Summary of Valence Bond Theory for Organic Molecules

Learning Objectives:

• Understand hybridisation in multiple bonded molecules.
• Formation of σ (sigma) bonding frameworks.
• Formation of π (pi) bonding frameworks.

Hybridisation of Methane (CH₄):

• sp³ hybridisation leads to tetrahedral geometry.

Hybridisation of Boron Trifluoride (BF₃):

• sp² hybridisation results in trigonal planar geometry.

Multiple Bonds in Organic Chemistry:

• Carbon can form multiple bonds.
• Double Bonds:
• 4 bonding electrons (2 pairs).
• Requires sp² hybridisation (three orbitals at 120°).
• Ex: Ethene (C₂H₄): each carbon has sp² orbitals and one unhybridized p orbital.

Sigma (σ) and Pi (π) Bonds:

• σ bonds formed by overlapping hybrid orbitals (sp² + 1s for C-H bonds, sp² + sp² for C-C bonds).
• π bonds formed by sidewise overlap of unhybridised p orbitals.
• C=C double bond consists of one σ and one π bond; π bond is weaker and breaks more easily in reactions.

Hybridisation in Methanal (H₂C=O):

• sp² hybrid orbitals for C and O to form σ bonds and accommodate lone pairs.
• C forms 3 σ bonds and 1 π bond; O forms 1 σ and 1 π bond with two lone pairs.

Hybridisation of Ethyne (HC≡CH):

• sp hybridisation leads to linear geometry (two sets of electrons).
• Utilises 2s and one 2p orbital.

Practice Exercises Suggested:

• Analyze NH₃, H₂O, BeCl₂, CO₂, N₂, O₂ for hybridisation types and lone pairs.