Photochemistry

The term is meant photochemistry chemical reactions initiated by exposure to light. The prerequisite for this is an absorption of light by the molecule to react. That is, the absorption properties of the molecule has to match the wavelength of the light used. Occasionally, a photosensitizer is used, the primary is excited and transfers energy to the molecules to be reacted.

In addition to the action of light can photochemistry also operate with higher energy quanta. With such photoinduced processes engaged and A. X-ray photochemistry or the high-energy photochemistry. Here is the synchrotron radiation in the chemical application.

The absorption of a photon leads to an energetically excited state of a molecule that its so energy gained either by photon emission from the excited singlet state ( fluorescence) or from the triplet state ( phosphorescence), losing by nonradiative relaxation or by a chemical reaction can.

The efficiency of each process can be calculated by quantum yields. These describe the fraction of molecules which have, in accordance with one of the above mentioned processes subdued. The sum of the quantum yields is - except for chain reactions - 1

Examples of photochemical reaction types are:

  • Cleavage, such as the removal of carbon monoxide from carbonyl compounds.
  • Surroundings, such as the isomerization of cycloheptatriene to toluene, for example.
  • Cyclization, such as the formation of carbazole of diphenylamine ( pericyclic reaction) and the cyclization of 1,3-butadiene to cyclobutene.
  • Light-induced chain reactions: Reaction of hydrogen and chlorine (chlorine detonating gas ) to hydrogen chloride, the reaction mechanism was particularly enlightened by Walther Nernst and Max Bodenstein
  • Photochlorination of alkanes, such as methane
  • Regioselective side-chain halogenation of alkylated aromatics ( the simplest example: chlorination of toluene at the methyl group ) according to the "SSS " rule (sun, boiling point, side chain)
  • Sulfochlorination of alkanes with sulfur dioxide and chlorine

A known example of a chemical reaction with the photochemical reaction steps of the photosynthesis.

The first summary books on the preparative organic photochemistry Alexander Schoenberg wrote.

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