Pi Bond

Definition

A pi (π) bond is a type of covalent chemical bond formed by the side-to-side overlap of atomic orbitals, resulting in electron density concentrated above and below the internuclear axis between two atoms. Pi bonds are crucial in the structure and reactivity of many organic compounds.

5 Must Know Facts For Your Next Test

1. Pi bonds are formed by the side-to-side overlap of p orbitals, resulting in electron density above and below the internuclear axis.
2. In sp$^2$ hybridized atoms, such as in ethylene, the carbon atoms form three sigma bonds and one pi bond.
3. In sp hybridized atoms, such as in acetylene, the carbon atoms form two sigma bonds and two pi bonds.
4. Molecular orbital theory describes pi bonds as being formed by the constructive interference of p orbitals, creating a region of high electron density between the bonded atoms.
5. The addition of HBr to ethylene is an example of a polar reaction, where the pi bond is broken, and a new sigma bond is formed between the carbon and bromine atoms.

Review Questions

• Explain the role of pi bonds in the structure and stability of alkenes, such as ethylene.
  • In ethylene, the carbon atoms are $sp^2$ hybridized, forming three sigma bonds and one pi bond. The pi bond is formed by the side-to-side overlap of the p orbitals on the carbon atoms, resulting in a region of high electron density above and below the internuclear axis. This pi bond contributes to the planar structure and increased stability of the alkene, as the pi electrons are delocalized and can be easily involved in various chemical reactions, such as electrophilic addition reactions.
• Describe how the presence of pi bonds in alkynes, such as acetylene, affects their molecular structure and reactivity.
  • In acetylene, the carbon atoms are $sp$ hybridized, forming two sigma bonds and two pi bonds. The two pi bonds, formed by the side-to-side overlap of the p orbitals, contribute to the linear structure of the molecule and increase its reactivity. The presence of the pi bonds makes alkynes more reactive than alkanes, as the pi bonds can be easily broken and involved in addition reactions, such as the addition of HX or $X_2$, where the pi bonds are converted to sigma bonds.
• Analyze the role of pi bonds in the mechanism of electrophilic addition reactions of alkenes, such as the addition of HBr to ethylene.
  • In the addition of HBr to ethylene, the electrophilic bromine atom first attacks the pi bond of the alkene, breaking the pi bond and forming a carbocation intermediate. The negatively charged bromine atom then attacks the carbocation, forming a new sigma bond between the carbon and bromine atoms. This mechanism, which involves the breaking of the pi bond and the formation of new sigma bonds, is characteristic of electrophilic addition reactions of alkenes and is crucial in understanding the reactivity and products of these important organic transformations.

Related terms

Sigma Bond: A sigma (σ) bond is a type of covalent chemical bond formed by the head-to-head overlap of atomic orbitals, resulting in electron density concentrated along the internuclear axis between two atoms.

Hybridization: Hybridization is the mixing of atomic orbitals to form new hybrid orbitals, which are used to describe the bonding in molecules and polyatomic ions.

Molecular Orbital Theory: Molecular orbital theory is a model that describes the behavior of electrons in molecules, where electrons are considered to occupy molecular orbitals rather than being localized between individual atoms.