A model of the organic compound allene is shown below. (a)...

A model of the organic compound allene is shown below. (a) Explain why the allene molecule is not flat. That is, explain why the $\mathrm{CH}_{2}$ groups at opposite ends do not lie in the same plane. (b) What is the hybridization of each of the carbon atoms in allene? (c) What orbitals overlap to form the bonds between carbon atoms in allene? (FIGURE CAN'T COPY)

A model of the organic compound allene is shown below.
(a) Explain why the allene molecule is not flat. That is, explain why the $\mathrm{CH}_{2}$ groups at opposite ends do not lie in the same plane.
(b) What is the hybridization of each of the carbon atoms in allene?
(c) What orbitals overlap to form the bonds between carbon atoms in allene?
(FIGURE CAN'T COPY)

Key Concepts

Orbital Overlap

Orbital overlap is the interaction where atomic orbitals come together to form chemical bonds through the sharing of electrons. In allene, the sigma bonds between the carbon atoms are formed by the end-to-end overlap of the sp hybrid orbitals from the central carbon with the appropriate orbitals on the terminal carbons. The pi bonds, on the other hand, are formed by the side-to-side overlap of unhybridized p orbitals on the central carbon with the unhybridized p orbitals on each terminal carbon. The perpendicular arrangement of these p orbitals results in the pi bonds being oriented in orthogonal planes.

Hybridization

Hybridization in molecules describes the mixing of atomic orbitals to form new hybrid orbitals that are oriented in specific geometries to maximize bonding. In cumulene structures like allene, the central carbon atom is sp hybridized because it forms two double bonds with its neighbors, using two linearly arranged sp orbitals for sigma bonding. The terminal carbon atoms are typically sp2 hybridized, using three hybrid orbitals in a trigonal planar arrangement for their bonding with hydrogen and the central carbon, leaving one unhybridized p orbital available for forming a pi bond.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, dictated by the types of bonds and the hybridization of the atoms involved. In allene, the unique double-bond arrangement causes the two p orbitals on the central sp hybridized carbon to be mutually perpendicular. This results in the terminal CH2 groups being oriented in different, orthogonal planes, which explains why the molecule is not flat.

Transcript

00:04 When it comes to molecules like aline, there are two adjacent double bonds.

00:11 And this molecule is not flat, even though every carbon here has three bonds to three different places, except for this one.

00:21 This one has a linear structure.

00:24 Or sorry, this would be spp, and this would be sp2.

00:30 Sp2, sp2, sp2, and sp2.

00:35 However, although it should technically be plainer, it isn't.

00:39 And it has to do with energy.

00:43 And if you were to look at probably, if you were to look through this view, just looking at it head on at those atoms, what you would see is you would see this has your central atom and the rest of the molecules of the aline are behind right there.

01:06 And either you could have these two atoms be on the same plane.

01:13 Those are both hydrogens.

01:16 Either they could be in the same plane and cause so much steric hindrance, well not technically stericindrance because they're hydrogens, which are really small, but so much just energy of having that orientation.

01:30 Or you could have it be really energy efficient.

01:35 And instead of having them like completely parallel, have them perpendicular, have them far away from causing so much torsional, energy.

01:48 Like there's so much energy from torsion that it would be really unfavorable for it to be vertical like that, which is why they would be mismatched in that sense.

02:03 And when we talk about allene's orbitals, let's talk about this central carbon.

02:12 So we have this carbon and it has sp orbitals.

02:16 So it has an sp orbital here and an sp orbital there.

02:21 And this carbon has an sp2 and this carbon has an sp2.

02:24 And it shares two electrons...

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