If two atoms covalently bonded to each other in an organic molecule can be rotated around each other in the same or opposite direction up to 360° without weakening or breaking the bond, there is said to be free rotation around the bond.  There is free rotation around a bond formed by linear overlap.  Thus, there is free rotation around a sigma bond.  There is no free rotation around a bond formed by parallel overlap.  Thus, there is no free rotation around a pi bond.  Consequently, there is free rotation around a single bond; there is no free rotation around an isolated multiple bond.

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