Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → Organic chemistry ↓


Pericyclic Reactions


Pericyclic reactions are a class of organic reactions that proceed via a coordinated process and usually involve the cyclic redistribution of bond electrons. These reactions do not involve ionic or radical intermediates and occur via a closed-loop mechanism. Understanding pericyclic reactions is important in organic chemistry because they provide information about molecular transformations and the behavior of electrons in cyclic processes.

Types of pericyclic reactions

1. Cycloaddition reactions

Cycloaddition reactions involve the interaction of two π-systems, forming a new ring. A common example is Diels-Alder reaction, in which a conjugated diene reacts with a dienophile to form a cyclohexene system.

   
    │ │ │ + │ │ │ → cyclohexene

The Diels-Alder reaction is a classic example of a [4+2] cycloaddition reaction, where four π-electrons from the diene and two π-electrons from the dienophile participate in the formation of the cyclic product.

2. Electrocyclic reactions

Electrocyclic reactions involve the conversion of π-bonds to σ-bonds or vice versa, resulting in ring formation or ring breaking. These reactions are reversible and their direction depends on thermal or photochemical conditions.

Open-chain closed-ring

An example of an electrocyclic reaction is the ring closure of hexatrienes to form cyclohexadienes. According to the Woodward–Hoffmann rules, the reaction can proceed via a thermally allowed conorotatory pathway or a photochemically allowed disorotatory pathway.

3. Sigmatropic rearrangement

Sigmatropic rearrangements involve the migration of a σ-bond adjacent to one or more π-systems, resulting in a structural change without changing the total number of π or σ bonds. These reactions are represented by two numbers in parentheses, such as [1,3] or [3,3], indicating the migration path.

R1─R2─R3 → R1─R3─R2
    [1,3]

A typical example of a sigmatropic rearrangement is the Cope rearrangement, which is characterized by a [3,3] shift of the carbon atoms.

Woodward–Hoffmann rules

The Woodward–Hoffmann rules are important in determining the stereochemistry and feasibility of pericyclic reactions. By using symmetry properties and conservation of orbital symmetry, these rules allow chemists to predict the outcome and permitted or forbidden nature of pericyclic reactions.

Conservation of orbital symmetry

Conservation of orbital symmetry is a fundamental principle that states that the symmetry of the interacting orbitals must be maintained throughout the course of a shielding reaction.

Molecular orbital theory and pericyclic reactions

Molecular orbitals play an important role in understanding pericyclic reactions. The overlap of frontier orbitals such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is relevant to the progress of these reactions.

Cycloaddition reactions and frontier orbitals

In cycloaddition reactions, the interaction of the HOMO and LUMO of the reacting partners determines the order of the reaction. For example, in the Diels-Alder reaction, the HOMO of the diene interacts with the LUMO of the dienophile.

Electrocyclic reactions and molecular orbitals

For electrocyclic reactions involving ring closure or opening, the symmetry of the HOMO determines whether the reaction will proceed via a conortatory or disortatory pathway. For thermal reactions, if the HOMO has symmetric orbitals, conortatory is allowed; for antisymmetric orbitals, disortatory is allowed.

Examples and applications

1. Synthesis of natural products

Pericyclic reactions, especially the Diels-Alder reaction, are important in the synthesis of complex natural products. Their ability to form multiple stereocenters makes them highly valuable in synthetic organic chemistry.

2. Synthesis of pharmaceuticals

In pharmaceuticals, pericyclic reactions enable the construction of complex molecules essential for drug development, often providing a more environmentally friendly route than ionic reactions.

Conclusion

Pericyclic reactions are an important part of organic chemistry, which utilizes the beautiful principles of molecular orbital theory and symmetry. Understanding these reactions allows chemists to manipulate molecular architecture, enabling advances in fields ranging from materials science to drug development.


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Pericyclic Reactions

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