HYBRIDIZATION

Hybridization

Covalent bonds are formed when atomic orbital overlap.  There are two types of orbital overlap. Sigma, s, overlap occurs when there is one bonding interaction that results from the overlap of two orbital.  Pi, p, overlap occurs when two bonding interactions result from the overlap of orbital.

sigma and pi bond

The organic chemist also needs to realize how these orbital overlaps relate to the type of bonding that is occurring between atoms:

Single bond      s overlap
double bond     s and p overlaps 

triple bond       s and two p overlaps

If one tries to correlate the overlap of atomic orbital to the shape of a molecule, however, the expected geometry does not correspond to a maximum orbital overlap.   Take a look at methane, CH_4. VSEPR predicts a tetrahedral geometry about the carbon atom but this is not achieved when one considers a maximum orbital overlap between four 1s orbital of H and the 2s, 2p_x, 2p_y and 2p_z orbital of carbon. 

Hybridization is a solution to this problem.  It is the imaginary mixing of the 2s, 2p_x, 2p_y and 2p_z atomic orbital of carbon to form a new set of 'hybrid' orbital that orient themselves in the desired VSEPR geometry.  The hybrid orbital are equivalent to one another making all orbital overlaps equivalent, therefore, all C-H bonding interactions equivalent.

Hybrid orbitals are named by considering the type and number of atomic orbitals from which they arose.  For CH₄ then the hybridization for the carbon is sp³.   One sees that the hybridization of an atom can be determined very quickly by considering the number of electron groups about an atom.  Hybrid orbitals are responsible for all the s bonding overlaps in a molecule. Unhybridised orbitals are responsible for all the p bonding overlaps in a molecule.

 
  You should be able to predict the hybridization of all non-hydrogen and non-terminal atoms in a molecule and draw the bonding interactions in that molecule based on the hybridization model.

There are three Types of hybridization that are important in organic chemistry.

sp hybridization

Type of hybrid	Diagram	Atomic orbitals used	Number of hybrid orbitals formed	Number of atoms bonded to the C	Geometry
s		s, p	2	2	linear

There is still a third type of hybridization, it is sp hybridization. In it the s orbital and one of the p orbitals from carbon's second energy level are combined together to make two hybrid orbitals. Those hybrid orbitals form a straight line. There is a 180 degree angle between one orbital and the other orbital. They are exactly opposite one another from the center of the carbon atom. Because this type of sp hybridization only uses one of the p orbitals, there are still two p orbitals left which the carbon can use. Those p orbitals are at right angles to one another and to the line formed by the hybrid orbitals. If they were shown in this diagram, one would go straight up and down and the other would go straight toward and away from you. This kind of hybridization occurs when a carbon atom is bonded to two other atoms.

A very important aspect of sp² and sp hybridization is that they allow carbon atoms to form double and triple bonds. Let's look at those next.

 

 

 

sp² hybridization

Type of hybrid	Diagram	Atomic orbitals used	Number of hybrid orbitals formed	Number of atoms bonded to the C	Geometry
sp2		s, p, p	3	3	flat triangular

Another kind of hybridization uses the s orbital and two of the p orbitals from the second energy level of carbon to form three hybrid orbitals. This kind of hybridization is called sp² hybridization. It has three hybrid orbitals and there is also an unchanged p orbital that is not shown here. The geometric arrangement of these three sp² hybrid orbitals is in a flat plane with 120 degree angles between them. The leftover p orbital lies at a 90 degree angle to the hybrid orbitals. If it had been included in this diagram, its two lobes would be pointing directly at and away from you. This kind of hybridization occurs when a carbon atom is bonded to three other atoms. If it is a very simple molecule with just the carbon atom and the other three atoms, it would be a flat triangular molecule. If this is part of a larger molecule, this part would have a flat triangular shape.

sp³ hybridization

Type of hybrid	Diagram	Atomic orbitals used	Number of hybrid orbitals formed	Number of atoms bonded to the C	Geometry
sp3		s, p, p, p	4	4	tetrahedral

The first is the sp³ hybridization that we have already talked about. sp³ hybridization results from the combination of the s orbital and all three p orbitals in the second energy level of carbon. It results in four hybrid orbitals and occurs when a carbon atom is bonded to four other atoms. The geometric arrangement of those four hybrid orbitals is called tetrahedral