<P> A 1, 2 - rearrangement or 1, 2 - migration or 1, 2 - shift or Whitmore 1, 2 - shift 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 . In the example below the substituent R moves from carbon atom C to C . </P> <P> 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 . </P>

When does a 1 2 hydride shift occur