Rearrangement involves the migration of atoms or group of atoms to another position within the molecule under suitable conditions.
In most of the organic reactions, the functional group of the substrate undergoes a structural change without affecting the C-skeleton of the molecule. But in some other organic reactions atoms, groups (R or Ar), double bond, or functional group migrate within the molecule. Such type of reactions are called rearrangement or molecular rearrangement reactions.
Migration of atoms or groups occurs usually from one adjacent atom to another within the same molecule.
The shift of M from X to Y is called 1, 2-shift. If M is H, Me or Ph it is called 1,2-hydride shift, 1,2-methyl shift, or 1,2-phenyl shift respectively. The rearrangement may take place through carbocation, carbanion, or free radical intermediates. The rearrangement through carbocation ¯¯¯e - deficient species is more common.