Rearrangement reaction

(nuclear physics) A nuclear reaction in which nucleons are exchanged between nuclei.
(organic chemistry) A chemical reaction involving a change in the bonding sequence within a molecule. Also known as molecular rearrangement.

A reaction in which an atom or bond moves or migrates, having been initially located at one site in a reactant molecule and ultimately located at a different site in a product molecule. A rearrangement reaction may involve several steps, but the key feature defining it as a rearrangement is that a bond shifts from one site of attachment to another. The simplest examples of rearrangement reactions are intramolecular, that is, reactions in which the product is simply a structural isomer of the reactant [reaction (1)].
1

More complex rearrangement reactions occur when the rearrangement is accompanied by another reaction, for example, a substitution reaction (2).
2

Rearrangement reactions are classified and named on the basis of the group that migrates and the initial and final location of the migrating bond. The initial bond location is designated as position 1, and the final location as position i, where the number of atoms is simply counted along the connection from 1 to i. Such a migration is called a [1,i] rearrangement or [1,i] shift. If the migrating group also reattaches itself at a different site from the one to which it had originally been attached, then both shifts are indicated, as in [i,j] shift. Reaction (1) is an example of a [3,3] rearrangement, because the initial carbon-oxygen (CO) bond that breaks designates the 1 position for each component, and both components then rearrange and form a new CC bond by reattaching at position 3 for each component. Reaction (2) is an example of a [1,3] rearrangement with substitution, commonly called an S_N2′ reaction.

In addition, classification can be based on how many electrons move with the migrating group. Of the two electrons in the initial bond that breaks, the migrating group may bring with it both electrons (nucleophilic or anionotropic), one electron (radical), or no electrons (electrophilic or cationotropic). If the rearrangement is a concerted reaction in which there is a cyclic delocalized transition state that results in shifts of pi bonds as well as sigma bonds, the reaction is called a sigmatropic rearrangement [for example, reaction (1)]. See also Delocalization; Pericyclic reaction.

The great majority of rearrangement reactions occur when a molecule develops a severely electron-deficient site. Shift of a nearby atom or group, with its pair of electrons, can serve to satisfy the electron deficiency at the original site, although it typically leaves behind another site with electron deficiency. As long as the final site can bear the electron deficiency better than the original site, the rearrangement will be favorable. See also Chemical bonding.

A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give 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:

Intermolecular rearrangements also take place.

A rearrangement is not well represented by simple and discrete electron transfers (represented by curly arrows in organic chemistry texts). The actual mechanism of alkyl groups moving, as in Wagner-Meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bond-breaking and forming. In pericyclic reactions, explanation by orbital interactions give a better picture than simple discrete electron transfers. It is, nevertheless, possible to draw the curved arrows for a sequence of discrete electron transfers that give the same result as a rearrangement reaction, although these are not necessarily realistic.

Some key rearrangement reactions:

• 1,2-rearrangements
• pericyclic reactions
• olefin metathesis

Examples are the Wagner-Meerwein rearrangement:

the Beckmann rearrangement:

hydride shifts

and the Claisen rearrangement:

References

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

v • d • e Topics in Organic Reactions Addition reaction - Elimination reaction - Polymerization - Reagents - Rearrangement reaction - Redox reaction - Regioselectivity - Stereoselectivity - Stereospecificity - Substitution reaction - List of organic reactions

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