The defining characteristic of an alkene molecule is the double bond. This bond is composed of a σ and a π covalent bond. Because π bonds are formed exclusively by the side‐to‐side overlap of atomic p orbitals, any rotation along the σ‐bond axis requires disruption of the π‐bond system. Breaking the π‐bond system requires an energy input of 63 kilocalories per mole. Due to this energy barrier, free rotation about the carbon‐carbon double bond is not possible. Multiple bonds between two atoms always lead to restricted rotation.
Compounds containing double bonds are said to be unsaturated because they are capable of reacting with hydrogen in the presence of a catalyst. The resulting compounds, which contain no multiple bonds, are said to be saturated. For each π bond destroyed, two σ bonds are formed, and energy is normally released. Thus chemists refer to a π bond as an element of unsaturation. Each element of unsaturation corresponds to two fewer hydrogen atoms than are found in the formula of the saturated compound of the same carbon‐chain length.