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 ↓


Photochemical reactions


Photochemical reactions are chemical reactions that occur in the presence of light, usually involving the absorption of photons. These reactions play an important role in a variety of chemical and biological processes, and they are important in many different areas of chemistry, including organic chemistry.

A photochemical reaction begins when a molecule absorbs a quantum of light, often represented as a photon, and becomes excited from its ground state to a higher energy state. This excited state can cause the molecule to participate in reactions it would not normally do.

Mechanism of photochemical reactions

When a molecule absorbs light, it moves from a lower energy state (ground state) to a higher energy state (excited state). This process can be summarized using a simple equation:

reactant + hν → excited reactant

Here, hν denotes the energy of the absorbed light photon. The excited reactant can undergo several possible processes:

  1. Emission: The molecule can return to its ground state by emitting a photon, a process known as fluorescence or phosphorescence.
  2. Fragmentation: The excited molecule may split into two or more fragments.
  3. Rearrangement: The molecule may undergo structural rearrangement resulting in the formation of new products.
  4. Reaction with another molecule: The excited molecule can react with another molecule to form new products.

Example: photosynthesis

One of the best-known photochemical reactions is photosynthesis, the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose:

6 CO 2 + 6 H 2 O + light energy → C 6 H 12 O 6 + 6 O 2

In this process, chlorophyll absorbs light, and excites electrons that help convert carbon dioxide and water into glucose and oxygen.

Types of excited states

Molecules can have a variety of excited states, mainly singlet and triplet states, determined by the spin of the electrons.

Singlet and triplet states

Singlet state: In this state, the spins of the electrons in the excited state are paired.

Singlet state Ground state

Triplet state: Here the spins of the electrons are unpaired, and because of its structure it is generally lower in energy than the singlet state.

Triplet state Singlet state

The transition from the singlet to the triplet state is known as intersystem crossing. Triplet state reactions are often slower than singlet state reactions, but can proceed by different pathways.

Photochemical reaction kinetics

The kinetics of photochemical reactions can be complex due to the involvement of excited states. These reactions are generally faster than thermal reactions because they involve highly reactive intermediate states.

Rate laws: Photochemical reaction rates are affected by many factors, including the intensity of the light, the concentration of the reactants, and the presence of any quenchers or inhibitors that may interact with the excited states.

Quantum yield

An important concept in photochemical reactions is quantum yield, which measures the number of molecules that react per photon of absorbed light. Quantum yield can help determine the efficiency of a photochemical reaction.

Quantum yield (Φ) = (number of molecules reacting) / (number of photons absorbed)

Applications of photochemical reactions

Photochemical reactions have numerous applications in various fields, including medicine, materials science, and environmental studies.

Photochemistry in medicine

In medicine, photochemical reactions are used in photodynamic therapy (PDT) to treat certain types of cancer. In this treatment, a photosensitizing agent is activated by light to generate reactive oxygen species that target and destroy cancer cells.

Environmental photochemistry

Photochemical reactions play a vital role in the degradation of pollutants in the environment, resulting in the disintegration of harmful compounds in the presence of sunlight.

Common photochemical reactions in organic chemistry

In organic chemistry, several photochemical reactions are commonly studied, including isomerization, dimerization, and various addition reactions.

Isomerization

Photochemical isomerization involves the conversion of a molecule from one isomeric form to another upon absorption of light.

Example: cis-2-butene (exposed to light) → trans-2-butene

Dimerization

Dimerization is a process in which two molecules combine to form a dimer. In photochemistry, it can be induced by UV light.

Example: 2 anthracene molecules (UV light) → anthracene dimer

[2+2] Cycloaddition

[2+2] Cycloadditions are a class of reactions where two alkenes or alkynes react in the presence of light to form cyclobutane products.

Example: 2 ethylene molecules (UV light) → cyclobutane

Exploring energy diagrams

Energy diagrams can help visualize transitions from the ground state to the excited state and possible reaction paths.

Ground state Excited state

In the above figure, we see an energy profile with the ground state and excited state, showing the energy change during the photochemical reaction.

Conclusion

Photochemical reactions are essential in organic chemistry and other scientific fields due to their unique properties and applications. Understanding the mechanisms, kinetics, and applications of these reactions can provide insight into their enormous potential and relevance.


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Photochemical reactions

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