Sigmatropic reactions are a type of chemical reaction where a sigma bond (σ bond) is formed or broken while a π bond (pi bond) is rearranged. These reactions typically involve the migration of a substituent across a molecule, leading to a new structural arrangement. They are often classified as pericyclic reactions, which means they occur in a cyclic transition state. 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.