Rearrangement reaction

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A rearrangement reaction are organic reactions where the carbon skeleton of a molecule is rearranged. The result is a structural isomer of the original molecule.[1] Often a substituent moves from one atom to another atom in the same molecule. In the example below, the substituent R moves from carbon atom 1 to carbon atom 2:

General scheme rearrangement

Intermolecular rearrangements also take place.

Isochorismate Pyruvate Lyase converts Isochorismate into salicylate and Pyruvate

Sometimes chemists draw diagrams with arrows that show how electrons are transferred between bonds during a rearrangement reaction. Many organic chemistry text books have such diagrams. But they do not tell the full story of the reaction mechanism. The actual mechanism of a rearrangement with an alkyl group moving is for the group to slide smoothly along a bond, not ionic bond-breaking and forming. One example of this is the Wagner-Meerwein rearrangement:

Isoborneol Camphene Conversion

In pericyclic reactions, the orbital interactions are important. The reactions can not be explained by a series of simple discrete electron transfers. But, the curved arrows showing a sequence of discrete electron transfers can give the same result as a rearrangement reaction. Yet, the diagrams are not necessarily realistic. In allylic rearrangement, the reaction is ionic.

Three important rearrangement reactions are 1,2-rearrangements, pericyclic reactions and olefin metathesis.

1,2-rearrangements[change | edit source]

A 1,2-rearrangement is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift, the substituent moves between two adjacent atoms. But moves over larger distances are possible. Examples are the Wagner-Meerwein rearrangement:


and the Beckmann rearrangement:

Beckmann rearrangement

Pericyclic reactions[change | edit source]

A pericyclic reaction is a type of reaction with multiple carbon-carbon bond making and breaking. The transition state of the molecule has a cyclic geometry. The reaction progresses in a concerted fashion. Examples are hydride shifts

Sigmatropic Hydride Shifts

and the Claisen rearrangement:

Claisen rearrangement

Olefin metathesis[change | edit source]

Olefin metathesis is a formal exchange of the alkylidene fragments in two alkenes. It is a catalytic reaction with carbene. It has transition metal carbene complex intermediates.

General mechanism olefin metathesis

References[change | edit source]

  1. March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7