Stereospecificity of Ring-opening Reactions
The rules for the conrotation or disrotation of a ring-closing reaction are different than for a ring-opening reaction. In effect, each entry of the table is switched from conrotatory to disrotatory, and vice versa.
If the reaction shown above (octatriene to cis-cyclohexadiene) was performed in reverse, then cis-cyclohexadiene would undergo a ring-opening reaction. Examining the rules for ring-openings, the thermal reaction of a 4n molecule would proceed through a disrotatory mechanism. Further analysis shows that the product would be (trans,cis,trans)-octa-2,4,6-triene. This demonstrates how these reactions are actually an equilibrium between the cyclic and acyclic molecules.
| System | Thermally Induced (ground state) | Photochemically Induced (excited state) |
|---|---|---|
| "4n" electrons | Disrotatory | Conrotatory |
| "4n + 2" electrons | Conrotatory | Disrotatory |
Read more about this topic: Electrocyclic Reaction
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