Sigmatropic reactions are a type of chemical reaction where a sigma bond is formed or broken while a π bond is rearranged. These reactions typically involve the movement of atoms or groups within a molecule, leading to the formation of new structural isomers. They are often classified as pericyclic reactions, which means they occur in a concerted manner without intermediates. A common example of a sigmatropic reaction is the Cope rearrangement, where a 1,5-hexadiene undergoes a rearrangement to form a different isomer. Sigmatropic reactions are important in organic chemistry for synthesizing complex molecules and understanding reaction mechanisms.