Chemistry:Propadienone

Propadienone is an organic compound with the molecular formula H
2C=C=C=O. It is the parent compound of the alkylidene ketenes, and the first member of the homologous series of cumulenones H
2C=(C=)
nC=O.

Synthesis

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.

Structure

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^-1, much larger than the higher cumulenones H
2C=(C=)
2C=O and H
2C=(C=)
3C=O, which are predicted to also be bent but with very small barriers to backbone linearization (<30 cm^-1).

Its C1 bond angle (C–C–O) is 166–173° and its C2 bond angle (C–C–C) is 142–144°, bending in opposite directions to each other in a zigzag geometry. The large deviation of the C2 bond angle from linearity is attributable to H
2C=C−
–C≡O+
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 (H
2C=C=N+
=N−
), which is predicted to also be bent.

Reactions

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 °C.

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.