Beginning Organic Chemistry (BOC)
1. Basic Knowledge
e. Molecular Orbitals and Bonding.
2. Double Bonds.
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Note that ethene is a flat molecule. The trigonal planar shape at each carbon is predicted by the VSEPR method, but that all atoms be in the same plane is not so predicted. Bonding theory suggests that, after hybridizing sp2, the remaining p orbitals can be lined up, when the molecule is flat, to form a second bond between the two carbon atoms. This second bond looks like an elongated p orbital, and is not symmetrical about the bond axis. It is referred to as a pi (p) bond. This overlap to form a bonding mo when the molecule is flat means that to rotate one carbon relative to the other, changing the molecule from planar, requires the input of energy to take the two electrons in the bonding mo and put them into the higher energy atomic p orbitals. When this energy is not available (which it is not at room temperature) the bond rotation will not occur. This leads, in substituted molecules, to the possibility of two isomers differing only in the spacial location of the atoms bound to the two carbons.
An antibonding p orbital is also available, but since the two electrons can be accommodated in the bonding p orbital, it is unoccupied in the ground state.
Date created: 2005 06 09.