A rotational barrier refers to the energy barrier that prevents a molecule from freely rotating around a bond. This phenomenon occurs due to the presence of steric hindrance, where atoms or groups of atoms in a molecule physically obstruct each other's movement. As a result, certain conformations of the molecule are favored over others, leading to distinct shapes and properties. In organic chemistry, rotational barriers are significant in determining the stability and reactivity of compounds. For example, in alkenes, the double bond creates a rotational barrier that restricts the rotation of the attached groups, influencing the molecule's overall geometry and behavior in chemical reactions.