Organic Chemistry: Molecular Orbitals

Section 1: Understanding Molecular Orbitals

Molecular orbitals are formed when atomic orbitals overlap during the process of bonding. Atomic orbitals in the same energy level combine to form molecular orbitals.
The Pauli Exclusion Principle states that at maximum, a molecular orbital can hold only two electrons. They also must have opposite spins.
Bonding molecular orbitals are lower in energy and more stable than the atomic orbitals from which they were formed.
Anti-bonding molecular orbitals are higher in energy and less stable than the atomic orbitals from which they were formed.

Section 2: Types of Molecular Orbitals

Sigma (σ) molecular orbitals occur when the overlap along the axis between the two bonding atoms. This type of bond is formed by either: end-to-end overlap, which is also known as axial overlap of s, pz or hybrid orbitals, known as the Sigma bond.
Pi (π) molecular orbitals occur when parallel atomic orbitals overlap. These are formed by the sidewise or lateral overlap of two p-orbital lobes, known as the Pi bond.
Delta (δ) molecular orbitals are much less common, occurring when four lobes of one filled orbital overlap four lobes of another filled orbital.

Section 3: Interpreting Molecular Orbital Diagrams

Molecular orbital diagrams visually represent the arrangement of molecular orbitals. Each orbital is represented by a line, with the energy level of the orbital indicated by the vertical position of the line.
In diagrams, if the atomic orbitals of two atoms have the same energy level or are ‘degenerate’, the resulting molecular orbitals will be one bonding orbital with lower energy and one anti-bonding orbital with higher energy.
If the atomic orbitals aren’t ‘degenerate’, or don’t have the same energy, then the molecular orbitals will also not have the same energy. The difference will be less than the difference in energy of the atomic orbitals.
The Aufbau Principle describes how electrons fill up atomic orbitals starting from the lowest energy state before moving to the next higher state. The same principle is applied in filling electrons in molecular orbital energy order.
Hund’s Rule states that each orbital in the same energy level will be filled with one electron first before electrons are paired up.

Section 4: Importance of Molecular Orbitals in Organic Chemistry

Molecular Orbitals help explain the chemical behaviour of molecules, particularly how they bond and their geometry.
Knowledge of molecular orbitals aids in understanding and predicting the outcomes of chemical reactions.
They also provide an understanding of the electron distribution in a molecule, which impacts a molecule’s polarity, reactivity, colour, magnetic properties, etc.