Atom light induced

Various light-induced reactions including hydrogen atom abstraction, electron transfer and (3-scission occur under the influence of UV light. Certain... 

Schematic representation of the various reaction modes for the dissolution of Fe(III)(hydr)oxides a) by protons b) by bidentate complex formers that form surface chelates. The resulting solute Fe(III) complexes may subsequently become reduced, e.g., by HS c) by reductants (ligands with oxygen donor atoms) such as ascorbate that can form surface complexes and transfer electrons inner-spheri-cally d) catalytic dissolution of Fe(III)(hydr)oxides by Fe(II) in the presence of a complex former e) light-induced dissolution of Fe(III)(hydr)oxides in the presence of an electron donor such as oxalate. In all of the above examples, surface coordination controls the dissolution process. (Adapted from Sulzberger et al., 1989, and from Hering and Stumm, 1990.)...

The initiation step is the light-induced formation of chlorine atoms as the radicals. Only a few chlorine molecules will suffer this fate, but these highly reactive radicals then rapidly interact with the predominant molecules in the system, namely cyclohexane. 

Finally, when we are running out of cyclohexane, the process terminates by the interaction of two radical species, e.g. two chlorine atoms, two cyclohexyl radicals, or one of each species. The combination of two chlorine atoms is probably the least likely of the termination steps, since the Cl-Cl bond would be the weakest of those possible, and it was light-induced fission of this bond that started off the radical reaction. Of course, once we have formed cyclohexyl chloride, there is no reason why this should not itself get drawn into the radical propagation steps, resulting in various dichlorocyclohexane products, or indeed polychlorinated compounds. Chlorination of an alkane will give many different products, even when the amount of chlorine used is limited to molar ratios, and in the laboratory it is not going to be a particularly useful process. 

In both light and high energy irradiation induced reactions, phenyl-thiol, 2-mercaptomesitylene, and their sulfides inhibit nonchain processes,201202 e.g., the light-induced conversion of benzophenone in 2-propanol to benzopinacol and acetone.99 217 The reaction converts compounds into radicals by removal or addition of hydrogen atoms. The sulfur compounds in rapid hydrogen transfer processes convert the free radicals to stable molecules and may do so repeatedly.46 46... 

In the light-induced reactions with diazoalkanes, methylene, the photolytic fragment from diazomethane, may be interposed between the halogen atom in the organic halide and the carbon atom to which it is attached,289 290 or may add to the double bonds in the aromatic ring.58 69... 

In terms of tons of chemicals per year, this is by far the most important industrial process in which a light-induced reaction plays a key role. The photochemical reaction itself is extremely simple, being the homolytic dissociation of molecular chlorine, Cl2, into Cl atoms,... 

Photophysics and photochemistry are relatively young sciences, a real understanding of light-induced processes going back some 50 or 60 years. The development of quantum mechanics was an essential step, as classical physics cannot account for the properties of excited states of atoms and molecules. In the past 30 years the advent of new experimental techniques has given a major impetus to research in new areas of photochemistry, and these are the subject of this final chapter. It must of course be realized that these developments advance all the time, and that we talk here of a moving frontier, as it is in 1992. 

I. e net result of CH4 + Cl- — CH3- + HC1 and CH3- + Cl2 —> CH3CI + Cl- is formation of chloromethane arid hydrogen chloride from methane and chlorine. Notice that the chlorine atom consumed in the first step is replaced by another one in the second step. This kind of sequence of reactions is called a chain reaction because, in principle, one atom can induce the reaction of an infinite number of molecules through operation of a chain or cycle of reactions. In our example, chlorine atoms formed by the action of light on... 

The chlorination of saturated hydrocarbons can be induced by light, but also can be carried out at temperatures of about 300° in the dark. Under such circumstances the mechanism is similar to that of light-induced chlorination, except that the chlorine atoms are formed by thermal dissociation of chlorine molecules. Solid carbon surfaces catalyze thermal chlorination, possibly by aiding in the cleavage of the chlorine molecules. 

Exercise 10-27 A radical of structure CH3C=CHBr is involved in the light-induced addition of HBr to propyne (see above). What geometry would you expect it to have Draw an atomic-orbital picture of the radical with particular attention to the hybridization of the orbitals at the radical center. 

A light-induced redox change of the central atom and an axial ligand, followed by escaping of the redox changed ligand, can be expressed as follows... 

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