Propadiene

Propadiene or allene  is the organic compound with the formula H2C\dC\dCH2. It is the simplest allene, i.e. a compound with two adjacent carbon double bonds. As a constituent of MAPP gas, it has been used as a fuel for specialized welding.

Production and equilibrium with methylacetylene
Allene exists in equilibrium with methylacetylene (propyne) and the mixture is sometimes called MAPD for methylacetylene-propadiene:
 * H3CC\tCH <-> H2C\dC\dCH2

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.