UNIT- I: Rearrangement reaction
- A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is
rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to
another atom in the same molecule. In the example below the substituent R moves from carbon atom 1 to carbon
atom 2:
- Types of Rearrangements
1. Rearrangement to Electron Deficient Carbon
 Carbon Migration
- Wagner-Meerwein Rearrangement
- Pinacol Rearrangement
- Benzilic Acid Rearrangement
- Arndt-Eistert Homologation Reaction
 Halogen, Oxygen, Sulfur, and Nitrogen Migration
2. Rearrangement to Electron Deficient Nitrogen
 Hofmann Rearrangement
 Curtius Rearrangement
 Schmidt Rearrangement
 Lossen Rearrangement
 Beckmann Rearrangement
3. Rearrangement to Electron Deficient Oxygen
 Baeyer Villiger Reaction
 Hydroperoxide Rearrangement
 Dakin Reaction
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4. Rearrangement to Electron-Rich Carbon
 Stevens Rearrangement
 Sommelet-Hauser Rearrangement
 Wittig Rearrangement
 Favorskii Rearrangement
5. Aromatic Rearrangements
Intermolecular Migration from Nitrogen to Carbon
Fries Rearrangement
Intramolecular Migration from Nitrogen to Carbon
Claisen Rearrangement
1. Rearrangement to Electron Deficient Carbon

Carbon Migration
 Wagner-Meerwein Rearrangement: It is one of the simplest systems where an alkyl group migrates, with its
bonding pair, to an electron-deficient carbon atom.
Mechanism
 Pinacol Rearrangement: Treatment of 1,2-diols (pinacol) with acid lead to rearrangement to give ketone.
Mechanism
 Halogen, Oxygen, Sulfur, and Nitrogen Migration
 In the system X-C-C-Y, an atom X with an unshared pair of electrons can assist the heterolysis of the C-Y bond. In
case of unsymmetrical system, nucleophilic attack predominates at the less substituted carbon of the bridged ion
that leads to rearranged skeleton.
Mechanism
2. Rearrangement to Electron Deficient Nitrogen
 Hofmann Rearrangement: This rearrangement provides an effective method for the synthesis of primary aliphatic
and aromatic amines from primary amides.
Mechanism
3. Rearrangement to Electron Deficient Oxygen
 Dakin Reaction: Benzaldehyde or acetophenone bearing hydroxyl substituent in the ortho or para position proceed
rearrangement to give catechol or quinol, respectively.
Mechanism
4. Rearrangement to Electron-Rich Carbon
- This group of reaction has been less explored, and is less of synthetic importance compared to the
rearrangements to electron deficient carbons. The rearrangements to electron deficient hetero atom may be
generally explained as:
 Wittig Rearrangement: Ethers undergo [1,2]-sigmatropic rearrangement in the presence of strong base such as
amide ion or phenyllithium to give more stable oxyanion.
Mechanism
5. Aromatic Rearrangements
- A number of rearrangements occur in aromatic compounds of the type:
- X is usually nitrogen or oxygen. Both intermolecular and intramolecular migrations are known.
- These compounds undergo [5,5]-sigmatropic rearrangement in the presence of acid to give benzidines.
Mechanism
 Claisen Rearrangement Aryl allyl ethers undergo [3,3]-sigmatropic rearrangement on being heating to
allylphenols.
Mechanism