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.
RC1-C2-C3 → C1-RC2-C3 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. A 1,2-rearrangement is often initialised by the formation of a reactive intermediate such as: a carbocation by heterolysis in a nucleophilic rearrangement or anionotropic rearrangementa carbanion in a electrophilic rearrangement or cationotropic rearrangementa 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.
1,2-rearrangement mechanism is found in
Wagner-Meerwein rearrangement Pinacol rearrangement Hofmann rearrangement Curtius rearrangement SN1 reaction (generally) halogen dance rearrangement
references The common basis of molecular rearrangements Frank C. Whitmore; J. Am. Chem. Soc.; 1932; 54(8); 3274-3283.
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