Sigmatropic rearrangements are a type of chemical reaction where a molecule undergoes a structural change involving the migration of a sigma bond and the reorganization of π bonds. This process typically occurs in a concerted manner, meaning that the bond-breaking and bond-forming steps happen simultaneously without intermediates. These rearrangements are classified based on the number of atoms involved in the migration, such as 1,3-sigmatropic or 1,5-sigmatropic rearrangements. They are important in organic chemistry for synthesizing complex molecules and understanding reaction mechanisms, often leading to the formation of new functional groups or structural isomers.