buckl, Andreas Meiswinkel "Propyne" in Ullmann's Encyclopedia of Industrial Chemistry, 2008, Wiley-VCH, Weinheim. doi:10.1002/14356007.m22_m01