Why do double bonds restrict rotation in molecules?

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Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction. — covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.

Double bonds restrict rotation in molecules because they consist of both a sigma bond and a pi bond. While a sigma bond allows for free rotation, the pi bond is formed by electrons occupying orbitals located in regions parallel to the bond axis.12

Because the pi bond requires the alignment of these orbitals, rotating the atoms around the bond axis would disrupt the orbital overlap, which requires a significant amount of energy to break. Consequently, this structural constraint prevents free rotation and imparts rigidity to the molecule.13