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