pi bond

(noun)

covalent chemical interactions where two lobes of one involved atomic (p) orbital overlap two lobes of the other involved atomic (p) orbital

Related Terms

Examples of pi bond in the following topics:

  • 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.
    • Pi, or $\pi$, bonds occur when there is overlap between unhybridized p orbitals of two adjacent atoms.
    • A triple bond involves the sharing of six electrons, with a sigma bond and two $\pi$ bonds.
    • Triple bonds are stronger than double bonds due to the the presence of two $\pi$ bonds rather than one.
    • Overlap between adjacent unhybridized p orbitals produces a pi bond.

  • 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.
    • The pi bond between the carbon atoms forms by a 2p-2p overlap.
    • 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.
    • 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.
    • The remaining, non-hybridized p-orbitals overlap for the double and triple pi bonds.

  • Explanation of Valence Bond Theory

    • There are two types of overlapping orbitals: sigma ($\sigma$) and pi ($\pi$).
    • $\pi$ bonds occur when two (unhybridized) p-orbitals overlap.
    • The p-orbitals, in one $\pi$ bond, are located above and below the nuclei of the atoms.
    • By occupying the region of space that is above, below, and on the sides of an atom's nuclei, two $\pi$ bonds can form.
    • Double bonds consist of one $\sigma$ and one $\pi$ bond, while triple bonds contain one $\sigma$ and two $\pi$ bonds.

  • Covalent Bonds

    • Covalent bonding interactions include sigma-bonding (σ) and pi-bonding (π).
    • Pi bonds are a weaker type of covalent interactions and result from the overlap of two lobes of the interacting atomic orbitals above and below the orbital axis.
    • 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).
    • Unlike an ionic bond, a covalent bond is stronger between two atoms with similar electronegativity.

  • 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.

  • Bonding in Coordination Compounds: Valence Bond Theory

    • Valence bond theory is used to explain covalent bond formation in many molecules.
    • Valence bond theory is a synthesis of early understandings of how chemical bonds form.
    • Lewis proposed that the basis of chemical bonding is in the ability of atoms to share two bonding electrons.
    • 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.

  • Configurational Stereoisomers of Alkenes

    • The carbon-carbon double bond is formed between two sp2 hybridized carbons, and consists of two occupied molecular orbitals, a sigma orbital and a pi orbital.
    • Rotation of the end groups of a double bond relative to each other destroys the p-orbital overlap that creates the pi orbital or bond.
    • Because the pi bond has a bond energy of roughly 60 kcal/mole, this resistance to rotation stabilizes the planar configuration of this functional group.
    • In the first example, the left-hand double bond carbon has two identical substituents (A) so stereoisomerism about the double bond is not possible (reversing substituents on the right-hand carbon gives the same configuration).
    • Since alkynes are linear, there is no stereoisomerism associated with the carbon-carbon triple bond.

  • Atomic and Molecular Orbitals

    • 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.
    • Thus, pi-bonding is generally found only as a component of double and triple covalent bonds.
    • Two sp2 hybridized carbon atoms are then joined together by sigma and pi-bonds (a double bond), as shown in part B.
    • In both these cases three 2p atomic orbitals combine to form a sigma and two pi-molecular orbitals, each as a bonding and antibonding pair.
    • 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.

  • Properties of Aromatic Compounds

    • Aromatic compounds are ring structures with delocalized $\pi$ electron density that imparts unusual stability.
    • They are often represented as resonance structures containing single and double bonds.
    • Aromatic compounds are cyclic structures in which each ring atom is a participant in a$\pi$ bond, resulting in delocalized $\pi$ electron density on both sides of the ring.
    • Due to this connected network of $\pi$ bonds, the rings are planar, unlike the boat or table structures typical of cycloalkanes.
    • The reaction preserves the pi system of electrons and therefore the aromatic character of the benzene ring.

  • The Fisher Effect

    • We can use the bond market to show the Fisher Effect.
    • If the investors and businesses expect higher inflation in the future, then investors buy fewer bonds while businesses sell more bonds.
    • Consequently, the demand for bonds shifts toward the left while the supply for bonds shifts rightward.
    • We show the impact on the bond market in Figure 9.
    • Thus, the greater inflationary expectations cause greater bonds prices and lower bond interest rates as we discount bond prices.