Шешілу-қосылу реакциялары: цикл құру және бұзу механизмі

Chemical reaction in which a ring is formed/broken by adding/removing a single atom
[[File:Introduction. png|thumb|300px|right|Example cheletropic reactions:
Case 1: the single atom is the carbonyl carbon (|C\dO) that ends up in carbon monoxide (|C\tO). Case 2: the single atom is the nitrogen atom in the diazenyl group (|N\dN), which ends up as dinitrogen (|N\tN). The above are known as cheletropic eliminations because a small, stable molecule is given off in the reaction. are a type of pericyclic reaction (a chemical reaction that involves a transition state with a cyclic array of atoms and an associated cyclic array of interacting orbitals). Specifically, cheletropic reactions are a subclass of cycloadditions. The key distinguishing feature of cheletropic reactions is that on one of the reagents, both new bonds are being made to the same atom.

In the pericyclic transition state, a small molecule donates two electrons to the ring. The reaction process can be shown using two different geometries, the small molecule can approach in a linear or non linear fashion. In the linear approach, the electrons in the orbital of the small molecule are pointed directly at the π system. In the non linear approach, the orbital approaches at a skew angle. The π system's ability to rotate as the small molecule approaches is crucial in forming new bonds. The direction of rotation will be different depending on how many π electrons are in the system. Shown below is a diagram of a two electron fragment approaching a four electron π system using frontier molecular orbitals. The rotation will be disrotatory if the small molecule approaches linearly and conrotatory if the molecule approaches non linearly. Disrotatory and conrotatory are sophisticated terms expressing how the bonds in the π system are rotating. Disrotatory means opposite directions while conrotatory means the same direction. This is also depicted in the diagram below. Using Hückel's rule, one can tell if the π system is aromatic or antiaromatic. If aromatic, linear approaches use disrotatory motion while non linear approaches use conrotatory motion. The opposite goes with an anti aromatic system. Linear approaches will have conrotatory motion while non linear approaches will have disrotatory motion.