Pericyclic reaction facts for kids

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Example of a pericyclic reaction the  Cyclohexatriene norcaradiene rearrangement

In organic chemistry, a pericyclic reaction is a type of chemical reaction between organic compounds. In the case of pericyclic reactions, the transition state of the molecule is a ring (has a cyclic geometry), and the reaction goes forward in a concerted way. Pericyclic reactions are usually rearrangement reactions. The most important groups of pericyclic reactions are:

In general, pericyclic reactions are equilibrium processes. However, it is possible to push the reaction in one direction if the product is at a significantly lower energy level. This is applying Le Chatelier's principle to a reaction involving a single molecule.

Many pericyclic reactions have similar stepwise radical processes connected with them. Chemists disagree whether some reactions are pericyclic reactions. For example, it is not definitively known whether the [2+2] cycloaddition mechanism is concerted (or may depend on the reactive system). Many pericyclic reactions have similar reactions that are metal-catalyzed. But these metal-catalyzed reactions are also not really pericyclic. The metal catalysts stabilize the reaction intermediates. So the reaction is not concerted, but rather metal-stabilized.

A large photoinduced hydrogen sigmatropic shift was utilized in a corrin synthesis performed by Albert Eschenmoser containing a 16π system.

Due to the principle of microscopic reversibility, there is a parallel set of "retro" pericyclic reactions, which perform the reverse reaction.

Pericyclic reactions in biochemistry

Pericyclic reactions also happen in several biological processes:

  • Claisen rearrangement of chorismate to prephenate in almost all prototrophic organisms.
  • [1,5]-sigmatropic shift when precorrin-8x is being changed into hydrogenobyrinic acid
  • non-enzymatic, photochemical electrocyclic ring opening and a (1,7) sigmatropic hydride shift in vitamin D synthesis.
  • a conversion of Isochorismate into salicylate and Pyruvate in a catalyzed, true pericyclic reaction.
Isochorismate Pyruvate Lyase converts Isochorismate into salicylate and Pyruvate

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