Rearrangement reaction

A rearrangement reaction is an organic reaction where the carbon skeleton of a molecule is rearranged. The result is a structural isomer of the original molecule. 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:


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. 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
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. Moves over larger distances are possible. Examples are the Wagner-Meerwein rearrangement:

500px|Wagner-Meerwein

and the Beckmann rearrangement:

500px|Beckmann rearrangement

Pericyclic reactions
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

300px|Sigmatropic Hydride Shifts

and the Claisen rearrangement:

300px|Claisen rearrangement

Olefin metathesis
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.

600px|General mechanism olefin metathesis

References