Meso-ionic compounds

Publisher Summary This chapter reviews meso-ionic heterocycles. It discusses the currently known types of meso-ionic heterocycles, which can then be considered in relation to the total number of possibilities associated with certain structural conditions. A compound may be appropriately called meso-ionic if it is a five or possibly a six-membered heterocyclic compound that cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of electrons in association with the atoms comprising the ring. The ring bears a fractional positive charge balanced by a corresponding negative charge located on a covalently attached atom or group of atoms. The adoption of the term meso-ionic and its frequent use has demonstrated its usefulness. The five-membered mesoionic heterocycles can be divided into two general classes: type A and type B. This chapter is concerned with the currently known 44 members of type A and eight members of type B.