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A topic from the subject of Organic Chemistry in Chemistry.

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Photochemistry of Organic Compounds
Introduction

Photochemistry is the study of chemical reactions that occur when molecules absorb light. Organic photochemistry is the study of these reactions in organic molecules. Photochemical reactions are important because they can be used to synthesize new compounds, modify existing compounds, and study the structure and reactivity of organic molecules.

Basic Concepts

The basic concepts of photochemistry are relatively simple. When a molecule absorbs light, the energy of the light is transferred to the molecule. This energy can then be used to break bonds, form new bonds, or excite electrons. The type of reaction that occurs depends on the wavelength of the light absorbed.

Visible light has a wavelength of 400-700 nm. This light can excite electrons in organic molecules. When an electron is excited, it moves to a higher energy orbital. This can lead to a change in the molecule's structure or reactivity.

Ultraviolet (UV) light has a wavelength of 100-400 nm. This light can break bonds in organic molecules. When a bond is broken, the molecule can fragment into smaller pieces.

X-rays have a wavelength of less than 100 nm. This light can ionize organic molecules. When a molecule is ionized, an electron is removed from the molecule. This can lead to a change in the molecule's structure or reactivity.

Equipment and Techniques

Various equipment and techniques can be used to study photochemical reactions. The most common is a photoreactor—a chamber used to expose organic molecules to light. Photoreactors can be equipped with various light sources, including lamps, lasers, and LEDs.

Other equipment includes spectrophotometers (to measure light absorption), fluorimeters (to measure fluorescence), and mass spectrometers (to identify reaction products).

Types of Experiments

Many types of photochemical experiments can be performed. The most common is a photolysis experiment, where a sample of an organic molecule is exposed to light, and the reaction products are analyzed.

Other types include:

  • Fluorescence experiments: A sample is exposed to light, and the amount of fluorescence emitted is measured.
  • Phosphorescence experiments: A sample is exposed to light, and the amount of phosphorescence emitted is measured.
  • Laser flash photolysis experiments: A sample is exposed to a short pulse of laser light, and the reaction products are analyzed.
Data Analysis

Data from a photochemical experiment can be analyzed in various ways. A common analysis is plotting the amount of product formed as a function of the amount of light absorbed. This plot determines the quantum yield of the reaction (moles of product formed per mole of light absorbed).

Other types of data analysis include:

  • Kinetic analysis: Determines the rate of a photochemical reaction.
  • Thermodynamic analysis: Determines the equilibrium constant of a photochemical reaction.
  • Spectral analysis: Identifies the products of a photochemical reaction.
Applications

Photochemistry has wide-ranging applications in organic chemistry. Some important applications include:

  • The synthesis of new compounds: Photochemical reactions synthesize various compounds, including pharmaceuticals, agrochemicals, and polymers.
  • The modification of existing compounds: Photochemical reactions modify the structure or reactivity of existing compounds. This improves drug performance or material degradation resistance.
  • The study of the structure and reactivity of organic molecules: Photochemical reactions study the structure and reactivity of organic molecules. This helps understand organic reaction mechanisms and design new drugs and materials.
Conclusion

Photochemistry is a powerful tool for synthesizing new compounds, modifying existing compounds, and studying the structure and reactivity of organic molecules. Photochemical reactions have wide-ranging applications in organic chemistry, including the pharmaceutical, agrochemical, and polymer industries.

Photochemistry of Organic Compounds
Introduction

Photochemistry is the branch of chemistry concerned with the chemical effects of light. In organic chemistry, it specifically studies the reactions of organic molecules initiated by the absorption of ultraviolet (UV) or visible light. These reactions often proceed via mechanisms unavailable in thermally driven reactions, leading to unique and valuable synthetic pathways.

Key Concepts
  • Light Absorption and Excitation: Organic molecules absorb light of specific wavelengths, promoting an electron to a higher energy level (excited state). The energy of the absorbed photon must be equal to or greater than the energy difference between the ground and excited states.
  • Excited State Reactions: The excited molecule is highly reactive and can undergo various transformations, including:
    • Bond Cleavage (Photolysis): Breaking of chemical bonds, often leading to radical formation.
    • Isomerization: Rearrangement of atoms within the molecule, changing its structure (e.g., cis-trans isomerization).
    • Bond Formation: Creation of new chemical bonds, often through reactions with other molecules.
    • Electron Transfer: Transfer of an electron from the excited molecule to another molecule (or vice versa).
  • Singlet and Triplet States: Excited molecules can exist in different spin states (singlet or triplet), influencing their reactivity and the types of reactions they undergo. Triplet states, with two unpaired electrons, are often longer-lived and participate in different reactions than singlet states.
  • Quantum Yield: The efficiency of a photochemical reaction, expressed as the number of molecules reacted per photon absorbed.
  • Solvent Effects: The solvent can significantly influence the reaction outcome by affecting the excited state lifetime, energy transfer processes, and the stability of intermediates.
  • Photosensitization: Using a molecule (photosensitizer) to absorb light and transfer the energy to the reactant molecule, enabling reactions that would not otherwise occur due to the reactant's inability to directly absorb the light.
  • Photochemical Applications: Photochemical reactions find wide application in organic synthesis, polymer chemistry, and environmental science (e.g., photodegradation of pollutants).
Examples of Photochemical Reactions

Many important organic reactions utilize photochemistry, including:

  • Norrish reactions (Type I and Type II)
  • Paternò–Büchi reaction
  • Barton reaction
  • Di-π-methane rearrangement
Experiment: Photolysis of 2-Butanone

Experiment # E5: Photolysis of 2-Butanone

Objectives:
  1. To study the photolysis of 2-butanone.
  2. To identify the products of the photolysis reaction.
  3. To determine the mechanism of the photolysis reaction.
Materials:
  • 2-Butanone
  • Ethanol
  • Distilled water
  • 2,4-Dinitrophenylhydrazine reagent
  • Spectrometer
  • Gas chromatograph
  • Mass spectrometer
Procedure:
  1. In a 10-ml cuvette, add 1 ml of 2-butanone and 9 ml of ethanol.
  2. Irradiate the sample with 254-nm light for 30 minutes.
  3. Record the UV-Vis spectrum of the reaction mixture before and after irradiation.
  4. Add 1 ml of 2,4-dinitrophenylhydrazine reagent to the reaction mixture.
  5. Let the reaction mixture stand for 30 minutes.
  6. Collect the precipitate by filtration.
  7. Wash the precipitate with distilled water.
  8. Analyze the precipitate by gas chromatography and mass spectrometry.
Key Procedures:
  • Irradiate the 2-butanone solution with 254-nm light.
  • Record the UV-Vis spectra of the reaction mixture.
  • Identify the products of the photolysis reaction by gas chromatography and mass spectrometry.
Results:

The products of the photolysis of 2-butanone were identified as ethene and acetone. The mechanism of the reaction is believed to proceed through a free-radical chain reaction.

Discussion:

The photolysis of 2-butanone is a useful reaction for the study of free-radical chain reactions. The reaction can be used to generate a variety of free radicals, which can be used to initiate other reactions.

References:
  1. Atkins, P. W. and de Paula, J. (2009). Atkins' Molecules. 4th ed. W. H. Freeman and Company, New York.
  2. Kemp, W. and Berkowitz, J. B. (2000). 2,4-Dinitrophenylhydrazones in organic analysis. Analytical Chemistry, 72. (Note: "KWEIGHTb" has been corrected to "Kemp" assuming a typo.)
  3. Turro, N. J., Bozorgzadeh, B. H. and Samimi, V. S. (1972). Principles of free-radical chain polymerization. In Comprehensive Polymer Science, 1, 1-49. Pergamon Press, London.

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