Examples of Photochemical Reactions

The kinetics of two photochemical reactions will be compared with the kinetics of the thermal reactions. In the absence of light, hydrogen iodide decomposes by the elementary reaction  

In the initial stages the reverse reaction can be ignored. In either case the rate of reaction can be written 

In the photochemical reaction, at wavelengths below about 327 nm, the mechanism is 

Other possible elementary reactions either have much higher activation energies or require three-body collisions. The rate of disappearance of HI is 

The photochemical reaction between and Btj f ollows a kinetic law which resembles that 

Two commercial photochemical processes quoted in most texts are the syntheses of caprolactam and vitamin D3. The caprolactam process. 

Other than energy considerations, on which there is little comparative data, the most important green role for photochemistry is in improving atom economy. Although only a preliminary research result, an excellent example of this is the avoidance of the need for stoichiometric amounts of Lewis acid catalysts in the synthesis of some acylated aromatic compounds. Benzoquinone can be reacted with an aldehyde under a sunlamp to yield an acylhydroquinone in up to 88% yield. The alternative procedure would involve reaction of an acyl chloride with hydroquinone and a 

Like many of the topics discussed in this book, photochemical reactions are most likely to be used in niche applications for commercial and environmental reasons. Unless there is a major breakthrough in reactor and lamp design, widespread use of this technology is unlikely. Perhaps the best hope of producing high-intensity monochromatic sources of radiation rests with lasers, but currently equipment costs are too high to justify their use for commercial chemical production. 

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Ozone in the atmosphere is a good example of photochemical reactions. Atmospheric ozone is not due to equilibrium. The production and decomposition of ozone are largely by photochemical process, and the concentration of ozone in the stratosphere is at steady state, controlled by the kinetics of photochemical production and decomposition. 

Despite these apparent difficulties, there are now a number of examples for photoinduced electron transfer reactions that are significantly catalyzed. It is the purpose of this chapter to present fundamental concepts and the application of catalysis of photoinduced electron transfer reactions. The photochemical redox reactions, which would otherwise be unlikely to occur, are made possible to proceed efficiently by the catalysis on the photoinduced electron transfer steps. First, the fundamental concepts of catalysis on photoinduced electron transfer are presented. Subsequently, the mechanistic viability is described by showing a number of examples of photochemical reactions that involve catalyzed electron transfer processes as the ratedetermining steps. 

Here are several examples of photochemical reactions, for which the thermal equivalents were forbidden. In most cases the corresponding thermal reactions simply did not occur, like the [2 +2] suprafacial cycloadditions 8.24 -+ 8.25,1097 the [4 +4] suprafacial cycloadditions 8.26 — 8.271099 and 8.28 — 8.29,1100 and the 1,3-suprafacial sigmatropic rearrangement 8.32 —> 8.33.1101 In others, the thermal reactions have a different stereochemistry, as in the antarafacial cheletropic extrusions of sulfur dioxide 8.30 > 8.31,1102 or a different regiochemistry, as in the 1,7-suprafacial sigmatropic rearrangement 8.34 > 8.35.1103 1104... 

There are several well known examples of photochemical reactions which exhibit a discontinuous change in rate constants and quantum yields when they are carried out in solid matrices and not in solution (see Table 2.5). The deviation from first-order kinetics for a reaction carried out in a matrix below its glass transition temperature is a characteristic feature of solid-state reactions. 

It is evident from the scheme shown in Fig. 3.21 that the chemical structures of pesticides are quite diverse they undergo various physico-chemical effects in the environment after application (solar radiation, heat, air, soil, water) as well as being subjected to various metabolic transformations in plants, microorganisms, insect and animals. Common metabolic transformations are schematically surveyed in Table 3.20, which shows that primarily oxidation, hydrogenation, reduction and hydrolytic reactions are concerned. Also among the individual chemical compounds mutual chemical reactions take place. Some examples of photochemical reactions of pesticides in water are presented in Figs 3.22 to 3.26. Biochemical reactions of DDT and DDE are shown in Fig. 3.27. The number of individual chemical species is hence significantly multiplied in the hydrosphere due... 

Many photochromic systems can be handled by the equations given in the previous section. They represent essential examples of photochemical reactions. Their principles and their applications are discussed in detail in the literature. A good review is given in Ref. [162], Typical examinations deal with the photokinetics of fulgides [163-165] and spiropyrans [166-168]. 

Another factor which affects the reactivity of polymer molecules is that the reactants must be expected to achieve a particular geometrical relationship with respect to each other in order to form the excited complex from which reaction occurs. If mobility is severely restricted, this geometry may not be achievable within the lifetime of the excitation, and chemists not familiar with macromolecular reactions might assume that the restrictions of motion in the solid state would be so great that reaction would not occur at all. In fact, this is not the case, and there are many examples of photochemical reactions which are just as efficient in solid polymers as they are in dilute solutions of small-... 

Chain reactions were discovered by M. Bodenstein [1] in 1913 by the example of photochemical reaction of hydrogen chloride formation from molecular chlorine and hydrogen. 

The photodimerisation of anthracene-2-sulphonate in water gives four products A, B, C, D (Scheme 124). However, if the reaction is carried out in presence of p-cyclodextrin, only the isomer A is obtained. " Some other examples of photochemical reactions are ... 

It is well recorded that benzopinacol can be obtained quantitatively on photoirradiation of 4,4 -dimethyl benzophenone in isopropylalcohol. However, in the solid state photoirradiation gives the dimeric compound (Scheme 132). Besides the representative example of photochemical reactions in solid phase a large number of illustrations are available. " ... 

The chemistry of fluorinated 1,3,5-triazines is not as well studied as the chemistry of their chloro derivatives. In case of fluorotriazines the reactions directed on the ring nitrogen atoms, displacement of fluorine atoms and reactions on carbon atoms on the ring with retention of the fluorine atoms appear to be the most characteristic ones. In this section the N-alkylation and N-acylation reactions, as well as replacement of fluorine atoms by a variety of nucleophiles will be considered. Metallation of fluorotriazines and synthesis on the basis of organometallic compounds, as well as the cross-coupling reactions were described. Also several examples of photochemical reactions and transformations are presented. 

Surface modification of the surface of latexes carried out via photochemistry is also presented by Nakashima et al. [126]. In this work, two examples of photochemical reactions on the surface of PS latex particles were reported with the objective of understanding various phenomena that take place in practical coating systems (e.g., photodegradation of pigments and binders). 

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