The atoms in a single bond can rotate about the internuclear axis without breaking the bond. The atoms in a double and triple bond cannot rotate about the internuclear axis unless the bond is broken. Why?

A single bond between two atoms consists of a single sigma (σ) bond, which is formed by the overlap of two atomic orbitals - typically, two s-orbitals, two p-orbitals, or one s and one p-orbital. These orbitals contain two electrons, one from each atom, and form the basis for the bond between the atoms. A double bond comprises one sigma bond and one pi (π) bond, resulting from the overlap of two parallel p-orbitals. Since the pi bond exists above and below the internuclear axis, it creates a region of electron density that restricts the rotation of atoms around each other. A triple bond is formed by one sigma (σ) bond and two pi (π) bonds. Here, the overlap of three atomic orbitals forms a bond with six electrons distributed across three regions of high electron density. These regions act as barriers that prevent rotation without breaking the bond.

In a single bond, the electron density along the internuclear axis is uniformly distributed. The overlap of orbitals is direct, and the electron cloud is symmetrical, allowing the bond to rotate freely without breaking.

In double and triple bonds, since the pi bonds exist above and below the internuclear axis, they restrict the possibility of rotation around that axis without breaking the bond. In order to rotate, the electron cloud in the pi bonds would have to break and re-establish itself, breaking the bond in the process.

Another aspect to consider is the steric hindrance created by multiple bonds. In a molecule with multiple bonds, the atoms are held closer together due to the increased electron density. This closer proximity makes the atoms and their electron clouds more likely to interact, causing additional resistance to bond rotation. In summary, single bonds allow for free rotation around the internuclear axis due to their direct orbital overlap and symmetrical electron cloud along the axis. In contrast, double and triple bonds restrict rotation due to their pi bond electron densities above and below the internuclear axis, as well as the increased steric hindrance resulting from the closer atomic proximity within the bond.