Halogenation photochemical free-radical

Replacement of halogen by an allyl moiety via thermal or photochemical free radical reaction with trialkylallylstannanes. 

Of course, in the atmosphere S(IV) and halogens will be present simultaneously and their chemistries become intertwined. In addition, other oxidants such as 03, H202, and OH are present in the gas phase and can be taken up into solution. Subsequent photochemical reactions of 03 and H202 generate a variety of free radicals, including OH and H02. This chemistry is summarized in the following chapter. 

Free-radical halogenation of hydrocarbons induced thermally or photochemically can be performed with all four halogens, each exhibiting certain specificities. Because of the thermodynamics of the process, however, only chlorination (and bromination) are of practical importance.31,106-108 Fluorination with elemental fluorine is also possible. This reaction, as discussed above (see Section 10.1.1), follows an electrophilic mechanism in the solution phase.109,110 Under specific conditions, however, free-radical fluorination can be performed. 

A third transition of less significance is the n-+o transition this is observed in the photolysis of halogenated compounds.7 The a energy level is unusually unstable and the molecule undergoes bond cleavage with the formation of free-radical species. Many photoreactions can, in fact, be thought of in radical terms, and close analogies exist between certain photochemical reactions and free-radical processes. 

The reverse process, photochemical transfer of an electron from the solvent molecule to the central metal ion, has been postulated in the case of several of the more strongly oxidizing metal ions, and, more recently, on the basis of an extensive study of the spectra of free radicals such as halogen atoms and OH. By analogy with Eq. 

Studies of the reactions of many atmospherically important atomic and free radical species were described in Section 9 this Section deals primarily with important molecular species. A brief review of the progress achieved recently in the field of atmospheric chemistry has been provided by Cox, " with emphasis on the reactions of O3 and important H-, N-, C-, halogen-, and S-containing species. Waynehas reviewed extraterrestrial atmospheric photochemistry and Strobel " has reviewed the photochemistries of the atmospheres of Jupiter, Saturn, and Titan. Kaye and Strobeldescribed a 1-dimensional photochemical model of PHj chemistry in the atmosphere of Saturn. A study of the photochemical reactions of H2O and CO in the Earth s primitive atmosphere has been presented by Bar-Nun and Chang. " They concluded that even if the primitive atmosphere initially contained no H2 and contained carbon only in the form of CO and CO2, photochemical processes would have enriched the environment with a variety of organic compounds. 

Chignell, C.F. and Sik, R.H (1989) Spectroscopic studies of cutaneous photosensitizing agents. XIV. The spin trapping of free radicals formed during the photolysis of halogenated salicylanilide antibacterial agents, Photochem. Photobiol., 50, 287-295. 

The most important applications of photochemical reactions in organic technol-ogy/synthesis are in the field of free radical reactions such as halogenation (mainly chlorination), sulfochlorination, sulfoxidation, and nitrosation. There is an increasing trend now toward development of nonradical photoreactions, particularly in the synthesis of vitamins, drugs, and fragrances. Table 26.2 lists representative syntheses of commercial value that have been carried out with light (see, among others, Coyle, 1986 Pfoertner, 1991, for more examples). 

The observation of PSC indicates a high cation-radical lifetime, t > (al) (a = absorption cross section). If a = 10 -10 cm [5], I = 10 cm s , an estimate of t > 10 s is obtained. It should be noted that the resrrlts of this study evidences that there is a range of the cation-radical states from labile (t = 1-10 s) to a fully stable ones (t > s) which are involved in the process of photostimirlated generation. Similar stabilized cation-radicals and related PSCs for the PI based on substituted triphenylamines resulting from irreversible photochemical transformation of free radical type with the halogen hydrocarbons are observed in [45]. 

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