Q.What is a pi (pi) bond?

A pi bond arises from the sideways overlap of parallel p-orbitals, giving electron density above and below (not along) the bond axis, and is the second/third bond present in double/triple bonds.

When two atoms bond, they can form two distinct types of covalent bonds based on how their atomic orbitals overlap:

• Sigma (sigma) bond: formed by head-on (axial) overlap of orbitals directly along the line joining the two nuclei (the internuclear axis). This is always the first bond formed between any two directly bonded atoms, and it is generally the stronger of the two bond types due to greater orbital overlap.

• Pi (pi) bond: formed by sideways (lateral) overlap of two parallel, unhybridised p-orbitals on the two bonded atoms, with each p-orbital lobe lying perpendicular to (above and below) the internuclear axis. The resulting electron density is concentrated in two regions, one above and one below the sigma-bond axis, rather than directly between the nuclei.

Key properties of a pi bond:

1. It is always found in ADDITION to a sigma bond between the same two atoms — never alone. A double bond consists of one sigma + one pi bond; a triple bond consists of one sigma + two pi bonds.
2. Because the sideways overlap is less effective than head-on overlap, a pi bond is generally WEAKER than a sigma bond and more easily broken (which is why alkenes/alkynes are more reactive than alkanes, tending to undergo addition reactions at the pi bond). …