What Is an Organic Rearrangement Reaction Definition Types Mechanism and Examples
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 Reactions in Chemistry
1. What is an organic rearrangement reaction?
An organic rearrangement reaction is a type of reaction in which the carbon skeleton of a molecule is reorganized to form a structural isomer without changing its molecular formula. In these reactions, atoms or groups migrate from one atom to another within the same molecule.
- The molecular formula remains the same.
- A new connectivity of atoms is formed.
- They often proceed through carbocation, carbanion, or radical intermediates.
2. What are the types of organic rearrangement reactions?
The main types of organic rearrangement reactions include 1,2-shifts, ring expansions, and named rearrangements involving migration of groups. These can be classified as:
- 1,2-Hydride shift (migration of H with its bonding electrons).
- 1,2-Alkyl shift (migration of an alkyl group).
- Pinacol–Pinacolone rearrangement.
- Beckmann rearrangement.
- Claisen rearrangement (a pericyclic rearrangement).
- Wagner–Meerwein rearrangement.
3. What is a 1,2-hydride shift in organic chemistry?
A 1,2-hydride shift is a rearrangement in which a hydrogen atom with its bonding pair of electrons moves from one carbon to an adjacent carbocation center. This shift increases the stability of the carbocation.
- Occurs in reactions involving carbocation intermediates.
- Converts a less stable carbocation (e.g., secondary) into a more stable one (e.g., tertiary).
- Common in acid-catalyzed reactions of alkenes and alcohols.
4. Why do carbocation rearrangements occur?
Carbocation rearrangements occur to form a more stable carbocation intermediate. Stability increases in the order: primary < secondary < tertiary, due to hyperconjugation and inductive effects.
- A less stable carbocation rearranges via hydride or alkyl shift.
- The driving force is increased thermodynamic stability.
- Rearrangements often compete with nucleophilic attack.
5. What is the difference between a rearrangement reaction and an isomerization reaction?
A rearrangement reaction is a specific type of isomerization in which atoms or groups migrate within a molecule via a defined mechanism, often involving intermediates. The key differences are:
- Rearrangement reaction: Involves bond migration and specific mechanistic steps (e.g., 1,2-shift).
- Isomerization reaction: Broad term for conversion of one isomer to another.
- All rearrangements are isomerizations, but not all isomerizations involve migration steps.
6. What is the Pinacol–Pinacolone rearrangement?
The Pinacol–Pinacolone rearrangement is an acid-catalyzed reaction in which a vicinal diol is converted into a ketone through a 1,2-alkyl shift. For example:
- (CH3)2C(OH)–C(OH)(CH3)2 → (CH3)3C–CO–CH3 (in acidic medium)
- Protonation of –OH group.
- Loss of water to form a carbocation.
- 1,2-alkyl shift.
- Formation of a carbonyl group.
7. What is the Beckmann rearrangement?
The Beckmann rearrangement is the acid-catalyzed conversion of an oxime into an amide. In this reaction, the group anti to the –OH group migrates to nitrogen.
- Starting material: ketoxime or aldoxime.
- Reagent: strong acid such as H2SO4.
- Product: amide (or lactam in cyclic systems).
8. What is the Claisen rearrangement?
The Claisen rearrangement is a pericyclic [3,3]-sigmatropic rearrangement in which an allyl vinyl ether is converted into a γ,δ-unsaturated carbonyl compound. It occurs through a concerted mechanism.
- No carbocation intermediate is formed.
- It is a thermal rearrangement.
- Proceeds via a cyclic six-membered transition state.
9. How can you predict if a rearrangement will occur in a reaction?
A rearrangement is likely if the reaction forms a carbocation that can become more stable by a 1,2-shift. To predict:
- Identify if a carbocation intermediate is formed.
- Compare stability before and after possible hydride or alkyl shift.
- Check for formation of a tertiary carbocation or resonance-stabilized carbocation.
10. What is the Wagner–Meerwein rearrangement?
The Wagner–Meerwein rearrangement is a carbocation rearrangement involving a 1,2-alkyl shift that changes the carbon skeleton of a molecule. It commonly occurs in acid-catalyzed reactions of alcohols and alkenes.
- Involves migration of an alkyl group.
- Forms a more stable carbocation intermediate.
- Often results in ring expansion or contraction.
