ELECTROCYCLIC - REACTIONS

The better ways to understanding how orbitals symmetry affects the pericyclic reactions is.? For some examples, let's see first at a group of polyene rearrangements is called  "electrocyclic reaction" . The electrocyclic reaction is a pericyclic process that's involving the cyclization of a conjugated acyclic polyene. One (pi) π bond is broken, the other π bonds change position, a new σ bond is formed, and a cyclic compounds results. For example, a conjugated triene can be converting into a cyclohexadiene, and a conjugated diene can be converting into a cyclobutene.

Actually pericyclic reactions are reversible, and the position of the equilibrium depending on the specific case. In generally, the triene ⇔ cyclohexadiene and equilibrium favors by the cyclic product, whereas the diene ⇔cyclobutene equilibrium favors the lesser strained open-chain product.

                        The most striking of features of electrocyclic reactions is their stereochemistry. For example, (2E, 4Z, 6E)- 2, 4, 6 - octatriene yieldd only cis- 5, 6 - dimethyl - 1,3 - cyclohexadiene when heated and (2E, 4Z, 6Z) - 2, 4, 6- octatriene yields only trans - 5,6- dimethyl - 1,3 cyclohexadiene. However, remarkably, the streochemical results change to completely when the reaction is carried out and under what are called, photochemical, rather than the thermal, conditions. Photolysis or irradiation of (2E,4Z,6E) - 2, 4, 6- octatriene with UV light yields trans-5,6- dimethyl - 1,3 cyclohexadiene.

Electrocyclic interconversion of 2, 4, 6- both

Octatriene and 5,6- dimethyl - 1,3- cyclohexadiene isomers.

                       The result is obtained for the thermal electrocyclic ring-opening of 3,4-dimethylcyclobutene. The trans isomers the yields  only that (2E,4E)-2, 4- hexadiene, when heating and the cis isomers yields only (2E,4Z) - 2,4- hexadiene. On ultra voilet irradiation, however, the results are opposite side. The Cyclization of 62E, 4E isomers under the photochemical conditions yields cis product.

Electrocyclic interconversion of 2,4- hexadiene isomers and 3,4- dimethylcyclobutene isomers. 

 we can see that the two outermost lobes of the polyene MOs- the lobes that the interact when the cyclization occurred. These are two possibilities : lobes of like sign can be either on the same side or on opposite sides of the molecule. 

A bond to form, the outermost π electrons lobes must be rotate so that favorable bonding interaction is achieved - a position lobe with a positive lobes or a negative lobes with a negative lobe. If two lobes of signs, on the same side of the molecule, the two orbitals must be rotate in opposite direction - one clockwise and another one anti-clockwise. This kind of motion is referring to as " disrotatory" 

If the lobes of signs, on the opposite sides of the molecule, both orbitals must rotate in the same direction, either both clockwise or both anti-clockwise. This kinds of motion is called " conrotatory"