Organic Rearrangement Reaction

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What are Rearrangement Reactions?

Rearrangement reactions in Organic Chemistry refer to two types of organic chemical reactions. A rearrangement might involve the one step migration of a hydrogen or H atom or a larger molecular fragment in a relatively short period of time. However, a rearrangement may refer to a multi-step reaction including migration of Hydrogen or H atoms or a larger molecular fragment forming one of its steps. Rearrangement in Organic Chemistry refers to a vast array of chemical reactions where the carbon structure of the molecule is rearranged to make way for the structural isomer of the original molecule. Rearrangements in organic chemistry occur to give the more stable tertiary carbocation which is then attacked by the nucleophile. 

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Types of Rearrangement Reactions

There are several types of rearrangement reactions in organic chemistry. They may be broadly classified into the following groups-

1. Curtius Rearrangement or Curtius Reaction

Curtius Rearrangement refers to the heating of acyl azide which on losing its hydrogen transforms into an isocyanate. 

If this rearrangement reaction occurs in an alcoholic or aqueous medium, the isocyanate further transforms to form urethane amine or substituted urea. The conversion of acyl azide to isocyanate occurs under Curtius Rearrangement. While on the other hand, Curtius reaction refers to the conversion of acids to amines, urethane or substituted urea with the help of Curtius Rearrangement.

RCON₃ → R-N=C=O + N₂

Acyl Azide is obtained in the following manner-

RCOCl + NaN₃ → RCON₃ + NaCl


2. Claisen Rearrangement  

Claisen Rearrangement means the first step of isomerisation of allyl aryl ethers to ortho alkylated phenols.  A cyclohexadiene is produced in this rearrangement process which is a [3,3] sigmatropic rearrangement. Three valence electrons are shifted in this procedure simultaneously. 

Claisen Rearrangement refers to the thermal rearrangement of allyl aryl ethers and allyl vinyl ethers. Claisen has first discovered this rearrangement in allyl vinyl and later experimented and applied it to allyl aryl to form ally phenols. 

3. Beckmann Rearrangement

Under the Beckmann Rearrangement an oxime gets transformed into an amide. An oxime can be obtained from treating aldehyde or ketone with hydroxylamine. In this rearrangement, cyclic oxides produce lactams. The Beckmann rearrangement is useful in the insertion of an NH group among the carbonyl carbon and the alpha carbon. This rearrangement of the oxime of cyclohexanone is done in a vast scale in major industries owing to the fact that the product caprolactum is the direct predecessor of nylon 6 which has many utilities like production of carpets and textiles. In this reaction, concentrated sulphuric acid is used as an acid catalyst as well as a solvent. 

4. Hoffman Rearrangement

The Hofmann Rearrangement is the result of the treatment of primary amide with bromine and hydroxide ion with water. This leads to the production of an amine which has lost its carbonyl group. Thus, this rearrangement helps to shorten the carbon chain by one atom. It also brings about a change in the functional group from an amide to an amine. 

5. Pericyclic Rearrangement

Pericyclic Rearrangements may be classified as the reactions that take place due to a concerted cyclic shift of electrons. The two critical points of this pericyclic rearrangement are as follows- 

  1. Pericyclic Rearrangement is concerted. Thus refers to the fact that in this reaction, the reactant bonds are broken, and product bonds are formed simultaneously without intermediaries. 

  2. Pericyclic Reactions involve a cyclic shift of electrons wherein the cyclic transitions include pi bonds.

The activation energy in these reactions is supported by heat or by Uv light.               These reactions are stereospecific and are likely that they yield products of opposite stereochemistry.

Three properties of Pericyclic Rearrangements which are related to each other are as follows-

  1. Pericyclic Reactions are induced by heat or UV light. However, many of these reactions require heat but are not necessarily initiated by light or vice versa. 

  2. The number of pi bonds present in Pericyclic Reaction.

  3. The stereochemistry of Pericyclic Reaction.

6. Photochemical Rearrangement

In general, the term rearrangement is used in place of isomerisation. However, the reactions which are classified under Photochemical Rearrangements do not seek a differentiation between the two terms. 

However, for convenience, Photochemical reactions are classified as Cis Trans Isomerization, Sigmatropic Rearrangements, Electrocyclic Rearrangements and Structural Rearrangements. Structural Rearrangements result from intramolecular Cycloadditions. 

Solved Examples

1. What Do You Mean by Molecular Rearrangement in Organic Synthesis?

Molecular Rearrangement in Organic Synthesis helps chemists all over the world understand and decipher the Rearrangement Reactions to elevate the synthesis of useful compounds.

2. What are the Different Types of Photochemical Reactions?

The different types of Photochemical Reactions are as follows- Cis Trans Isomerization, Sigmatropic Rearrangements, Electrocyclic Rearrangements and Structural Rearrangements. 

FAQ (Frequently Asked Questions)

1. What Do You Mean by Organic Rearrangement Reactions?

Ans: Organic Rearrangement Reactions refer to the vast class of chemical reactions which deal with chemical reactions whereby there is a change in the carbon structure of the molecule to make way for the isomer structure of the original molecule. It might be a single step reaction where there occurs the migration of the Hydrogen or H atom. On the other hand, it might be a multi-step process where the migration of the hydrogen atom is just one step out of many. Organic Rearrangement Reactions are sigmatropic reactions while their numbers 1,2 refer to the subclass they belong to.

2. Give a List of Rearrangement Reactions in Organic Chemistry.

Ans: Some of the well known Rearrangement Reactions Organic Chemistry are Beckmann Rearrangement, Hofmann Rearrangement, Curtius Rearrangement, Photochemical Rearrangement, Pericyclic Rearrangements etc. However, some of the lesser-known Rearrangement Reactions in Organic Chemistry are as follows- Alkaline Zipper reaction, Allen Miller Trippet Rearrangement, Electrocyclic Reaction, Alpha Ketol Rearrangement, Fischer Hepp Rearrangement, Ring Expansion and Contraction Rearrangement, Benzilic Acid Rearrangement, Mumm Rearrangement, Wolff Rearrangement, etc.

These rearrangements deal mainly with chemical reactions which change the carbon structure of the molecule to make way for the isomer structure of the original molecule. These reactions may be a single step or multi-step in nature.