Allene (1,2-propadiene), H2 C=C=CH2, has two adjacent double bonds. What kind of hybridization m

step 1 Let's look at the molecule aline, or which is also 1 -2 propodyene, which has a structure H2C, C

Allene (1,2-propadiene), H2 C=C=CH2, has two adjacent double...

Allene (1,2-propadiene), $\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2}$, has two adjacent double bonds. What kind of hybridization must the central carbon have? Sketch the bonding $\pi$ orbitals in allene. What shape do you predict for allene?

Key Concepts

? Bonding Orbitals

The bonding ? orbitals in cumulenes like the given compound involve side-on overlap of unhybridized p orbitals. For the central carbon that is sp hybridized, the two remaining perpendicular p orbitals interact with corresponding p orbitals on the terminal carbons. This results in a set of two ? bonds that lie in perpendicular planes, a distinctive feature that contributes to the unique electronic structure of cumulenes.

Molecular Geometry

The overall geometry of a molecule containing cumulated double bonds is determined by both the linear arrangement of the sp hybridized central atom and the orthogonal arrangement of the adjacent ? bonds. This leads to a linear or near-linear arrangement for the connected atoms along the central axis, while the terminal groups connected by the ? bonds tend to lie in perpendicular planes. The compound, therefore, exhibits a shape that is often described as linear with twisted terminal groups, a consequence of the orthogonal orientation of the ? bonding systems.

sp Hybridization

In molecules with two adjacent double bonds sharing a central carbon, the central atom adopts sp hybridization. This allows for the formation of two linear sp orbitals that form sigma bonds along the axis of the molecule, while leaving two unhybridized p orbitals available to participate in pi bonding. In the case of cumulenes, this sp hybridization is essential for aligning the p orbitals in the correct orientation to form consecutive ? bonds.

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