Question 7.12: Describe the hybridization of the carbon atoms in allene, H2......
Describe the hybridization of the carbon atoms in allene, H_{2}=C=C=CH_{2} ,and make a rough sketch of the molecule showing its hybrid orbitals.
STRATEGY
Draw an electron-dot structure to find the number of charge clouds on each atom.
\quad\quad\quad\quad\quad\quad\quad\quad\underset{\downarrow}{\text{Two charge clouds}} \\ \quad\quad\quad\quad\quad\quad\quad\quad\quad H_{\setminus } \quad\quad\quad\quad _{/}H \\ \text{Three charge clouds}_{\searrow } \quad\quad\quad\quad\quad\quad _{\swarrow}\text{Three charge clouds}\\ \quad\quad\quad \quad\quad\quad\quad\quad\quad\quad C=C=C \\\quad\quad\quad\quad\quad\quad\quad\quad\quad H^{/} \quad\quad\quad\quad\quad ^{\setminus }HThen predict the geometry around each atom using VSEPR theory.
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 tri-gonal planar geometry and are sp² -hybidized . 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.
(The carbon orbitals shown in blue are sp and sp² hybrids; those shown in green are unhybridized p orbitals.)
