Ultraviolet radiation free-radical reactions

Most of the reactions the inorganic chemist encounters in the laboratory involve ionic species such as the reactants and products in the reactions just discussed or those of coordination compounds (Chapter 13). However, in the atmosphere there are many free radical reactions initiated by sunlight. One of the most important and controversial sets of atmospheric reactions at present is that concerning stratospheric ozone. The importance of ozone and the effect of ultraviolet (UV) radiation on life has been much discussed. Here we note briefly that only a small portion of the sun s spectrum reaches the surface of the earth and that parts of the UV portion that are largely screened can cause various ill effects to living systems. 

Ethyl- and methyldifluoramine have been prepared by reaction of the respective iodides with tetrafluorohydrazine excited by ultraviolet radiation (5). We therefore investigated the free radical reaction of tert-butyl iodide with tetrafluorohydrazine and found that it produced the desired tert-butyldifluoramine routinely in 40% yield. The reaction is believed to take place by the following steps ... 

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet li t and solar radiation. Free radicals formed by the homol3rtic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involving the production of HCl and conjugated double bonds. 

The oxidation of hydrocarbons, including hydrocarbon polymers, takes the form of a free-radical chain reaction. As a result of mechanical shearing, exposure of ultraviolet radiation, attack by metal ions such as those of copper and manganese as well as other possible mechanisms, a hydrocarbon molecule breaks down into two radicals... 

During the long Antarctic night, appreciable amounts of molecular chlorine, Cl, and hypochlorous acid, HOCl, accumulate within the polar vortex. When the sun returns during the spring (in September in Antarctica), ultraviolet radiation decomposes the accumulated molecular chlorine and hypochlorous acid to produce atomic chlorine. Cl. Atomic chlorine is a free radical. Free radicals are atoms or molecules that contain an unpaired or free electron. The Lewis structures of free radicals contain an odd number of electrons. The unpaired electron in free radicals makes them very reactive. The free radical Cl produced from the decomposition of CI2 and HOCl catalyzes the destruction of ozone as represented by the reaction ... 

At low concentrations of chlorine, dimeric nitrosoalkanes free from chlorine are produced when alkanes are treated also with nitric oxide. Under these circumstances, molecular chlorine is first converted into atomic chlorine which attacks the alkane to form alkyl radicals and hydrogen chloride. The alkyl radicals, in turn, form nitrosoalkanes with nitric oxide. This reaction is most effectively carried out when the ultraviolet radiation is between 380 and 420 mp. [43, 56],... 

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed. 

In addition to ESR spectroscopy, which is a general method for detecting radicals, Dole et al. (9, 10, 11, 12) have developed a method of ultraviolet spectroscopy at low temperatures, which is specific for allylic and polyenylic radicals. Numerous papers have dealt with changes in polymers on irradiation, and all of these conclude that the reactions, in one way or another, arise from the formation of free radicals. Only a few papers describe experiments in which the radicals have been observed directly by ESR or ultraviolet spectroscopy at low temperatures. This article merely summarizes the present knowledge of the nature of radicals formed in polyolefins by irradiation in vacuum (ionizing radiation and ultraviolet light) and discusses some new trends in studying these radicals. 

CFC-12. CFCs escape into the atmosphere and, because of their inertness, remain without further reaction until they reach the stratosphere and the ozone layer. In the stratosphere the high-energy ultraviolet radiation causes a chlorine atom to split off from the CFC molecule. This chlorine atom, or free radical, then reacts with the ozone. 

Free radicals Atoms or groups of atoms with unpaired electrons and which are therefore highly reactive. They can be produced by high-energy radiation such as ultraviolet light in photochemical reactions. 

Phototoxicity occurs when skin exposed to sunlight, especially in the UVA region of 320 to 400 nm, reddens and develops blisters as a consequence of the presence of certain chemical species. The phototoxic chemical species that result in such reactions are ones to which an individual is exposed either directly on the skin or systemically. These compounds absorb ultraviolet radiation and, like the porphyrins discussed above, enter excited states interacting with 02 to generate destructive oxidant species and free radicals. Numerous chemical species, including furocoumarins, polycyclic aromatic hydrocarbons, tetracyclines, and sulfonamides, can be phototoxic. 

The free radical photoinitiator may be any compound that produces a free radical on exposure to radiation, such as ultraviolet or visible radiation, and thereby initiates a polymerization reaction. 

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