UNIT-5
Organic Chemistry
Classification of Rearrangements
1. Introduction
Rearrangement reactions are those in which the carbon skeleton of a molecule is
reorganized to form a structural isomer of the original compound. These reactions often
involve migration of atoms or groups within the molecule.
Rearrangements are broadly classified into:
(a) Electron-deficient rearrangements
(b) Electron-rich rearrangements
(c) Radical rearrangements
This section focuses on electron-deficient skeletal rearrangements, which mainly proceed
via carbocation intermediates.
2. Electron-Deficient Skeletal Rearrangements
These rearrangements involve the migration of atoms or groups in intermediates that are
electron deficient, such as carbocations, carbenium ions, or sometimes carbonium ions.
The driving force is the formation of a more stable carbocation or relief of ring strain.
Electron-deficient skeletal rearrangements are organic reactions where a molecule
with an electron-deficient center, such as a carbocation, reorganizes its carbon
skeleton to form a new, more stable isomer. Examples include the Wagner-
Meerwein rearrangement, where an alkyl group migrates, and the Pinacol
rearrangement, which involves a 1,2-diol undergoing a carbocation-mediated 1,2-
shift
3. Structure and Stability of Carbocations
Carbocations are sp² hybridized species with a planar structure. The positively charged
carbon has an empty p-orbital perpendicular to the plane of the molecule.
Stability order: 3° > 2° > 1° > methyl
Special stabilization: Allylic and benzylic carbocations are stabilized by resonance.
Tropylium ion and cyclopropylcarbinyl cations are also unusually stable.
,4. Classical and Non-Classical Carbocations
Classical carbocations have a localized positive charge on one carbon atom (e.g., tert-butyl
cation, benzyl cation).
Non-classical carbocations have delocalized charge over two or more atoms via a bridged
structure (three-center two-electron bonds), e.g., norbornyl cation, cyclopropylmethyl
cation.
5. Neighbouring Group Participation (NGP)
Neighbouring group participation occurs when a lone pair or π-electrons of an adjacent
atom or group assist in the formation of an intermediate, leading to a rearranged product.
Participating groups: –OH, –OR, –Cl, –NH₂, aromatic rings, alkenes, etc.
Mechanism:
1. The neighbouring group interacts with the developing positive charge, forming a bridged
intermediate.
2. Rearrangement or substitution then occurs with retention or inversion depending on the
structure.
The power of neighboring group participation; it can dramatically enhance the rate
of reactions versus standard nucleophilic substitution reactions.
Primary alkyl chlorides with water. In the absence of the rate
information, we would assume these are both simple SN2
reactions.
6. Important Rearrangement Reactions
1. a) Wagner–Meerwein Rearrangement:
, Type: 1,2-alkyl or hydride shift in carbocations.
A less stable carbocation rearranges to a more stable one via a 1,2-shift of an alkyl or
hydrogen atom.
Example: Conversion of tert-amyl cation to neopentyl cation.
2. b) Pinacol–Pinacolone Rearrangement:
Type: Acid-catalyzed rearrangement of vicinal diols to carbonyl compounds.
Mechanism:
1. Protonation of one –OH group.
2. Loss of water → formation of carbocation.
Organic Chemistry
Classification of Rearrangements
1. Introduction
Rearrangement reactions are those in which the carbon skeleton of a molecule is
reorganized to form a structural isomer of the original compound. These reactions often
involve migration of atoms or groups within the molecule.
Rearrangements are broadly classified into:
(a) Electron-deficient rearrangements
(b) Electron-rich rearrangements
(c) Radical rearrangements
This section focuses on electron-deficient skeletal rearrangements, which mainly proceed
via carbocation intermediates.
2. Electron-Deficient Skeletal Rearrangements
These rearrangements involve the migration of atoms or groups in intermediates that are
electron deficient, such as carbocations, carbenium ions, or sometimes carbonium ions.
The driving force is the formation of a more stable carbocation or relief of ring strain.
Electron-deficient skeletal rearrangements are organic reactions where a molecule
with an electron-deficient center, such as a carbocation, reorganizes its carbon
skeleton to form a new, more stable isomer. Examples include the Wagner-
Meerwein rearrangement, where an alkyl group migrates, and the Pinacol
rearrangement, which involves a 1,2-diol undergoing a carbocation-mediated 1,2-
shift
3. Structure and Stability of Carbocations
Carbocations are sp² hybridized species with a planar structure. The positively charged
carbon has an empty p-orbital perpendicular to the plane of the molecule.
Stability order: 3° > 2° > 1° > methyl
Special stabilization: Allylic and benzylic carbocations are stabilized by resonance.
Tropylium ion and cyclopropylcarbinyl cations are also unusually stable.
,4. Classical and Non-Classical Carbocations
Classical carbocations have a localized positive charge on one carbon atom (e.g., tert-butyl
cation, benzyl cation).
Non-classical carbocations have delocalized charge over two or more atoms via a bridged
structure (three-center two-electron bonds), e.g., norbornyl cation, cyclopropylmethyl
cation.
5. Neighbouring Group Participation (NGP)
Neighbouring group participation occurs when a lone pair or π-electrons of an adjacent
atom or group assist in the formation of an intermediate, leading to a rearranged product.
Participating groups: –OH, –OR, –Cl, –NH₂, aromatic rings, alkenes, etc.
Mechanism:
1. The neighbouring group interacts with the developing positive charge, forming a bridged
intermediate.
2. Rearrangement or substitution then occurs with retention or inversion depending on the
structure.
The power of neighboring group participation; it can dramatically enhance the rate
of reactions versus standard nucleophilic substitution reactions.
Primary alkyl chlorides with water. In the absence of the rate
information, we would assume these are both simple SN2
reactions.
6. Important Rearrangement Reactions
1. a) Wagner–Meerwein Rearrangement:
, Type: 1,2-alkyl or hydride shift in carbocations.
A less stable carbocation rearranges to a more stable one via a 1,2-shift of an alkyl or
hydrogen atom.
Example: Conversion of tert-amyl cation to neopentyl cation.
2. b) Pinacol–Pinacolone Rearrangement:
Type: Acid-catalyzed rearrangement of vicinal diols to carbonyl compounds.
Mechanism:
1. Protonation of one –OH group.
2. Loss of water → formation of carbocation.
