Question 5.4: Predicting the Hybridization of an Atom Describe the hybridi......
Predicting the Hybridization of an Atom
Describe the hybridization of the carbon atoms in allene, H_2C=C=CH_2, and make a rough sketch of the molecule showing its hybrid orbitals.
STRATEGY
Draw an electron-dot structure of allene to find the number of charge clouds on each atom.
\quad \quad \quad \quad \quad \quad \quad \quad \quad \ \ \ \underset{\downarrow}{\text{Two Charge clouds}} \\ \text{Three charge clouds} \rightarrow \overset{\overset{H \ \ \ \ \ }{ \ \ \ ╲}}{\underset{\underset{H \ \ \ \ \ \ \ \ \ }{╱}}{ \ \ \ \ C}}=C=\overset{\overset{ \ \ \ \ \ \ \ \ H }{╱}}{\underset{\underset{ \ \ \ \ \ \ H }{╲}}{C \ \ \ \ \ }} \ \leftarrow \text{Three charge clouds}Then predict the geometry around each atom using the VSEPR model in Table 5.1.
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 C-H bonds), they have trigonal planar geometry and are sp² -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. Each terminal carbon atom uses an sp² orbital for σ bonding to carbon, a p orbital for π bonding, and its two remaining sp² 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.
