Organic Chemistry – Some Basic Principles and Techniques

Chemistry · Class 11

Simulation · Chemistry · Class 11

Counting σ and π bonds

From the lesson Carbon's tetravalence and the shapes of organic molecules in Organic Chemistry – Some Basic Principles and Techniques. Change the values and watch what happens.

Counting σ and π bondsChemistry · Class 11

The idea behind it

NCERT § "Tetravalence of Carbon: Shapes of Organic Compounds"

  • Carbon is tetravalent and forms covalent bonds using hybrid orbitals: sp³ (tetrahedral, as in CH₄), sp² (trigonal planar, as in C₂H₄) and sp (linear, as in C₂H₂).
  • s character is 25% in sp³, 33% in sp² and 50% in sp; more s character holds electrons closer to the nucleus.
  • An sp-hybridised carbon is therefore more electronegative than sp², which is more electronegative than sp³.
  • More s character also means shorter and stronger bonds formed by that carbon.
  • A π bond forms by sideways overlap of unhybridised p orbitals; the two atoms joined by it and the atoms attached to them must lie in one plane, and rotation about the double bond is restricted.
  • π electrons lie above and below the bond axis and are loosely held, so multiple bonds are reactive centres attacked by electron-seeking reagents.