FYUGP B.Sc Chemistry Semester 1: Unit 4 - Structure of Organic Molecules

This unit explores the nature of bonding in organic molecules, focusing on hybridization, valence bond theory, and molecular orbital theory. These notes are designed to help students understand how hybridization affects bond properties and molecular structure.

1. Nature of Bonding in Organic Molecules

Organic molecules are primarily held together by covalent bonds, which involve the sharing of electron pairs between atoms. The nature of these bonds is explained using:

• Valence Bond Theory (VB): Describes bonding as the overlap of atomic orbitals.
• Molecular Orbital Theory (MO): Describes bonding in terms of molecular orbitals formed by the combination of atomic orbitals.

2. Hybridization of Atomic Orbitals

Hybridization is the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding. Common types of hybridization include:

a. sp³ Hybridization

• One s orbital and three p orbitals combine to form four sp³ hybrid orbitals.
• Example: Methane (CH₄).
• Bond angle: 109.5° (tetrahedral geometry).

b. sp² Hybridization

• One s orbital and two p orbitals combine to form three sp² hybrid orbitals.
• Example: Ethene (C₂H₄).
• Bond angle: 120° (trigonal planar geometry).

c. sp Hybridization

• One s orbital and one p orbital combine to form two sp hybrid orbitals.
• Example: Ethyne (C₂H₂).
• Bond angle: 180° (linear geometry).

Fig 1: Types of Hybridization (sp³, sp², sp)

3. Valence Bond Theory (VB)

Valence Bond Theory explains bonding as the overlap of atomic orbitals. Key points:

• Sigma (σ) Bonds: Formed by head-on overlap of orbitals (e.g., sp³-sp³ overlap in ethane).
• Pi (π) Bonds: Formed by side-on overlap of p orbitals (e.g., p-p overlap in ethene).

4. Molecular Orbital Theory (MO)

Molecular Orbital Theory describes bonding in terms of molecular orbitals formed by the combination of atomic orbitals. Key points:

• Bonding Molecular Orbitals: Lower in energy than the original atomic orbitals.
• Antibonding Molecular Orbitals: Higher in energy than the original atomic orbitals.
• Example: In H₂, the 1s atomic orbitals combine to form σ_1s (bonding) and σ_1s^* (antibonding) molecular orbitals.

Fig 2: Molecular Orbitals in H₂

5. Effect of Hybridization on Bond Properties

Hybridization influences bond length, bond strength, and bond angle:

• Bond Length: sp³ > sp² > sp (sp hybridized bonds are the shortest).
• Bond Strength: sp > sp² > sp³ (sp hybridized bonds are the strongest).
• Bond Angle: Determined by the geometry of the hybrid orbitals (e.g., 109.5° for sp³, 120° for sp², 180° for sp).

Hybridization	Bond Length	Bond Strength	Bond Angle
sp3	Longest	Weakest	109.5°
sp2	Intermediate	Intermediate	120°
sp	Shortest	Strongest	180°

Practical Applications

• Predicting Molecular Geometry: Hybridization helps predict the shape of molecules, which is crucial for understanding reactivity and properties.
• Organic Synthesis: Knowledge of hybridization is essential for designing synthetic routes and understanding reaction mechanisms.
• Material Science: Hybridization influences the properties of materials, such as conductivity and strength.

These notes provide a comprehensive understanding of the structure of organic molecules and the role of hybridization in determining bond properties. Practice problems and molecular modeling exercises will help reinforce these concepts.