in the pericyclic transition state, a small molecule donates two electrons to the ring. two possible geometries can explain the reaction. the small molecule can approach in either a linear or non-linear way. in the linear approach, the electrons in the orbital of the small molecule are pointed directly at the π system of the big molecule. in the non-linear approach, the orbital approaches at a slightly off angle. the π-systems 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. a diagram of a two-electron fragment approaching a four-electron π-system using frontier molecular orbitals is shown below. the rotation will be disrotatory if the small molecule approaches linearly and conrotatory if the molecule approaches non-linearly. disrotatory and conrotatory tell how the bonds in the π-system are rotating. disrotatory means opposite directions while conrotatory means the same direction. this is also shown in the diagram below.