What are rearrangement reactions?

Rearrangement reactions are organic reactions in which the structure of a molecule changes by the migration of atoms or groups within the molecule, resulting in an isomeric product.

What triggers a rearrangement reaction?

Rearrangement reactions are often triggered by the formation of a reactive intermediate, such as a carbocation, carbanion, or radical, where atoms or groups migrate to form a more stable intermediate or product.

What is the Beckmann rearrangement?

The Beckmann rearrangement converts oximes into substituted amides. It typically occurs in the presence of an acid or reagents like thionyl chloride, tosyl chloride, or phosphorus pentachloride.

Why do carbocation rearrangements occur?

Carbocation rearrangements occur to form a more stable carbocation. Stability is often increased by hydride shifts, alkyl shifts, or resonance stabilization, leading to a lower energy intermediate.

Rearrangement Reaction: Types, Important Reactions and FAQs

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JEE Chemistry

Rearrangement Reactions

Rearrangement reactions are reactions where atoms or groups within a molecule shift from one position to another, resulting in an isomeric product. Unlike typical substitution or elimination reactions where bonds break and form with different molecules, rearrangement reactions involve internal migration, leading to a new structural arrangement but retaining the same molecular formula.

1.0Types of Rearrangement Reactions

Carbocation Rearrangement:

These rearrangements occur via a carbocation intermediate, where a shift of hydrogen (hydride shift) or an alkyl group (alkyl shift) happens to form a more stable carbocation.
Carbocation rearrangement reactions often occur to stabilize the carbocation intermediate. In these rearrangements, atoms or groups of atoms move within a molecule to convert a less stable carbocation into a more stable one. Hydride shifts, which involve the migration of a hydrogen atom along with its bonding electrons, are common in such reactions. The driving force behind these shifts is the tendency of the system to achieve greater stability, usually by increasing the degree of substitution of the carbocation or achieving resonance stabilization.

Nucleophilic Rearrangement:

In nucleophilic rearrangements, a nucleophile induces the migration of an atom or group within the molecule.

Electrophilic Rearrangement:

Here, an electrophile induces the migration of atoms or groups. Often, electrophiles like protons can create carbocation intermediates that facilitate the rearrangement.
Example: Pinacol-Pinacolone Rearrangement, where a diol is treated with acid, causing one of the hydroxyl groups to leave as water, creating a carbocation that then rearranges.

Sigmatropic Rearrangement:

These are pericyclic reactions involving the simultaneous shift of sigma (σ) bonds and pi (π) electrons. The atoms involved in the rearrangement move along with the migration of electrons.
Example: Claisen Rearrangement.

2.0Important Rearrangement Reactions

Claisen Rearrangement: This is a sigmatropic rearrangement in which an allyl vinyl ether undergoes heat-induced rearrangement to form an unsaturated carbonyl compound.

Favorskii Rearrangement: This rearrangement occurs when α-halo ketones are treated with a strong base, resulting in the formation of carboxylates or esters.

Beckmann rearrangement The Beckmann rearrangement involves the conversion of an oxime functional group into a substituted amide. The rearrangement can also be performed on haloimines and nitrones, yielding similar products. In the case of cyclic oximes and haloimines, the reaction produces lactams (cyclic amides). This reaction was named after the German chemist Ernst Otto Beckmann. While the Beckmann rearrangement is typically catalyzed by acid, various other reagents can promote the reaction, including tosyl chloride (TsCl), thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), and phosphorus pentoxide (P₂O₅), among others.

Beckmann rearrangement of Cyclohexanone oxime to Caprolactam:

Beckmann rearrangement of Cyclohexanone oxime to Caprolactam

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