Propadienone is an organic compound with the molecular formula . It is the parent compound of the alkylidene ketenes, and the first member of the homologous series of cumulenones .
Propadienone may be generated via flash vacuum pyrolysis of several possible precursors, including acrylic anhydride and 3-diazotetronic acid. While it is only transiently present under standard conditions, it may be captured via matrix isolation in solid argon at 14 K.
While propadienone is planar, it has a nonlinear CCCO backbone, in contrast to most other ground-state cumulenes and heterocumulenes such as propadiene and carbon suboxide. The barrier to linearization is at least 360 cm<sup>-1</sup>, much larger than the higher cumulenones and , which are predicted to also be bent but with very small barriers to backbone linearization (<30 cm<sup>-1</sup>).
Its C1 bond angle (CâÂÂCâÂÂO) is 166âÂÂ173ð and its C2 bond angle (CâÂÂCâÂÂC) is 142âÂÂ144ð, bending in opposite directions to each other in a zigzag geometry. The large deviation of the C2 bond angle from linearity is attributable to being a major resonance contributor, making propadienone a 1,3-dipole, a property that is shared by the higher cumulenones. Its dipole moment is 2.3 D.
Propadienone is isoelectronic to diazoethene (), which is predicted to also be bent.
Exposure of matrix-isolated propadienone to UV light of wavelength 230 ± 10 nm leads to its photodissociation into acetylene and carbon monoxide via a postulated methylidenecarbene intermediate. The same decarbonylation reaction occurs during vacuum pyrolysis at temperatures at or above 570 ðC.
Warming matrix-isolated propadienone yields a white insoluble polymer of unknown composition. The same polymer is also obtainable from direct low-temperature condensation of the undiluted pyrolysis gas stream.