Why free rotation occurs in sp2 hybridization?
Hybridization is the restructuring of atomic orbitals necessary for formation of molecules. Through this, we end up with hybridized orbitals that have a different allocation of electrons than the original orbitals. For example, in methane, we have 4 equivalent orbitals which are formed by excitation of 1 electron from s orbital to p orbital. This results in sp3 hybridization that has 4 hybridized orbitals, each of which contains 1 electron. These electrons can each combine with 1 hydrogen atom to form methane. Similarly, there are sp2 and sp hybridizations as well.
There is no free rotation in case of sp2 hybridization or even in sp hybridization. It is only possible in case of sp3 hybridization. The reason is simple, sigma bonds are unaffected by rotation, while pi-bonds are not. Pi-bonds are formed in case of double (contains 1 sigma and 1 pi bond) and triple (contains 1 sigma and 2 pi bonds) bonds or, in other words, in case of sp2 and sp hybridization. This is the reason, compounds like ethene and ethyne have no free rotation.
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