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. 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-
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.
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.
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.
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.
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.
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.
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.
FAQs on Organic Rearrangement Reaction
1. What is an organic rearrangement reaction as per the CBSE syllabus?
An organic rearrangement reaction is a type of chemical reaction where the carbon skeleton of a molecule is rearranged to form a structural isomer of the original molecule. This process often involves the migration of an atom (like hydrogen) or a group (like an alkyl group) from one atom to another within the same molecule. The main purpose of such a rearrangement is typically to achieve a more stable intermediate, such as a tertiary carbocation, before the final product is formed.
2. What are the major types of organic rearrangement reactions studied in Class 12 Chemistry?
Several key rearrangement reactions are important for the Class 12 curriculum. These reactions are often named after their discoverers and include:
- Hofmann Rearrangement: Converts a primary amide into a primary amine with one less carbon atom.
- Curtius Rearrangement: Involves the thermal decomposition of an acyl azide to an isocyanate.
- Beckmann Rearrangement: Transforms an oxime into a substituted amide.
- Claisen Rearrangement: A [3,3]-sigmatropic rearrangement of an allyl vinyl ether or allyl aryl ether.
- Pinacol-Pinacolone Rearrangement: Involves the conversion of a 1,2-diol to a ketone via a carbocation intermediate.
- Wolff Rearrangement: Converts an α-diazoketone into a ketene.
3. How does the Beckmann rearrangement convert an oxime into an amide?
In the Beckmann rearrangement, an oxime is treated with an acid catalyst (like concentrated sulphuric acid). The acid protonates the hydroxyl group on the oxime, turning it into a good leaving group (water). As the water molecule leaves, an alkyl or aryl group from the opposite side (anti-periplanar) migrates to the electron-deficient nitrogen atom. The resulting nitrilium ion is then attacked by water, and after a tautomerization step, it yields a stable substituted amide or a lactam if the starting oxime was cyclic.
4. What is the key difference between the Hofmann and Curtius rearrangements?
The primary difference between the Hofmann and Curtius rearrangements lies in their starting materials, even though they both proceed through a similar isocyanate intermediate to form an amine. The Hofmann rearrangement starts with a primary amide (R-CO-NH₂) and uses bromine (Br₂) and a strong base (like NaOH). In contrast, the Curtius rearrangement begins with an acyl azide (R-CO-N₃), which is typically heated to initiate the reaction. Both reactions result in a primary amine with one less carbon than the starting material.
5. What are the real-world applications of the Curtius rearrangement reaction?
The Curtius rearrangement is highly valuable in synthetic organic chemistry, particularly in the pharmaceutical industry. Its main advantage is its high tolerance for various functional groups and the complete retention of the molecule's stereochemistry during the reaction. This makes it extremely useful for synthesising complex medicinal agents and natural products that contain amines, urethanes, and ureas, as it provides a reliable method for introducing these functional groups with high precision.
6. Why is carbocation stability the primary driving force for many rearrangement reactions?
Many rearrangement reactions, like the Pinacol-Pinacolone rearrangement, proceed through a carbocation intermediate. The stability of carbocations follows the order: tertiary (3°) > secondary (2°) > primary (1°). If a reaction forms a less stable carbocation (e.g., secondary), and a simple shift of an adjacent hydrogen (1,2-hydride shift) or alkyl group (1,2-alkyl shift) can create a more stable tertiary carbocation, this rearrangement will occur spontaneously. This more stable intermediate is lower in energy, making it the preferred pathway for the reaction to proceed, ultimately leading to the major product.
7. How does the mechanism of a Claisen rearrangement exemplify a pericyclic reaction?
The Claisen rearrangement is a classic example of a pericyclic reaction because it occurs in a single, concerted step without any ionic or radical intermediates. It involves a cyclic redistribution of electrons through a single transition state. Specifically, it is a [3,3]-sigmatropic rearrangement, where a sigma bond is broken, pi bonds are reorganized, and a new sigma bond is formed simultaneously in a six-membered ring-like transition state. This concerted mechanism explains why the reaction is often stereospecific and is induced by heat.
8. In a Hofmann rearrangement, how is the carbon chain shortened by one atom?
The carbon chain is shortened in a Hofmann rearrangement because the carbonyl carbon of the initial amide is lost as carbon dioxide during the final step. The process is as follows:
- The amide's alkyl/aryl group migrates from the carbonyl carbon to the nitrogen atom, forming an isocyanate (R-N=C=O) intermediate.
- This isocyanate is then hydrolysed by water.
- During hydrolysis, the isocyanate intermediate breaks down, releasing the carbonyl group as a molecule of carbon dioxide (CO₂).
- The remaining part of the molecule forms a primary amine (R-NH₂), which now has one less carbon atom than the original amide.
