1.7 Hybridization and Molecular Shapes
If we consider the bond angles of organic molecules using s and p orbitals, we expect bond angles of about 90° because s orbitals are nondirectional, and the p orbitals are oriented at 90° to one another. But experimental evidence shows, however, that bond angles in most of organic compounds are close to 109°, 120° or 180°.
According to valence-shell electron-pair repulsion theory (VSEPR theory), the bonds pair and the lone pair electrons repel each other and attain the largest possible angles. An angle of 109.5° is the largest possible separation for four pairs of electrons, 120° is the largest separation for three pairs, and 180° is the largest separation for two pairs.
To explain the shapes of common organic molecules, we assume that the s and p orbitals combine to form hybrid orbitals that separate the electron pairs more widely in space. Hybrid orbitals are mixed orbitals that they result from combining orbitals. The concept of combining orbitals is called hybridization.
1.7.1 sp3 Hybrid Orbitals
Methane is the simplest example of sp3 hybridization. Methane has four covalent bonds with same length. The bond angles of all the covalent bonds are same (109.5°) which gives a tetrahedron shape. So the four bonds in methane are identical.
These four degenerate orbitals are called sp3 hybrid orbitals formed by the combination of one s orbital and three p orbitals. Each orbital has 25% s character and 75% p character.
A sp3 orbital has two lobes with different size because the s orbital adds to one lobe of the p orbital and subtracts from the other lobe of the p orbital. The larger lobe of the orbital is used in covalent bond formation. The four orbitals arrange themselves as far away as possible in space to minimize the repulsion. They point toward the corners of a regular tetrahedron.
The negatively charged carbon in the methyl anion has three pairs of bonding electrons and one lone pair. The four pairs of electrons, three bond pair and two electrons, point toward the corners of a tetrahedron. In the methyl anion, three of carbon's sp3 orbitals overlap with the s orbital of a hydrogen and forms covalent bonds, and the fourth sp3 orbital holds the lone pair.
