Photochemical smog

Examples of Photochemical smog in the following topics:

• Photochemical Smog

  • Photochemical smog is a major contributor to air pollution.
  • The components of photochemical smog were established during the 1950s.
  • The mixture of these primary and secondary pollutants forms photochemical smog.
  • Photochemical smog is composed of primary and secondary pollutants.
  • Recall what causes photochemical smog and why it is a problem for humans

• Photochemistry

  • Two fundamental principles are the foundation for understanding photochemical transformations:
  • The first law of photochemistry, the Grotthuss-Draper law, states that light must be absorbed by a compound in order for a photochemical reaction to take place.
  • The efficiency with which a given photochemical process occurs is given by its Quantum Yield (Φ).
  • Since many photochemical reactions are complex, and may compete with unproductive energy loss, the quantum yield is usually specified for a particular event.
  • The primary photochemical reaction is the homolytic cleavage of a carbon-carbon bond shown in the top equation.

• Cyclohexadienone Reactions

  • Derivatives of 2,5-cyclohexadienone are common in nature, and their photochemical transformations posed a challenge to early researchers.
  • These photochemical rearrangements occur by way of triplet excited states, which are conveniently depicted as diradicals.
  • Derivatives of 6,6-disubstituted 2,4-cyclohexadienones are also photochemically reactive.
  • Lumisantonin itself undergoes additional solvent dependent photochemical transformations, shown in the bottom part of the diagram.
  • Suggested mechanisms for the photochemical transformation of santonin and the thermal reaction of lumisantonin are shown above in the third diagram.

• Air Pollution

  • Increased ozone concentrations at ground levels: an increase in surface ozone that contributes to smog.

• Electrocyclic Reactions

  • Photochemical ring closure can be effected, but the stereospecificity is opposite to that of thermal ring opening.

• Non-Ionic Reactions

  • The term "pericyclic reaction" encompasses a large and varied group of concerted thermal and photochemical transformations.

• Alkene Isomerization

  • A photochemical reaction occurs when internal conversion and relaxation of an excited state leads to a ground state isomer of the initial substrate molecule, or when an excited state undergoes an intermolecular addition to another reactant molecule in the ground state.
  • The cis-trans photochemical isomerization of stilbene is a reaction of the first kind, as shown in the first diagram below.

• Mechanistic Background

  • Consequently, ultraviolet light is most often used to effect photochemical change.
  • The light required for a photochemical reaction may come from many sources.
  • Photochemical sensitization commonly occurs by a T1 + S0 → S0 + T1 reaction, where the bold red-colored species is the sensitizer.

• Perplexing Features of Pericyclic Reactions

  • The thermal reaction is disrotatory, and the photochemical process is conrotatory.

• Conjugated π-Orbital Functions

  • Taken together with the very short lifetimes of these excited states (≤10 nsec), this suggests that photochemical products should reflect the rotamer composition of the ground state.
  • Furthermore, 1,2-divinylcyclopentene (drawn in the gray-shaded box) is photochemically unreactive and exhibits fluorescence in solution, demonstrating that twisting about the central (3,4-) double bond of the triene is an essential factor in these reactions.