1,2-rearrangement

A 1,2-rearrangement or 1,2-shift or Whitmore 1,2-shift is a 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. In the example below the substituent R moves from carbon atom C1 to C2.

Related Topics:
Organic reaction - Substituent - Chemical compound

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RC1-C2-C3 → C1-RC2-C3

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The rearrangement is intramolecular and the starting compound and reaction product are structural isomers. The 1,2-rearrangement belongs to a broad class of chemical reactions called rearrangement reactions.

Related Topics:
Intramolecular - Structural isomer - Rearrangement reaction

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A 1,2-rearrangement is often initialised by the formation of a reactive intermediate such as:

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• a carbocation by heterolysis in a nucleophilic rearrangement or anionotropic rearrangement
• a carbanion in a electrophilic rearrangement or cationotropic rearrangement
• a free radical by homolysis
• a nitrene.

The driving force for the actual migration of a substituent in step two of the rearrangement is the formation of a more stable intermediate. For instance a tertairy carbocation is more stable than a secondary carbocation and therefore the SN1 reaction of neopentyl bromide with ethanol yields tert-pentyl ethyl ether.

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1,2-rearrangement: 1,2 Rearrangements ►