Molecular rearrangement reactions are of two types; pericyclic rearrangements (e.g. Claisen and Cope rearrangements), and migrations of a group from one atom to another. Both types are intramolecular reactions. Rearrangements can be difficult to rationalise, particularly in unfamiliar cases.
Migrations are almost always from one atom to an adjacent atom ("1,2-migrations"), but longer movements can be achieved by a series of 1,2-migrations.
Pericyclic rearrangements are classified by a system based on migration of a sigma bond, and include [1,5]-, [2,3]- and [3,3]-sigmatropic rearrangements, among others.
This short section includes some of the more celebrated rearrangement reactions and those which can actually be synthetically useful in certain circumstances. Rearrangements have long been known and many of the examples in this section were discovered in the mid-to-late 19th Century, before the advent of organic structure theory. For example, Liebig observed the Benzil-Benzilic Acid rearrangement as far back as 1838.
Rearrangements can be classified broadly as nucleophilic, electrophilic or free radical:
In a nucleophilic rearrangement, the migrating group Y moves with its electron pair.
In an electrophilic rearrangement, the migrating group Y moves without its electron pair.
In a free radical rearrangement, the migrating group moves with a single electron.
Nucleophilic rearrangements are found to be the most common type and it is possible to rationalise this by considering the energy of the transition states for the three possibilities.
The migration of a group Y from atom A to atom B may be represented as:
The transition state (or intermediate) can be represented by the following, in which Y is partially bonded to both A and B:
In electrophilic or free radical rearrangements, the antibonding orbitals must be occupied and this raises the energy of the transition state. When these rearrangements are observed, the migrating group is usually aromatic, which can accommodate the extra electron(s).
The questions in this section are probably suited to the advanced user. More difficult examples show the electrons/atoms which move in red to help you.