Benzene orbitals

Orbitals in a six-membered aromatic ring.

One very important MCAT context for orbital theory is in understanding the hybrid orbitals of organic chemistry, their spatial geometry and energy characteristics. In forming chemical bonds, atomic orbitals undergo both inter- and intra-molecular orbital hybridization.

Intermolecular orbital hybridization occurs between orbitals of different atoms.

Intramolecular orbital hybridization occurs between orbitals of the same atom.

In organic chemistry, intra-molecular orbital hybridization (sp3, sp2, etc.) plays a roll in determining the structural and chemical properties of the functional groups.

The π orbitals of an extended sequence of sp2 hybridized atoms may combine to form a π system, wherein electrons are delocalized through orbital hybridization over a large number of atoms. Such resonant forms are lower energy, i.e. more stable than non-conjugated systems.

Why is that so. A simplistic way of looking at it is to say that extended conjugation allows electron charge to spread out and decrease in potential energy, a simplistic, but useful way of looking at it.

A more complete way is to say that in addition to the decrease in electrostatic potential energy, there is also greater 'quantum multiplicity' in extended hybrid orbitals.

For both of these reasons, resonance hybridization decreases the free energy of a molecular form, making it more stable.