Valence Bond Theory - Pi Framework
Overview
In Valence Bond (VB) Theory, pi ($\pi$) bonds form from the sideways overlap of atomic orbitals, contributing to double and triple bonds in molecules.
Formation of Pi Bonds
- Unlike sigma ($\sigma$) bonds, $\pi$ bonds form from parallel overlap of unhybridized p orbitals.
- Occurs in double and triple bonds, where one bond is a sigma bond and the remaining are $\pi$ bonds.
Examples
| Molecule | Bond Type | Sigma Bonds | Pi Bonds |
|---|---|---|---|
| Ethene ($C_2H_4$) | Double bond (C=C) | 1 | 1 |
| Ethyne ($C_2H_2$) | Triple bond (C≡C) | 1 | 2 |
| Benzene ($C_6H_6$) | Delocalized $\pi$ bonds | 6 $\sigma$ | 3 $\pi$ (delocalized) |
Properties of Pi Bonds
- Weaker than sigma bonds due to lesser orbital overlap.
- Restrict rotation around the bond axis, leading to cis-trans isomerism.
- Found in conjugated systems, contributing to delocalization (e.g., benzene, aromatic compounds).
Bonding in Conjugated Systems
- Delocalized $\pi$ bonds occur when p orbitals overlap across multiple atoms.
- Example: Benzene ($C_6H_6$) has alternating single and double bonds, forming a resonance structure.
Applications
- Explains rigidity in molecules (e.g., why ethene cannot freely rotate).
- Helps in predicting reactivity (e.g., electrophilic addition in alkenes, resonance stability in aromatic rings).
- Important in organic chemistry and material science (e.g., conductive polymers).