Organic Chemistry

24 CHAPTER 1 Electronic Structure and Bonding • Acids and Bases

p atomic orbital

of oxygen

p atomic orbital

of carbon

π∗ antibonding molecular orbital

π bonding molecular orbital

Energy

Figure 1.8N
Side-to-side overlap of a porbital of
carbon with a porbital of oxygen
to form a bonding molecular
orbital and a antibonding
molecular orbital.

p*

p

Now let’s look at the molecular orbital diagram for side-to-side overlap of a por-

bital of carbon with a porbital of oxygen—the orbitals are the same, but they belong

to different atoms (Figure 1.8). When the two patomic orbitals combine to form mo-

lecular orbitals, they do so unsymmetrically. The atomic orbital of the more electro-

negative atom contributes more to the bonding molecular orbital, and the atomic

orbital of the less electronegative atom contributes more to the antibonding molecular

orbital. This means that if we were to put electrons in the bonding MO, they would be

more apt to be around the oxygen atom than around the carbon atom. Thus, both the

Lewis theory and molecular orbital theory tell us that the electrons shared by carbon

and oxygen are not shared equally—the oxygen atom of a carbon–oxygen bond has a

partial negative charge and the carbon atom has a partial positive charge.

Organic chemists find that the information obtained from MO theory, where valence

electrons occupy bonding and antibonding molecular orbitals, does not always yield the

needed information about the bonds in a molecule. The valence-shell electron-pair

repulsion (VSEPR) modelcombines the Lewis concept of shared electron pairs and

lone-pair electrons with the concept of atomic orbitals and adds a third principle:the

minimization of electron repulsion. In this model, atoms share electrons by overlapping

ppp ppp

Enegry

σ

σ∗

π∗ π∗

ππ

Figure 1.7N
pOrbitals can overlap end-on to
form bonding and
antibonding molecular orbitals, or
can overlap side-to-side to form
bonding and antibonding
molecular orbitals. The relative
energies of the molecular orbitals
are s 6 p 6 p 6 s.

p*

p

s s*