Propadiene

Propadiene, also called allene, is a three-carbon hydrocarbon with two consecutive carbon–carbon double bonds. Its central carbon forms double bonds to both terminal carbons.

Allene geometry: The arrangement of atoms around an allene’s central carbon, with its two terminal carbon groups lying in perpendicular planes. Propadiene’s perpendicular terminal groups follow from the geometry of its central carbon.
Hydrocarbon: An organic compound composed only of carbon and hydrogen. Propadiene belongs to this broad class because it contains only carbon and hydrogen.
Chemical synthesis: The deliberate preparation of chemical compounds through one or more reactions. Propadiene serves as a starting material for making other organic compounds.
Propyne: A three-carbon alkyne with formula C₃H₄ and structure CH₃C≡CH. Propyne has propadiene’s molecular formula but a carbon–carbon triple bond instead of consecutive double bonds.
Axial chirality: Chirality arising from groups arranged around an axis rather than around a single stereogenic atom. Substituted propadienes can be chiral because their terminal substituents occupy perpendicular planes.
Alkene: A hydrocarbon containing a carbon–carbon double bond. Propadiene contains two alkene-like carbon–carbon double bonds, though its consecutive bonds define a distinct structure.
Organic reaction mechanism: A stepwise account of how electrons and bonds change during an organic chemical reaction. Its cumulene bonds participate in transformations whose outcomes depend on electron movement.
Cyclopropene: A three-carbon cyclic alkene with formula C₃H₄ and one carbon–carbon double bond. Cyclopropene is another C₃H₄ structural isomer, distinguished by its three-membered ring.
Molecular orbital theory: A framework describing molecular bonding through orbitals spread over atoms in a molecule. Its orbital picture explains how the central carbon forms two perpendicular pi bonds.
Cumulene: A compound containing two or more consecutive carbon–carbon double bonds. Propadiene is the simplest cumulene, with two consecutive double bonds.

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