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 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.

1,2 rearrangements

1,2-rearrangement mechanism is found in

• Wagner-Meerwein rearrangement
• Pinacol rearrangement
• Hofmann rearrangement
• Curtius rearrangement
• SN1 reaction (generally)
• halogen dance rearrangement
• 1,2-Wittig rearrangement
• Beckmann rearrangement

references

The common basis of molecular rearrangements Frank C. Whitmore; J. Am. Chem. Soc.; 1932; 54(8); 3274–3283.