To determine the number of electrons in a pi system, you need to count the electrons contributed by pi bonds and lone pairs participating in resonance within the system.
Understanding Pi Systems
A pi system is a region of overlapping p-orbitals in a molecule, extending laterally, that creates a space where electrons can delocalize. This delocalization contributes to the molecule's stability and reactivity. These systems are commonly found in alkenes, alkynes, aromatic compounds, and molecules containing conjugated double bonds.
Steps to Determine Pi Electrons
- Identify Pi Bonds: Each double bond (π bond) contains 2 pi electrons. Triple bonds contain 2 pi electrons (the remaining 2 electrons form a sigma bond). Count the total number of pi bonds within the molecule or ring.
- Identify Lone Pairs Involved in Resonance: Lone pairs on atoms adjacent to pi bonds can participate in resonance and contribute to the pi system. Only lone pairs in p-orbitals that can overlap with the adjacent pi system should be counted. A lone pair contributes 2 pi electrons.
- Sum the Electrons: Add the number of electrons from the pi bonds and the contributing lone pairs to find the total number of electrons in the pi system.
Examples
- Ethene (Ethylene): Contains one double bond. Therefore, it has 2 pi electrons.
- Benzene: Contains three double bonds. Therefore, it has 6 pi electrons.
- Aniline (Phenylamine): Contains three double bonds in the benzene ring (6 pi electrons) and the nitrogen atom has a lone pair that can participate in resonance with the ring, contributing 2 more pi electrons. Therefore, the pi system contains 8 electrons.
- Furan: Contains two double bonds (4 pi electrons). The oxygen atom has two lone pairs. One of these lone pairs is involved in the pi system, contributing 2 pi electrons. Therefore, Furan has a total of 6 pi electrons.
Table Summary
| Feature | Contribution to Pi System |
|---|---|
| Double Bond | 2 pi electrons |
| Lone Pair (in resonance) | 2 pi electrons |
By following these steps, you can accurately determine the number of electrons participating in the pi system of a molecule.
