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Rearrangement reaction

Chemical reaction where a molecule is turned into a structural isomer of itself

Chemical reaction where a molecule is turned into a structural isomer of itself

In organic chemistry, a rearrangement reaction is a broad class of organic reactions "that involves a change of connectivity". Usually the term rearrangement refers to intramolecular processes involving modification of carbon skeleton. Often a substituent moves from one atom to another atom in the same molecule, hence these reactions are usually intramolecular. In the example below, the substituent R moves from carbon atom 1 to carbon atom 2:

:{-}\underset{| \atop \displaystyle \color{Blue}\ce{R}}\ce{C}\ce{-C-C{-} - {-C-}}\underset{| \atop \displaystyle \color{Blue}\ce{R}}\ce{C}\ce{-C}{-}

A rearrangement is not well represented by simple and discrete electron transfers (represented by curved 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. In allylic rearrangement, the reaction is indeed ionic.

1,2-rearrangements

Main article: 1,2-rearrangement

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 movement involves two adjacent atoms but moves over larger distances are possible. Skeletal isomerization is not normally encountered in the laboratory, but is the basis of large applications in oil refineries. In general, straight-chain alkanes are converted to branched isomers by heating in the presence of a catalyst. Examples include isomerisation of n-butane to isobutane and pentane to isopentane. Highly branched alkanes have favorable combustion characteristics for internal combustion engines. :[[File:N-pentane isomerization.svg|500px|Isomerization of pentane]]

Further examples are the Wagner–Meerwein rearrangement: :[[File:Isoborneol2CampheneConversion.svg|500px|Wagner–Meerwein]]

and the Beckmann rearrangement, which is relevant to the production of certain nylons: :[[File:Beckmann-rearrangement overview.svg|300px|Beckmann rearrangement]]

Pericyclic reactions

Main article: pericyclic reactions

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

Sigmatropic Hydride Shifts

and the Claisen rearrangement:

Claisen rearrangement

Other rearrangement reactions

1,3-rearrangements

1,3-rearrangements take place over 3 carbon atoms. Examples:

  • the Fries rearrangement
  • a 1,3-alkyl shift of verbenone to chrysanthenone

References

References

  1. (2014). "molecular rearrangement".
  2. {{JerryMarch
  3. (2015). "Molecular Rearrangements in Organic Synthesis".
  4. (2002). "Hydrocarbons". Wiley-VCH.
  5. (2012). "Organic chemistry". Oxford University Press.
  6. (25 April 2013). "Ring-Opening Polymerization—An Introductory Review". Polymers.
  7. (December 1988). "The thermal, aliphatic Claisen rearrangement". Chemical Reviews.
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