Elements of photochemistry and radiochemistry- Part 4-

[benainouna]

The photolysis of hydrogen iodide:

When exposed to light, some diatomic molecules separate into atoms. The energy factors at play determine whether a chain reaction is likely to occur. As a result, when exposed to light, the chlorine molecules in the mixture of hydrogen and chlorine dissociate into atoms, triggering the initiation of a chain reaction. In contrast, when hydrogen iodide is exposed to light with a wavelength of at least 2500A, the process proceeds quite differently:
hv+ HI=== H+ I
H+ HI=== H2+ I
2I=== I2

• Because the reaction that produces the hydrogen atoms (I+ HI=== I2+ H) is endothermic and slow-moving, there is no chain reaction.

The photolysis of acetic aldehyde:

Light frequently initiates chain reactions that dissociate complex molecules into simpler molecules; for example, the photolysis of acetic aldehyde proceeds as follows:

[Created by using microsoft paint and word]

• The energy absorbed during the primary act of a polyatomic molecule is then distributed among the various parts of the molecule, making it practically impossible to predict the direction of evolution of the subsequent reactions. Light can cause processes in complex molecules that result in the restructuring of the entire molecule.

The photolysis of fumaric acid:

fumaric acid is converted into Maleic acid under the influence of light irradiation, and vice versa, establishing an equilibrium in a mixture of these bodies:

[Created by using microsoft paint and word]

maleic acid- Wikimedia Commons

• Polyenes, thioindigo, carotene, and other compounds exhibit similar behavior. Sometimes light causes photodimerization, as in:
  
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• According to the following scheme, radical formation causes this reaction to spread in a chain:
  
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• We also have reactions like:
  2RH==== R-R+2H
  which are dehydrodimerizations. This is particularly the case for the reaction:
  
  [Created by using microsoft paint and word]
  where Ar denotes an aryl.

References:

• [Smail Meziane: Livre Chimie générale- Structure de la matiére. Berti edition, Alger, 2006]

• [Principles & Applications of Photochemistry, Brian Wardle,Wiley, ISBN 0470014938]

• [R. OUAHES et B. DEVALLEZ- Chimie générale- Office des publacations universitaires- Alger]

• Monika Krysta (2006) Modélisation numérique et assimilation de données de la dispersion de radionucléides en champ proche et à échelle continentale, Thèse de doctorat de l’Université Paris XII - Val de Marne

• Victor Henri, René Wurmser. Le mécanisme élémentaire des actions photochimiques. J. Phys. Radium, 1927, 8 (7), pp.289-310. ff10.1051/jphysrad:019270080

• J. R. Mahan (2002). Radiation heat transfer: a statistical approach (3rd ed.). Wiley-IEEE. p. 58. ISBN 978-0-471-21270-6.

• Braslavsky, S. E. (2007-01-01). "Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)". Pure and Applied Chemistry. 79 (3): 293–465. doi:10.1351/pac200779030293. ISSN 1365-3075.

• [Norman S. Allen- Photochemistry and Photophysics of Polymeric Materials]

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