Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Organic chemistry → Reaction mechanism ↓


Pericyclic reactions


Pericyclic reactions are a fascinating area of study in organic chemistry, characterized by the coordinated reorganization of electrons through cyclic transition states. These reactions are unique in that they proceed without intermediates, occur via a single, coordinated mechanism, and are usually induced by heat or light. Understanding pericyclic reactions requires an understanding of concepts such as molecular orbitals and symmetry, and their study involves the Woodward–Hoffmann rule and the Dever–Zimmerman approach.

Features of pericyclic reactions

A fundamental feature of pericyclic reactions is their coordinated nature, meaning that the bond-breaking and bond-forming processes occur simultaneously. This "simultaneous" character is in contrast to stepwise mechanisms that involve discrete intermediates. Pericyclic reactions involve the redistribution of π electrons in the cyclic array of interacting orbitals, often making them stereospecific, maintaining or translating the stereochemistry from reactants to products in a predictable manner.

Types of pericyclic reactions

Pericyclic reactions can be divided into several types. Each type is defined by the nature of the bond changes and the mechanism involved:

  • Cycloaddition reactions: In these reactions two unsaturated molecules are combined to form a cyclic product. The most famous example is the Diels-Alder reaction, which is a [4+2] cycloaddition.
  • Electrocyclic reactions: In these, conjugated polyenes are converted into cyclic products by forming sigma bonds or by breaking down the cyclic compound into polyenes.
  • Sigmatropic rearrangement: It involves the transfer of the σ-bonded atom or group with a concurrent change in the position of the π-bonds.
  • Cheletropic reactions: These are special versions of cycloaddition reactions in which two bonds are formed or broken on the same atom.

Cycloaddition reactions

Cycloaddition reactions involve the union of two or more unsaturated molecules to form cyclic products, and are usually classified based on the number of electrons involved. The usual notation is [m+n] cycloaddition, where m and n are the number of electrons contributed by each reactant. The Diels-Alder reaction is the prototypical [4+2] cycloaddition reaction:

CH2=CH-CH=CH2 + CH2=CH2 → Cyclohexene

This can be viewed as follows:

In the Diels-Alder reaction, a diene reacts with a dienophile to form a six-membered ring. It is one of the most powerful synthetic tools in organic chemistry because of its ability to create complex ring systems rapidly and with high stereospecificity.

Electrocyclic reactions

Electrocyclic reactions involve the conversion of a conjugated polyene to a cyclic structure or vice versa. The reaction is often thermal (heat-induced) or photochemical (light-induced) in nature. For example:

cis,cis-1,3,5-hexatriene → cyclohexadiene

The electrocyclic reaction observed here can be visualized as follows:

Here, a π bond is transformed into a σ bond about a rotation axis of symmetry, resulting in the closing or opening of the circle.

Sigmatropic rearrangement

Sigmatropic rearrangements involve the migration of σ-bonded atoms or groups along the corresponding π-bonded structure of the same molecule. An example of this is the [3,3]-sigmatropic rearrangement known as the Cope rearrangement:

1,5-hexadiene ↔ 1,5-hexadiene

Here is a visual example:

This rearrangement involves the transfer of a hydrogen atom or an alkyl group between carbon atoms, changing the valency, while the system remains intact.

Cheletropic reactions

Cheletropic reactions focus on the formation of two sigma bonds on the same atom. An example of such a reaction is the reaction of sulfur dioxide (SO2) with butadiene:

CH2=CH-CH=CH2 + SO2 → Sulfolene

The following diagram shows an example of a cheletropic reaction:

In cheletropic reactions, the central atom passes through an excited state where it can make or break two bonds simultaneously, leading to the formation of specific products such as cyclic or sulfur-containing compounds.

Woodward–Hoffmann rules

The Woodward-Hoffmann rules define the stereochemistry of pericyclic reactions and are based on the principle of conservation of orbital symmetry. They show that the order of the pericyclic reaction is controlled by the symmetry properties of the frontier (most occupied and least unoccupied) orbitals. The rules predict whether a particular pericyclic process will be permitted under thermal or photochemical conditions.

Thermal and photochemical conditions

Various conditions affect the symmetry and behaviour of electrons in molecular orbitals:

  • Thermal reactions: These involve interactions between molecules at high temperatures. The allowed reactions involve conservation of σ orbitals and are usually thermally induced.
  • Photochemical reactions: These involve the excitement of electrons by light radiation, changing their energy and behaviour, and lead to reactions that are symmetry-forbidden under thermal conditions.

Dewar–Zimmerman approach

The Dever-Zimmerman approach, or aromatic transition state theory, provides an alternative explanation to orbital symmetry rules. This approach views pericyclic reactions in terms of the aromaticity of the transition state and emphasizes the effect of the cyclic network of interacting orbitals. Mapping these interactions can help understand the stability and wettability of the transition state.

Conclusion

Pericyclic reactions are an integral part of organic chemistry and provide a unique method for constructing complex molecular entities with specific stereochemistry. From fundamental principles governing orbital symmetry to practical applications in synthesis, the study of pericyclic reactions bridges theoretical and experimental chemistry, enhancing our understanding and expanding the possibilities within the field.


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Pericyclic reactions

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