sigma bond

Examples of sigma bond in the following topics:

• Single Covalent Bonds

  • Single covalent bonds are sigma bonds, which occur when one pair of electrons is shared between atoms.
  • The strongest type of covalent bonds are sigma bonds, which are formed by the direct overlap of orbitals from each of the two bonded atoms.
  • Regardless of the atomic orbital type, sigma bonds can occur as long as the orbitals directly overlap between the nuclei of the atoms.
  • A single covalent bond can be represented by a single line between the two atoms.
  • These are all possible overlaps between different types of atomic orbitals that result in the formation of a sigma bond between two atoms.

• Covalent Bonds

  • Covalent bonding interactions include sigma-bonding (σ) and pi-bonding (π).
  • Sigma bonds are the strongest type of covalent interaction and are formed via the overlap of atomic orbitals along the orbital axis.
  • Single bonds occur when two electrons are shared and are composed of one sigma bond between the two atoms.
  • Double bonds occur when four electrons are shared between the two atoms and consist of one sigma bond and one pi bond.
  • Triple bonds occur when six electrons are shared between the two atoms and consist of one sigma bond and two pi bonds (see later concept for more info about pi and sigma bonds).

• Bonding in Coordination Compounds: Valence Bond Theory

  • Valence bond theory is used to explain covalent bond formation in many molecules.
  • The two types of overlapping orbitals are sigma (σ) and pi (π) orbitals.
  • Where bond order is concerned, single bonds are considered to be one sigma bond, double bonds are considered to contain one sigma and one pi bond, and triple bonds consist of one sigma bond and two pi bonds.
  • Sigma bonds occur when the like orbitals of shared electrons overlap.
  • For instance, when two s-orbital electrons overlap, we see a sigma bond.

• Atomic and Molecular Orbitals

  • When these bonding orbitals are occupied by a pair of electrons, a covalent bond, the sigma bond results.
  • Since bonds consisting of occupied π-orbitals (pi-bonds) are weaker than sigma bonds, pi-bonding between two atoms occurs only when a sigma bond has already been established.
  • Since carbon atoms involved in double bonds have only three bonding partners, they require only three hybrid orbitals to contribute to three sigma bonds.
  • Two sp2 hybridized carbon atoms are then joined together by sigma and pi-bonds (a double bond), as shown in part B.
  • Two p-orbitals remain unused on each sp hybridized atom, and these overlap to give two pi-bonds following the formation of a sigma bond (a triple bond), as shown below.

• Double and Triple Covalent Bonds

  • Double and triple bonds, comprised of sigma and pi bonds, increase the stability and restrict the geometry of a compound.
  • The overlap does not occur between the nuclei of the atoms, and this is the key difference between sigma and pi bonds.
  • Multiple bonds between atoms always consist of a sigma bond, with any additional bonds being of the π type.
  • The double bond between the two carbon atoms consists of a sigma bond and a π bond.
  • A triple bond involves the sharing of six electrons, with a sigma bond and two $\pi$ bonds.

• sp2 Hybridization

  • Ethene (C2H4) has a double bond between the carbons.
  • The three hybridized orbitals explain the three sigma bonds that each carbon forms.
  • The two carbon atoms form a sigma bond in the molecule by overlapping two sp2 orbitals.
  • Two sp2 hybrids bond with the hydrogen atoms, and the other forms a sigma bond with the other carbon atom.
  • Recognize the role of sp2 hybridized atoms in sigma and pi bonding.

• Explanation of Valence Bond Theory

  • There are two types of overlapping orbitals: sigma ($\sigma$) and pi ($\pi$).
  • $\sigma$ bonds occur when orbitals overlap between the nuclei of two atoms, also known as the internuclear axis.
  • Single bonds have one sigma bond.
  • Double bonds consist of one $\sigma$ and one $\pi$ bond, while triple bonds contain one $\sigma$ and two $\pi$ bonds.
  • In the F2 molecule, the F–F $\sigma$ covalent bond is formed by the overlap of pz orbitals of the two F atoms, each containing an unpaired electron.

• Grob Fragmentation

  • An interesting and generally useful skeletal transformation, involving specific carbon-carbon bond cleavage with accompanying conversion of certain sigma-bonds to pi-bonds, is known as the Grob fragmentation.
  • As background for discussing this reaction, it is helpful to define the concept of ethylogy, which may be regarded as the sigma-bond equivalent of vinylogy.
  • Whenever functional group interactions occur through a chain of covalent bonds (sigma or pi), stereoelectronic factors will play an important role.
  • A Grob fragmentation takes place in the top example, because the orbitals of the bonding and non-bonding electron pairs participating in the reaction are aligned properly.
  • These are the non-bonding pair on nitrogen and the bonding pairs in the green-colored covalent bonds.

• Hybridization in Molecules Containing Double and Triple Bonds

  • sp2, sp hybridizations, and pi-bonding can be used to describe the chemical bonding in molecules with double and triple bonds.
  • In ethylene (ethene), the two carbon atoms form a sigma bond by overlapping two sp2 orbitals; each carbon atom forms two covalent bonds with hydrogen by s–sp2 overlapping all with 120° angles.
  • The chemical bonding in acetylene (ethyne) (C2H2) consists of sp-sp overlap between the two carbon atoms forming a sigma bond, as well as two additional pi bonds formed by p-p overlap.
  • Each carbon also bonds to hydrogen in a sigma s-sp overlap at 180° angles.
  • In ethene, carbon sp2 hybridizes, because one π (pi) bond is required for the double bond between the carbons, and only three σ bonds form per carbon atom.

• Stereoselectivity in Addition Reactions to Double Bonds

  • As illustrated in the drawing below, the pi-bond fixes the carbon-carbon double bond in a planar configuration, and does not permit free rotation about the double bond itself.
  • We see then that addition reactions to this function might occur in three different ways, depending on the relative orientation of the atoms or groups that add to the carbons of the double bond: (i) they may bond from the same side, (ii) they may bond from opposite sides, or (iii) they may bond randomly from both sides.
  • Since initial electrophilic attack on the double bond may occur equally well from either side, it is in the second step (or stage) of the reaction (bonding of the nucleophile) that stereoselectivity may be imposed.
  • If the two-step mechanism described above is correct, and if the carbocation intermediate is sufficiently long-lived to freely-rotate about the sigma-bond component of the original double bond, we would expect to find random or non-stereoselective addition in the products.