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Rearrangement reaction is a term used in organic chemistry which refers to two different types of chemical reactions. The rearrangement comprises one-step migration of an H atom or a bigger molecular fragment in a short period of time. In organic chemistry, rearrangement reactions are broadly classified into groups.
Key Terms: Rearrangement Reactions, Curtius Reaction, Claisen Rearrangement, Hoffman Rearrangement, Photochemical Rearrangement
What is Rearrangement Reaction?
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The rearrangement reaction is a broad group of organic reactions. In this reaction, a molecule’s carbon skeleton is rearranged in order to give a structural isomer to the molecule. Generally, a substituent moves from one atom to another in the same molecule, hence the reactions are mostly intramolecular. The rearrangement involves one-step migration of an H-atom or a bigger molecular fragment within a relatively short period of time.
Rearrangement Reaction
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Types of Rearrangement Reactions
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The different types of rearrangement reactions in organic chemistry are as follows:
Curtius Rearrangement
Curtius rearrangement or curtius reaction is associated with the heating of acyl azide which transforms into an isocyanate by losing its nitrogen. If the same rearrangement happens in an alcoholic or aqueous medium, the isocyanate again reacts to form urethane, amine or substitute urea. The conversion or transformation of acyl azide into isocyanate happens under the curtius rearrangement. Similarly, curtius reaction involves the conversion of acids into amines, substituted urea or urethane using curtius rearrangement.
RCON3 → R-N=C=O + N2
Acyl Azide is attained in the following way:
RCOCl + NaN3 → RCON3 + NaCl
RCOOC2H5 → RCONHNH2 → RCON3 + 2H2O
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Claisen Rearrangement
Claisen rearrangement refers to the very first step of isomerisation of allyl aryl ethers into ortho alkylated phenols. In this rearrangement, a cyclohexadiene is being produced which makes it a sigmatropic rearrangement. Here, three valence electrons are shifted simultaneously. This reaction also refers to the thermal rearrangement of allyl aryl ethers and allyl vinyl ethers.
Claisen Rearrangement
Hoffman Rearrangement
Hoffman rearrangement happens due to the treatment of primary amide with bromine and hydroxide ion with water. This creates an amine that has lost its carbonyl group. Hence, the rearrangement aids in shortening the carbon chain by one atom. This also brings a shift in the functional group from an amide to an amine.
Hoffman Rearrangement
Read More: Hoffmann bromamide rearrangement reaction
Beckmann Rearrangement
In the beckmann rearrangement, an oxime is converted into an amide. An oxime is obtained from treating ketone or aldehyde with hydroxylamine. Also, cyclic oxides produce lactams in the beckmann rearrangement. This rearrangement can be used in the insertion of the NH group among the alpha carbon and the carbonyl carbon. In beckmann rearrangement, a concentrated sulphuric acid is being used as a solvent as well as an acid catalyst.
Beckmann Rearrangement
Read More: Beckmann Rearrangement Reaction
Photochemical Rearrangement
Generally, the term is used for isomerisation. But the reactions under photochemical rearrangements do not have a differentiation between the two terms. Photochemical reactions are classified into different types such as cis-trans isomerisation, electrocyclic rearrangements, sigmatropic rearrangements and structural rearrangements (this results from intramolecular cycloadditions).
Photochemical Rearrangement
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Pericyclic Rearrangement
This reaction happens as a result of the concerted cyclic shift of electrons. The two key points that describe a pericyclic reaction are given below:
- The first point states that the reaction is concerted. In this reaction, the reactant bonds are broken. At the same time, the product bonds are formed without intermediates.
- The second point comprises a cyclic shift of electrons and the term pericyclic came from the cyclic shift of electrons. Thus, the reactions are characterized by a cyclic transition state consisting of the pi bonds. The energy needed for the activation of pericyclic reactions is given by UV light or heat. The three characteristics of any pericyclic reaction are interrelated:
- Pericyclic reactions are activated either by UV light or by thermal energy or heat.
- The number of pi bonds used in the reaction.
- The stereochemistry of the particular reaction.
Pericyclic Rearrangement
Read Also: NCERT Solutions Class 11 Chemistry
Things to Remember
- In organic chemistry, rearrangement reactions are used to determine a broad group of organic reactions.
- In a rearrangement reaction, the skeleton of a molecule’s carbon is rearranged to give a structural isomer to the molecule.
- The rearrangement consists of one step migration of an H-atom or a larger molecular fragment within a short period of time.
- Some of the different types of rearrangement reactions are curtius rearrangement, claisen rearrangement, hofmann rearrangement, photochemical rearrangement, pericyclic rearrangement, and beckmann rearrangement.
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Sample Questions
Ques: Define rearrangement reaction? (2 marks)
Ans: Rearrangement reaction is a term used in organic chemistry which defines a large class of organic reactions. In this reaction, the carbon skeleton of a molecule is rearranged to provide the original molecule a structural isomer.
Ques: Elaborate an example of a rearrangement reaction? (2 marks)
Ans: Normally, straight-chain alkanes are transformed by heating with the help of a catalyst to branched isomers. Examples consist of n-butane isomerization to isobutane and pentane to isopentane. Highly branched alkanes carry favorable properties for internal combustion engines.
Ques: Explain the applications of curtius rearrangement reaction? (3 marks)
Ans: Every rearrangement reaction has its own purpose. Similarly, curtius rearrangement has several applications used in the discovery of new medicines or drugs. Rearrangement reaction allows the full retention of stereochemistry of molecules included. Also, this reaction has a high tolerance for a variety of functional groups.
Ques: Explain beckmann rearrangement? (2 marks)
Ans: This rearrangement is used to replace amides by an oxime functional group. The beckmann rearrangement is generally catalyzed by acid. At the same time, other reagents were known to facilitate the rearrangement.
Ques: Explain sigmatropic rearrangement reactions? (3 marks)
Ans: This is a type of molecular rearrangement. In sigmatropic rearrangement reactions, a σ-bonded atom flanked by one or more π-electron systems, changes to a different location with a corresponding restructuring of the π-bonds. The number of π-bonds and σ-bonds remain unchanged.
Ques: Define organic photochemistry? (3 marks)
Ans: It is a part of chemistry dealing with organic reactions that are induced by exposure to UV radiation or sunlight. The catalyzing factor is the absorption of ultraviolet light by organic molecules. This often results in spontaneous reactions. Initially, sunlight was used to begin photochemical reactions. Nowadays, ultraviolet lamps are also being used. The reactants involved in the photochemical reaction could be either liquids or gases.
Ques: What is the concerted path of a reaction? (2 marks)
Ans: The coordinated reaction is a reaction where all breaking as well as making of bonds occur in one step. There are no other high energy unstable intermediates or intermediate reactive intermediates involved.
Ques: Define regioselective reaction? (2 marks)
Ans: Regioselectivity is the eminence of making or breaking a chemical bond in one direction against all other possible directions. This reaction can also be extended to various reactions like pi-ligands addition. This selectivity also happens in carbene insertions like the baeyer-villiger reaction.
Ques: What is bond cleavage? (3 marks)
Ans: The breaking of chemical bonds is known as bond cleavage. There are two types of bond cleavage based on the sharing of electrons between two atoms of a bond:
- Homolytic cleavage: The two electrons in a bond are equally shared between the two atoms.
- Heterolytic cleavage: One of the atoms receives both the electrons of the bond.
Ques: Is H2O a nucleophile or an electrophile? (1 mark)
Ans: The water (H2O) is an extreme polar compound with a huge density of electrons or electron-rich molecules. So, water is a good example of a nucleophile.
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