Sigma Bond

Definition

A sigma bond is a type of covalent chemical bond formed by the overlap of atomic orbitals along the internuclear axis between two atoms. Sigma bonds are the strongest type of covalent bonds and are responsible for the structural stability and geometry of molecules.

5 Must Know Facts For Your Next Test

1. Sigma bonds are formed by the head-on overlap of atomic orbitals, resulting in a high electron density along the internuclear axis.
2. The strength of a sigma bond is directly related to the degree of overlap between the participating atomic orbitals.
3. Sigma bonds can be formed by the overlap of s-s, s-p, or p-p atomic orbitals, depending on the hybridization of the atoms involved.
4. In the context of molecular geometry, sigma bonds determine the overall shape and structure of molecules, such as the tetrahedral structure of methane (CH4).
5. Sigma bonds are essential in the formation of alkanes, alkenes, and alkynes, as they provide the structural framework for these hydrocarbon compounds.

Review Questions

• Explain the role of sigma bonds in the structure of ethane (C2H6), a molecule with sp3 hybridized carbon atoms.
  • In ethane (C2H6), each carbon atom is sp3 hybridized, meaning it has four sigma bonds. The sigma bonds are formed by the head-on overlap of the sp3 hybridized orbitals, resulting in a tetrahedral arrangement of the bonds around each carbon. This tetrahedral geometry, stabilized by the sigma bonds, gives ethane its characteristic structure and contributes to the overall stability of the molecule.
• Describe how sigma bonds influence the cis-trans isomerism observed in alkenes, such as 2-butene.
  • Alkenes, like 2-butene, have a carbon-carbon double bond composed of one sigma bond and one pi bond. The sigma bond is formed by the head-on overlap of the sp2 hybridized orbitals, while the pi bond is formed by the side-to-side overlap of the remaining p orbitals. The presence of the rigid sigma bond, in addition to the pi bond, allows for the formation of cis-trans isomers, where the substituents can be positioned on the same side (cis) or opposite sides (trans) of the double bond. This isomerism is a direct consequence of the structural constraints imposed by the sigma bonds in alkenes.
• Analyze the role of sigma bonds in the context of molecular orbital theory and their influence on the stability and reactivity of organic compounds.
  • According to molecular orbital theory, sigma bonds are formed by the constructive interference of atomic orbitals, resulting in a high electron density along the internuclear axis. This high electron density contributes to the overall stability of the molecule by lowering the potential energy of the system. The strength and directionality of sigma bonds also play a crucial role in determining the reactivity of organic compounds. For example, the presence of sigma bonds in alkanes makes them relatively unreactive towards electrophilic addition reactions, whereas the presence of sigma bonds in alkenes and alkynes allows for their characteristic reactivity towards electrophilic addition and substitution reactions. The understanding of sigma bonds within the framework of molecular orbital theory is essential for predicting and explaining the chemical behavior of organic compounds.

Related terms

Covalent Bond:

A chemical bond formed by the sharing of one or more pairs of electrons between two atoms.

Atomic Orbitals:

The wave functions that describe the behavior of electrons in an atom, such as s, p, d, and f orbitals.

Hybridization:

The mixing of atomic orbitals to form new, equivalent orbitals with different shapes and energies.