An atom or group of atoms migrate from one position to another; such reactions are rearrangement reactions.
Post rearrangement, the molecule formed has the atoms arranged differently but has the same molecular formula as the parent. Therefore, such rearrangements create structural isomers, compounds that differ in structure but have the same molecular formula.
Related Reading - Structural Isomerism
For example, in the structural isomers shown below, the position of Hydrogen (shown in red) undergoes constant changes.
The position from which the group moves is the migration origin (the start point), and the migration terminus is the position to which the group migrates (the destination).
The most common migrations are 1,2 rearrangements in which migrating group moves to the adjacent atom. However, longer rearrangements are also possible.
The rearrangement reactions are of three types, depending on the number of electrons the migrating atom or group carries along with it.
a) Migration with electron-pair (nucleophilic)
When the migrating atom or group W moves with the bond electron pair, it is called nucleophilic or anionotropic rearrangement. The migrating group is regarded as a nucleophile.
Since the migrating group carries along with it an electron pair, the migration terminus B must have only six electrons in its outer shell. That is, it must have space to accommodate two extra electrons.
Such a sextet of electrons at the migration terminus is formed before the actual migration, mainly by two methods- the carbocation formation or nitrene formation.
Carbocation formation method
Rearrangements facilitated by carbocation formation are encountered in SN1 and Friedel Craft alkylation conditions.
Most rearrangement reactions involve the making or breaking of C-H, C-C, C-N, or C-O bonds and can accompany substitution, elimination, and addition reactions.
For example, in a substitution reaction involving 3-methylbutan-2-ol, the expected product differs from the obtained one. The reaction goes via the formation of carbocation and 1,2- migration of the H atom as a nucleophilic hydride, so that the molecule exists as a stable carbocation (20 less stable than 30). The substitution with Bromo follows the migration to obtain the final product.
In summary, the nucleophilic rearrangement of hydride occurred due to the carbocation formation, and the reaction ended with a substitution.
The other examples of carbocation-induced alkyl and hydride shifts are- Pinacol Rearrangement, Wagner-Meerwein shifts, Dienone-Phenol rearrangement, etc.
Nitrene formation method
The nitrene-induced alkyl migration is seen in Curtius rearrangement.
Similar Carbon-Nitrogen (C-N) migration of the alkyl group is seen in Lossen, Schmidt, and Hofmann Rearrangements.
b) Migration without electrons (electrophilic, rare)
The migrating atom or group W moves without the bond electron pair, which is called electrophilic or cationotropic rearrangements, and in the case of migrating hydrogen (as H+), prototropic rearrangement. The migrating group is regarded as an electrophile.
Rearrangement reactions like Favorskii, Ramberg-Backlund, Wolff, etc., are examples of carbanions or carbenes' formation.
c) Migration with one electron (free-radical)
The migrating atom or group W moves with only one electron, that is, as a radical.
Heinrich Otto Weiland reported the first radical 1,2 rearrangement in 1911, the bis(triphenylmethyl)peroxide (substrate) conversion to tetraphenylethane (product).
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Types of Rearrangement reactions in chemistry