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Pericyclic Reaction wiki intro by: JYOTI RAJPUT


 Pericyclic reactions : electrocyclisation, cycloaddition, sigmatropic rearrangements and other
 related concerted reactions. Principles and applications of photochemical reactions in organic chemistry.
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In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions. The major classes of pericyclic reactions are:

An electrocyclic reaction is the concerted cyclization of a conjugated π-electron system by converting one π-bond to a ring forming σ-bond. The reverse reaction may be called electrocyclic ring opening. Two examples are shown on the right. The electrocyclic ring closure is designated by blue arrows, and the ring opening by red arrows. Once again, the number of curved arrows that describe the bond reorganization is half the total number of electrons involved in the process.

Cycloaddition

cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity."[1] The resulting reaction is acyclization reaction. Many but not all cycloadditions are concerted. As a class ofaddition reaction, cycloadditions permit carbon–carbon bond formation without the use of a nucleophile or electrophile.
Cycloadditions can be described using two systems of notation. An older, but still common, notation is based on the size of linear arrangements of atoms in the reactants. It uses parentheses: (i + j + …) where the variables are the numbers of linear atoms in each reactant. The product is a cycle of size (i + j + …). In this system, the standardDiels-Alder reaction a (4 + 2)cycloaddition, the1,3-dipolar cycloaddition is a (3 + 2)cycloaddition and cyclopropanation of a carbene with an alkene a (2 + 1)cycloaddition.[1]
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Sigmatropic reaction

sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular process.[1] The name sigmatropic is the result of acompounding of the long-established sigma designation from single carbon–carbon bonds and the Greek word tropos, meaning turn. In this type of rearrangement reaction, asubstituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions are usually uncatalyzed, although Lewis acid catalysis is possible. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangementClaisen rearrangementCarroll rearrangement and the Fischer indole synthesis.

Group transfer reaction

                             The ene reaction


In organic chemistry, a group transfer reaction is a pericyclic process where one or more groups of atoms is transferred from amolecule to another. Unlike other pericylic reaction classes, group transfer reactions do not have a specific conversion of pi bondsinto sigma bonds or vice versa. Group transfer reactions are less frequently encountered processes and they can be sometimes confused with Diels–Alder cycloadditions. Like all pericyclic reactions, they must obey the Woodward–Hoffmann rules.[1]
The best known group transfer reaction is theene reaction.
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DESH DEEPAK A P S CHAUHAN

M.Sc. CHEMISTRY!

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