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 ↓


Pericyclic Reactions


Pericyclic reactions are a type of organic reaction mechanism characterized by coordinated, cyclic transition states, without any intermediates. These reactions have been extensively studied because they exhibit unique properties and provide profound insights into molecular orbital theory. The beauty of pericyclic reactions lies in their beautiful symmetry and the fact that, unlike other reaction types, they proceed through a coordinated mechanism without forming intermediate steps.

Fundamentals of pericyclic reactions

At the core of pericyclic reactions is the concept of conserved orbital symmetry. These reactions involve the rearrangement of electrons in the form of a cyclic array of orbitals passing through the transition state. This orbital symmetry conservation is well explained by the Woodward-Hoffmann rules, which give information on when these reactions are allowed.

The main types of pericyclic reactions include:

  • Cyclocombination reactions: Two or more unsaturated molecules (or parts of the same molecule) combine with rearrangement of electrons to form cyclic products.
  • Electrocyclic reactions: A single pi bond is converted into a sigma bond, or vice versa, when the system undergoes ring closure or ring opening.
  • Sigmatropic rearrangement: A sigma bond is shifted to a pi system, resulting in a new sigma bond and a recombined pi system.

Cycloaddition reactions

Cycloaddition reactions are important in synthetic organic chemistry, particularly the [2+2] and [4+2] cycloadditions. The [4+2] cycloaddition, also known as the Diels-Alder reaction, is one of the most ubiquitous examples. It involves a conjugated diene and a dienophile that form a six-membered ring.

+--------+
| diene  | +--+            +--+          |        +--+ + dienophile --> Cyclohexene ring

Electrocyclic reactions

Electrocyclic reactions involve the conversion of a pi bond to a sigma bond or vice versa, and these are generally ring opening or ring closing processes. Ring closure or opening occurs through a coordinated process that maintains the stereochemistry of the pi systems involved.

Conjugated pi system (open chain) --> sigma bond formation (ring closure)

Sigmatropic rearrangement

Sigmatropic rearrangement is a shift of the sigma bond in a pi system. Woodward-Hoffmann rules are applied here to predict the stereochemical outcome and feasibility of the rearrangement.

Sigma bond shift pi system --------------> (new sigma bond)

Woodward–Hoffmann rules

The Woodward–Hoffmann rules are fundamental in classifying whether a pericyclic reaction is thermally or photochemically allowed. They state that symmetry-allowed reactions are those where the symmetry of the molecular orbitals allows a spontaneous transition through the reaction pathway.

Using these rules, one can determine whether a reaction is possible by examining the symmetry properties of the molecular orbitals involved. Generally, reactions that preserve orbital symmetry during thermolysis are allowed.

Mechanism and theory

The pericyclic reaction mechanism can be understood through several theoretical approaches:

  • Orbital symmetry conservation: Pericyclic reactions involve coordinated changes in the molecular orbitals, which retain their symmetry properties throughout the reaction process.
  • Möbius and Hückel topology: The difference between allowed and forbidden reactions can often be seen in terms of Möbius vs. Hückel topology in molecular orbital diagrams.

Orbital symmetry conservation

In the pericyclic process, the atomic or molecular orbitals involved change in such a way that the symmetry is preserved. This theory helps to predict whether the pericyclic step is symmetry-permitted or forbidden according to the conservation laws.

Initial orbitals            Transition State            Final orbitals 
|----------|            Conservation of Symmetry            |-----------|

Example in detail

Here, we will take a closer look at specific examples of different types of pericyclic reactions to better understand their mechanistic details.

Example of Diels-Alder reaction

Consider the simple Diels-Alder reaction between 1,3-butadiene and ethene. This reaction proceeds through a cyclic transition state and forms a cyclohexene derivative.

Witch Dienophile Cyclohexene

Electrocyclic reaction examples

A classic example of an electrocyclic reaction is the conversion of hexatriene to cyclohexadiene. This process can proceed via either a conorotatory or disorotatory mechanism, depending on the thermal or photochemical conditions.

Hexatriene Cyclohexadiene

Sigmatropic rearrangement examples

An example of a sigmatropic rearrangement is the Cope rearrangement, in which a 1,5-diene is reorganized into another 1,5-diene via a cyclic transition state.

1,5-Diene 1,5-Diene(rearranged)

Importance in organic chemistry

Pericyclic reactions are fundamental in organic chemistry because of their utility in the construction of cyclic compounds, which are prevalent in natural products and synthetic materials. The predictability of these reactions through symmetry arguments also makes them invaluable for synthetic planning.

This ability to predict the stereochemistry and feasibility of reactions based on symmetry has greatly expanded the toolkit available to synthetic chemists, providing a deeper understanding and control of chemical reactivity.

Conclusion

In summary, pericyclic reactions represent a fascinating area of organic chemistry. They are unified by their coordinated mechanisms, conservation of orbital symmetry, and the predictive power of the Woodward-Hoffmann rules. These reactions not only provide synthetic utility for the construction of complex molecules, but also reinforce fundamental concepts in molecular orbital theory.


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Pericyclic Reactions

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