Valence & Hybridization (II)
In fact, there is only one isomer with the molecular formula CH2Cl2, and carbon has been confirmed to be tetrahedral, using modern x-ray diffraction methods. The observed tetrahedral geometry, however, does not agree with the prediction based on the orbital description, which would seem to predict a pyramidal structure. The explanation which is commonly given for this is that the four orbitals around carbon hybridize to form four equivalent orbitals having 75% p-character and 25% s-character. The resulting geometry is predicted to be tetrahedral and the driving force is electronic repulsion; placing the four orbitals in tetrahedral geometry provides the maximum separation between the electron pairs and minimizes electronic repulsion. A hybrid consisting of one s- and three p-orbitals is termed an sp3 hybrid and an sp3 center should always be considered to approximate tetrahedral geometry, with the overriding factor being the driving force for the molecule to assume the lowest energy geometry, which is readily accessible. Hydrocarbons containing only sp3 carbons are called alkanes and represent the simplest and least reactive class of organic compounds.
The driving force for "hybridization" is the formation of a bonding geometry with the lowest net potential energy, which is accessible by energetically feasible means. The tendency of molecules to seek to form the lowest energy structures (i.e., the most stable reaction intermediate) is a commonly occurring theme in organic reactions and a knowledge of intermediate stability will often allow the direction of a reaction pathway to be accurately predicted. Two additional orbital hybrids are also available for carbon; one in which the 2s-orbital combines with two of the available p-orbitals (sp2) and a second in which the 2s combines with one p (sp).
