Pi bonds (π bonds) form through the sideways overlap of atomic orbitals, resulting in electron density concentrated above and below the plane of the bonding atoms.
Understanding Pi Bond Formation
A pi bond is a type of covalent chemical bond where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital. This overlap occurs laterally (side by side), in contrast to sigma bonds where orbitals overlap head-on. The key aspects of pi bond formation are:
- Side-by-Side Overlap: Atomic orbitals align parallel to each other and overlap laterally, creating regions of electron density above and below the internuclear axis.
- Electron Density Location: The electron density in a pi bond is concentrated in two lobes, one above and one below the plane containing the nuclei of the bonding atoms. As the reference states, a pi bond is formed by the overlap of orbitals in a side-by-side fashion with the electron density concentrated above and below the plane of the nuclei of the bonding atoms.
- Weaker than Sigma Bonds: Because the overlap is not as direct as in sigma bonds, pi bonds are generally weaker.
- Formation Alongside Sigma Bonds: Pi bonds usually form after a sigma bond has already formed between the same two atoms. This is why double and triple bonds exist; a double bond consists of one sigma and one pi bond, while a triple bond consists of one sigma and two pi bonds.
Example of Pi Bond Formation: Ethene (C₂H₄)
Let's consider ethene (C₂H₄) as an example:
- Each carbon atom forms three sigma bonds: one C-C sigma bond and two C-H sigma bonds. This uses three sp2 hybrid orbitals on each carbon.
- Each carbon atom has one unhybridized p orbital remaining.
- These two p orbitals overlap side-by-side, forming the pi bond. This restricts rotation around the C-C bond, leading to the planar geometry of ethene.
Characteristics of Pi Bonds
| Characteristic | Description |
|---|---|
| Type of Overlap | Side-by-side (lateral) |
| Strength | Weaker than sigma bonds |
| Electron Density | Concentrated above and below the internuclear axis |
| Occurrence | Typically forms after a sigma bond |
| Impact on Rotation | Restricts rotation around the bond axis |
| Examples | Double bonds (e.g., in alkenes), triple bonds (e.g., in alkynes), and aromatic rings (e.g., in benzene) |
