Organic Rearrangement Reaction

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. 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. 

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 = 0 + N2

Acyl Azide is obtained in the following manner-

RCOCl + NaN3 → RCON3 + NaCl

RCOOC2H5 → RCONHNH2→ RCON3 + 2H2O

Mechanism of Curtius Rearrangement

We will now take a look at the mechanism of the Curtius rearrangement reaction. The mechanism involves shifting the alkyl group from the carbonyl carbon to the closest nitrogen atom in the molecule. This is accompanied by the release of nitrogen gas. An isocyanate compound is also formed as a result of this. This compound can further react in the presence of nucleophiles to form other new stable compounds that do not disintegrate in the solution. 

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 3,33,33,3 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 formally phenols. 

Mechanism of Claisen Rearrangement

The mechanism governing the Claisen rearrangement reaction is strikingly similar to the Diels-Alder reaction. Proton tautomerism is seen as part of this reaction. This implies that a proton is removed from one atom and is placed at a different position. 

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 on a vast scale in major industries because the product caprolactam is the direct predecessor of nylon 6 which has many utilities like the production of carpets and textiles. In this reaction, concentrated sulphuric acid is used as an acid catalyst as well as a solvent. 

Mechanism of Beckmann Rearrangement

The mechanism of the Beckmann rearrangement is governed by the same pattern as a pinacol reaction. The acid group present converts the oxime OH into a leaving group. On the other hand, we see that the alkyl group migrates onto the nitrogen atom as with the elimination of water.  A cation with the water that was eliminated reacts to give an amide. 

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 that 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. 

Mechanism of Hofmann Rearrangement

The mechanism behind the Hoffman rearrangement reaction is quite simple. Here, an amide is treated with bromine along with aqueous sodium hydroxide. In the addition of these, an intermediate isocyanate is formed. When water is further added to this mixture, the isocyanate loses a carbon dioxide molecule and forms the amine molecule.

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- Pericyclic Rearrangement is concerted. This refers to the fact that in this reaction, the reactant bonds are broken, and product bonds are formed simultaneously without intermediaries. 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 to yield products of opposite stereochemistry. Three properties of Pericyclic Rearrangements which are related to each other are as follows-

• 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. 

• The number of pi bonds present in Pericyclic Reaction.

• 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. 

7. 1,2-Rearrangement Reaction

A 1,2-rearrangement is another subsequent type of organic reaction. This kind of rearrangement is very common where a substituent shifts its position from one atom to another atom in a chemical compound. In a 1,2 shift, the movement primarily involves a shift between two adjacent atoms. However, shifting can even be seen over other large distances.