Question 8.5: Identifying hybridization and Orbital Overlap in Single and ...
Identifying Hybridization and Orbital Overlap in Single and Double Bonds
Describe the hybridization of the carbon atoms in allene, H_{2}C= C= CH_{2}, and make a rough sketch of the molecule showing orbital overlap in bond formation.
STRATEGY
Draw an electron-dot structure to find the number of charge clouds on each atom.
Then predict the geometry around each atom using the VSEPR model (Table 8.1)
| TABLE 8.1 Geometry Around Atoms with 2, 3, 4, 5, and 6 Charge Clouds | ||||
| Number of Bonds | Number of Lone Pairs | Number of Charge Clouds | Molecular Shape (Geometry) | Example |
| 2 | 0 | 2 |  |
 |
| \begin{bmatrix} 3 \\ 2 \end{bmatrix} | \begin{bmatrix} 0 \\ 1 \end{bmatrix} | 3 |  |
 |
| \begin{bmatrix} 4\\ 3 \\2 \end{bmatrix} | \begin{bmatrix} 0\\ 1 \\2 \end{bmatrix} | 4 |  |
 |
| \begin{bmatrix} 5 \\ 4 \\3\\2 \end{bmatrix} | \begin{bmatrix} 0 \\ 1\\2\\3 \end{bmatrix} | 5 |  |
 |
| \begin{bmatrix} 6 \\ 5\\4 \end{bmatrix} | \begin{bmatrix} 0 \\ 1\\2 \end{bmatrix} | 6 |  |
 |
Because the central carbon atom in allene has two charge clouds (two double bonds), it has a linear geometry and is sp-hybridized. Because the two terminal carbon atoms have three charge clouds each (one double bond and two single bonds), they have trigonal planar geometry and are sp^{2}–hybridized. The central carbon uses its sp orbitals to form two σ bonds at 180° angles and uses its two unhybridized p orbitals to form π bonds, one to each of the terminal carbons. The unhybridized p orbitals are oriented at 90° from each other and are designated at 2p_{x} and 2p_{y} to show this spatial arrangement. Each terminal carbon atom uses an sp^{2} orbital for s bonding to carbon, a p orbital for π bonding, and its two remaining sp^{2} orbitals for C-H bonds.
Note that the mutually perpendicular arrangement of the two π bonds results in a similar perpendicular arrangement of the two CH_{2} groups.
