Q47E - Organic Chemistry (Mcmurry)

Question

Allene, $HC=C=C{H}_2$ is somewhat unusual in that it has two adjacent double bonds. Draw a picture showing the orbitals involved in the s and p bonds of allene. Is the central carbon atom sp² or sp-hybridized? What about the hybridization of the terminal carbons? What shape do you predict for allene?

Step-by-Step Solution

Answer

Answer

Step 1: Orbitals involved in the sigma bond of allene

The central carbon atom is sp hybridized and has a linear geometry, while the outside carbon atoms are sp²  hybridized. Both the outer carbon atoms have trigonal planar geometry. However, their planes are rotated $90°$about the bond axis to be perpendicular to each other. The orbitals involved in the sigma bond of allene are represented as follows.

Step 2: Orbitals involved in the pi-bond of allene

The central carbon atom makes a pi bond with the carbon atom on the left using the p orbital that is oriented on the plane of the page. It makes a pi bond with the carbon atom on the right using the p orbitals that are oriented perpendicular to the plane of the page.

Step 3: Illustration of the plane of allene

As the central carbon atom of allene makes bond angles that are $180°$, the carbon atoms should be arranged in a straight line. 

The two double bonds of allene are perpendicular to each other because they involve p orbitals that are perpendicular to each other. Thus, the  orbitals of the outer carbon atoms are also oriented on planes that are perpendicular to each other.

The hydrogen atoms bonded with the carbon on the left should be located on a plane perpendicular to the hydrogen atoms located in the carbon on the right, represented as follows.