PHD
  1. Inorganic chemistry  1.1. Coordination chemistry  1.1.1. Ligand field theory  1.1.2. Crystal field theory  1.1.3. Molecular Orbital Theory for Coordination Compounds  1.1.4. Stability of Coordination Compounds  1.1.5. Electronic spectra of coordination compounds  1.1.6. Isomerism in Coordination Compounds  1.1.7. Kinetics and mechanism of coordination reactions  1.2. Organometallic Chemistry  1.2.1. Metal Carbonyls  1.2.2. Metal alkyl and aryl  1.2.3. Fischer and Schrock carbonaceans  1.2.4. Oxidative Addition and Reductive Elimination  1.2.5. Metal Hydrides  1.2.6. Catalysis by organometallic compounds  1.2.7. Metallocenes and Sandwich Complexes  1.2.8. CH activation  1.3. Solid state chemistry  1.3.1. Crystal structures and lattices  1.3.2. Band theory and electrical properties  1.3.3. Defects in the crystal  1.3.4. Synthesis of solid state materials  1.3.5. Magnetic and optical properties of solids  1.3.6. Superconductivity  1.3.7. Solid state ionics  1.4. Bio-inorganic chemistry  1.4.1. Metalloproteins and enzymes  1.4.2. Metal ion transport and storage  1.4.3. The role of metals in medicine  1.4.4. Bioinspired catalysis  1.4.5. Metal–DNA interactions  1.4.6. Metal Complexes in Medical Science  1.5. Lanthanides and Actinides  1.5.1. Electronic configuration and oxidation states in lanthanides and actinides  1.5.2. Spectral and Magnetic Properties in Lanthanides and Actinides  1.5.3. Separation and Extraction of Lanthanides and Actinides  1.5.4. Coordination chemistry of f-block elements  1.6. Main Group Chemistry  1.6.1. Chemistry of boron compounds  1.6.2. Chemistry of Silicon and Germanium Compounds  1.6.3. Chemistry of Phosphorus and Sulfur Compounds  1.6.4. Organosilicon chemistry  1.6.5. Noble Gas Chemistry
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition and substitution reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic Reactions  2.1.7. Photochemical reactions  2.2. Stereoscopic  2.2.1. Chirality and optical activity  2.2.2. Structural analysis  2.2.3. Stereoelectronic effects  2.2.4. Asymmetric synthesis  2.2.5. Dynamic Stereochemistry  2.3. Organometallic Chemistry in Organic Synthesis  2.3.1. Organolithium and organomagnesium reagents  2.3.2. Transition metal catalyzed coupling reactions  2.3.3. Palladium-catalyzed reactions  2.3.4. Olefin Metathesis  2.3.5. Gold and silver catalysis  2.4. Natural products chemistry  2.4.1. Alkaloids and terpenoids  2.4.2. Steroids and Flavonoids  2.4.3. Total synthesis of natural products  2.4.4. Biosynthesis of natural products  2.5. Supramolecular Chemistry  2.5.1. Host-Guest Chemistry  2.5.2. Self-assembly and molecular recognition  2.5.3. Supramolecular Catalysis  2.5.4. Molecular Machines
  3. Physical Chemistry  3.1. Thermodynamics  3.1.1. Laws of Thermodynamics  3.1.2. Chemical Potential and Equilibrium  3.1.3. Statistical thermodynamics  3.1.4. Non-equilibrium thermodynamics  3.2. Quantum Chemistry  3.2.1. Schrödinger equation and its applications  3.2.2. Molecular orbital theory  3.2.3. Density functional theory  3.2.4. Post-Hartree–Fock methods  3.3. Chemical kinetics  3.3.1. Reaction rate theory  3.3.2. Mechanism and rate laws  3.3.3. Catalysis and enzyme kinetics  3.3.4. Reaction dynamics  3.4. Spectroscopy and molecular structure  3.4.1. Infrared Spectroscopy  3.4.2. UV-visible spectroscopy  3.4.3. Nuclear Magnetic Resonance Spectroscopy  3.4.4. Mass Spectrometry  3.4.5. Raman Spectroscopy  3.5. Surface and Colloid Chemistry  3.5.1. Absorption and Catalysis  3.5.2. Surfactants and micelles  3.5.3. Colloidal Stability  3.5.4. Nanomaterials and Interfaces
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.1.4. Ion Chromatography  4.2. Electroanalytical techniques  4.2.1. Voltammetry and Polarography  4.2.2. Conductometry and potentiometry  4.2.3. Colometry  4.3. Spectroscopic Methods  4.3.1. Atomic absorption spectroscopy  4.3.2. Fluorescence Spectroscopy  4.3.3. X-ray diffraction  4.3.4. Electron paramagnetic resonance spectroscopy  4.4. Chemometrics  4.4.1. Data processing and statistical methods  4.4.2. Multidisciplinary analysis  4.4.3. Machine Learning in Analytical Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.2. Quantum chemistry methods  5.3. Computational drug design  5.4. Machine Learning in Chemistry  5.5. Ab initio molecular dynamics
  6. Biophysics and Medicinal Chemistry  6.1. Drug discovery and design  6.2. Enzyme kinetics and inhibition  6.3. Chemical biology approach  6.4. Protein-ligand interactions  6.5. Bioorthogonal chemistry
  7. Materials chemistry  7.1. Polymer chemistry  7.1.1. Polymerization mechanism  7.1.2. Functional Polymers  7.1.3. Biodegradable Polymer  7.1.4. Conducting and semiconducting polymers  7.2. Nano Chemistry  7.2.1. Nanoparticles and nanostructures  7.2.2. Carbon-based nanomaterials  7.2.3. Quantum dots  7.3. Energy and Environmental Chemistry  7.3.1. Battery and Fuel Cell Chemistry  7.3.2. Green chemistry and sustainable materials  7.3.3. Photocatalysis

PHD → Organic chemistry → Reaction mechanism ↓


Rearrangement reactions


Rearrangement reactions are a fascinating class of chemical reactions in organic chemistry where the structure of a molecule is rearranged to form a new isomer. Unlike substitution or elimination reactions, rearrangements involve the transfer of atoms or functional groups within the molecule, creating a new connectivity between atoms. These reactions can result in significant changes in the physical and chemical properties of compounds.

Understanding the basics

At their core, rearrangement reactions involve the reorganization of the carbon skeleton or functional groups within a molecule. This can occur through the movement of hydrogen atoms, alkyl groups, or other substituents. The driving force for these reactions is often the stabilization of a carbocation intermediate, a shift in electron density, or the formation of a more stable molecule.

Common types of rearrangement reactions

There are many well-known rearrangement reactions in organic chemistry. Some of the main types include the Wagner-Meerwein rearrangement, Beckmann rearrangement, Pinacol rearrangement, Hofmann rearrangement, and Curtius rearrangement. Each type has its own specific mechanism and applications.

1. Wagner–Meerwein rearrangement

This reaction involves the transfer of an alkyl group to stabilize the carbocation. This is commonly seen in reactions of alcohols under acidic conditions, where rearrangements can lead to the formation of more stable carbocations. For example, the conversion of a less stable secondary carbocation into a more stable tertiary carbocation.

    R-CH2-C^+H-CH2R' → R-CH-CH2-C^+HR'

2. Beckmann rearrangement

The Beckmann rearrangement involves the rearrangement of oximes into amides under acidic conditions. An example of this is the conversion of cyclohexanone oxime to caprolactam, an important industrial process in the production of nylon.

    C6H11N-OH → C6H11NH

3. Pinacol rearrangement

In the pinacol rearrangement, a vicinal diol is converted to a ketone under acidic conditions. This rearrangement generally occurs via a carbocation intermediate. The rearrangement from pinacol to pinacolone is an example.

    (CH3)2C(OH)-C(OH)(CH3)2 → (CH3)3CC=O

4. Hofmann rearrangement

The Hofmann rearrangement involves the conversion of primary amides into primary amines with the loss of a carbon atom. This rearrangement proceeds via the formation of an isocyanate intermediate. It is a useful reaction for decomposing large molecules by removing a carbon atom.

    R-CONH2 + Br2 + NaOH → R-NH2 + CO2 + NaBr + H2O

5. Curtius rearrangement

In the Curtius rearrangement the acyl azide is decomposed to isocyanate on heating, which can then be converted to amine, urea or carbamate. This reaction is used for the synthesis of various nitrogen-containing compounds.

    RCON3 → RN=C=O → RNH2

Mechanical details

The mechanisms of rearrangement reactions can vary considerably, but they often involve carbocation intermediates. The stability of these intermediates is important in determining whether rearrangement will occur. Stabilization can be achieved through hyperconjugation, resonance, or the formation of more stable cyclic structures.

Consider a general rearrangement mechanism proceeding via a carbocation intermediate:

    R-CH2-C^+-CH3 → R-CH-CH2-C^+

In this illustration, the positively charged carbon atom (the carbocation) allows rearrangement by facilitating the migration of alkyl or aryl groups from neighboring atoms. The resulting rearranged carbocation can further participate in reactions leading to product formation.

Typical case study

Wagner–Meerwein rearrangement: tertiary carbocation stability

Let's focus on the Wagner-Meerwein rearrangement to illustrate the stability of tertiary carbocations. This rearrangement is a common occurrence during the dehydration of alcohols. When an alcohol undergoes protonation and loses a molecule of water, a carbocation is formed. If a more stable carbocation can be produced through rearrangement, the molecular structure will change accordingly.

        (CH3)3C-OH → (CH3)3C^+ + H2O

Beckmann rearrangement: synthesis of lactams

The Beckmann rearrangement is particularly useful in the synthesis of lactams, which are cyclic amides. Under acidic conditions the cyclohexanone oxime undergoes a rearrangement to form caprolactam:

        C6H11N-OH → C6H11NH → caprolactam

This reaction is industrially important in the production of nylon-6, forming a key intermediate that, upon polymerization, forms a versatile synthetic fiber.

Pinacol rearrangement: from diol to ketone

Consider the pinacol rearrangement as a transformation of a vicinal diol to the corresponding ketone. Under acidic conditions, one of the hydroxyl groups is protonated and lost as water, forming a carbocation. Next, molecular rearrangement forms the ketone:

        (CH3)2C(OH)-C(OH)(CH3)2 → (CH3)3C-CO

Conclusion

Rearrangement reactions play an important role in organic synthesis, providing routes to reorganize molecules into more stable or differently configured isomers. As seen through specific examples such as the Wagner-Meerwein, Beckmann, and Pinacol rearrangements, these reactions enable chemists to create a wide variety of structural isomers and alter functionality within molecules. Each type of rearrangement has unique characteristics and conditions under which it occurs, illustrating the diversity of organic reactions possible through these interesting mechanisms.


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Rearrangement reactions

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