What makes a pi bond?
Aug 26, 2022, 16:45 IST
Some characteristic features of p bonds:
Sideways overlap of the two p orbitals on adjacent carbon atoms leads to the formation of a pi (p) bond. The necessary and sufficient condition for an effective sideways overlap is the parallel orientation of the two p orbitals. This demands that, in ethene (H2C = CH2) molecule, all the atoms must be in the same plane so that both the p orbitals are mutually parallel and are perpendicular to the plane of the molecule.
Rotation of one CH2 bonding unit relative to other prevents the maximum possible overlap of p orbitals and, therefore, such a rotation about carbon–carbon double bond [C = C] is restricted.
Estimates based on thermochemical calculations indicate that the strength of the p bond is 264 kJ mol-1. This barrier to rotation of the double bond is markedly higher than the rotational barrier of groups joined by carbon–carbon single bonds (13 – 26 kJ mol-1).
While group joined by single bonds rotate relatively freely at room temperature, those joined by double bonds do not.
Consequently, a disubstituted ethene such as C2H2Br2 can exist in two different forms (I and II) as show below because of the restricted rotation around the C = C bond.
For More Chemistry Doubts visit main page of Physics Wallah.
