A pi bond (π bond) in organic chemistry is a type of covalent chemical bond where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital. This overlap occurs laterally, meaning it is perpendicular to the axis of the bond, which is usually defined by a sigma (σ) bond between the same two atoms.
Understanding Pi Bonds
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Formation: Pi bonds form from the sideways overlap of p orbitals (and sometimes d orbitals) on adjacent atoms.
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Relationship to Sigma Bonds: Pi bonds always occur in addition to a sigma bond. A single bond is a sigma bond, a double bond is a sigma bond plus one pi bond, and a triple bond is a sigma bond plus two pi bonds.
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Strength: Pi bonds are generally weaker than sigma bonds because the lateral overlap of p orbitals is less effective than the head-on overlap that forms sigma bonds. This weaker overlap makes pi bonds more reactive.
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Rotation: The presence of a pi bond restricts rotation around the bond axis. This is because rotation would require breaking the pi bond, which requires energy. This restriction is key to the cis/trans isomerism seen in alkenes.
Pi Bonds and Molecular Properties
Pi bonds significantly influence a molecule's properties, including:
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Shape: Pi bonds contribute to the geometry of molecules. For example, the presence of a pi bond in ethene (C₂H₄) forces the molecule to be planar.
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Reactivity: Pi bonds are regions of high electron density, making them susceptible to attack by electrophiles (electron-seeking species). This explains why alkenes and alkynes (molecules with pi bonds) are more reactive than alkanes (molecules with only sigma bonds).
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Spectroscopy: Pi bonds absorb light in the UV-Visible region of the electromagnetic spectrum. This property is utilized in techniques such as UV-Vis spectroscopy, which can identify the presence and quantity of molecules containing pi bonds.
Examples of Molecules with Pi Bonds
| Molecule | Structure | Type of Bonds |
|---|---|---|
| Ethene | H₂C=CH₂ | 1 σ bond, 1 π bond |
| Ethyne | HC≡CH | 1 σ bond, 2 π bonds |
| Benzene | C₆H₆ | 6 σ bonds, 6 π bonds delocalized across the ring |
In summary, a pi bond is a covalent bond formed by the sideways overlap of p-orbitals, existing in addition to a sigma bond, and influencing the shape, reactivity, and spectroscopic properties of organic molecules.
